stratigraphy and strontium geochemistry of messinian evaporite bearing successions of the southern apennines foredeep italy implications for the mediterranean salinity crisis and regional palaeogeography an integrated study comprising detailed mapping stratigraphy evaporite facies analysis strontium content and 87sr 86sr ratio data is presented for two messinian evaporite bearing units cropping out in irpinia daunia mountains southern apennines italy these units record the tectonic and palaeogeographic evolution of the messinian southern apennine foreland basin system during the abrupt salinity crisis which occurred in the mediterranean region the daunia unit comprises the monte castello evaporites formed by euxinic diatomitic marls evaporitic limestones and shallow water gypsum having a 87sr 86sr average value of 0 70898 0 00005 close to the sr isotopic value of the coeval seawater these marginal evaporites are capped by an erosional unconformity and continental clastic deposits the vallone del toro unit is made up of thin bedded marls claystones calcilutites and diatomite layers with small gypsum crystals laminated gypsum and gypsarenite testifying deposition in relatively deep water settings gypsum has variable sr isotopic data with an average value of 0 70899 0 00005 for the argilliti policrome del torrente calaggio formation and an average value of 0 70863 0 00014 for the argilliti di mezzana di forte formation reflecting major riverine freshwater input the different patterns of lithofacies and sr isotopic composition of gypsum discriminate two distinctive evaporite units reflecting the tectonic and palaeogeographic controls within the apennines mountain belt and the mediterranean data for evaporites of the southern apennines testify the presence of basins in the central and eastern mediterranean sea which never desiccated during the entire salinity crisis and evolved to widespread lago mare conditions in the latest messinian

distribution characterization and geochemical controls of elements of concern in uranium mine tailings key lake saskatchewan canada abstract the distribution of as mo ni and se in the aqueous and solid phases of u mine tailings in the deilmann tailings management facility dtmf located at the key lake mine in northern saskatchewan canada was defined using as discharged tailings samples collected monthly starting in 1996 and core samples collected from the dtmf from 3 locations on 3 occasions between 2004 and 2009 these data indicated that the dtmf can be divided into two geochemical zones tailings generated from the deilmann ore body between 1996 and 2000 and tailings from the mcarthur river ore body mined after july 2000 the deilmann tailings solids are generally characterized by greater elemental concentrations than those in the mcarthur river tailings particularly for as co and ni these elevated concentrations are attributed to the abundance of ni co as s minerals in this ore the mean aqueous concentrations of as mo ni and se are 3 7 23 2 0 14 and 0 02 mg l 1 respectively in the deilmann porewaters and 0 20 4 16 0 06 and 0 03 mg l 1 respectively in the mcarthur river porewaters similarly the mean as mo ni and se solid phase concentrations are 5 89 10 3 69 9 3 20 10 3 and 17 4 g g 1 respectively for the deilmann tailings and 440 13 6 551 and 3 03 g g 1 in the mcarthur river tailings statistically ph remained unchanged from the time of discharge while eh values have remained oxic with no significant change between the two tailings types over 15 a of residence time results from sequential extractions thermodynamic modeling and results of complimentary research indicated that ferrihydrite solubility is the dominant control on aqueous as concentrations and also plays a role in controlling dissolved mo ni and se

conduit enlargement during the precursory plinian eruption of aira caldera japan increase in magma flux as the result of conduit enlargement is one of the key processes that triggered caldera collapse and eruption of the ito ignimbrite from aira caldera at 29 ka we examine the total volume of the pumice fall deposit vertical variations in grain size of pumice and the lithic content in the osumi pumice deposit to investigate the trigger for caldera collapse wider distribution of the later stage unit and the upward coarsening of grain size throughout the osumi pumice fall deposit indicate an increase in magma discharge toward the onset of collapse the total volume of lithic fragments in the osumi pumice fall deposit is estimated as 1 6 km3 based on the lithic content in several representative outcrops and the total volume of the osumi pumice fall deposit the lithic fragments in the osumi pumice fall deposit indicate intense mechanical erosion of the conduit during the plinian eruption prior to caldera collapse caldera collapse requires decompression of the magma chamber by withdrawal of magma effective enlargement of the conduit diameter during precursory eruptive phases is one of the important processes that subsequently allow the rapid discharge of a large volume of magma which in turn facilitates decompression of the reservoir and induces caldera collapse

morphometry of terrestrial shield volcanoes abstract shield volcanoes are described as low angle edifices built primarily by the accumulation of successive lava flows this generic view of shield volcano morphology is based on a limited number of monogenetic shields from iceland and mexico and a small set of large oceanic islands hawaii galapagos here the morphometry of 158 monogenetic and polygenetic shield volcanoes is analyzed quantitatively from 90 meter resolution srtm dems using the morvolc algorithm an additional set of 24 lava dominated shield like volcanoes considered so far as stratovolcanoes are documented for comparison results show that there is a large variation in shield size volumes from 0 1 to 1000 km3 profile shape height basal width h wb ratios mostly from 0 01 to 0 1 flank slope gradients average slopes mostly from 1 to 15 elongation and summit truncation although there is no clear cut morphometric difference between shield volcanoes and stratovolcanoes an approximate threshold can be drawn at 12 average slope and 0 10 h wb ratio principal component analysis of the obtained database enables to identify four key morphometric descriptors size steepness plan shape and truncation hierarchical cluster analysis of these descriptors results in 12 end member shield types with intermediate cases defining a continuum of morphologies the shield types can be linked in terms of growth stages and shape evolution related to 1 magma composition and rheology effusion rate and lava pyroclast ratio which will condition edifice steepness 2 spatial distribution of vents in turn related to the magmatic feeding system and the tectonic framework which will control edifice plan shape and 3 caldera formation which will condition edifice truncation

basaltic calderas collapse dynamics edifice deformation and variations of magma withdrawal 1 the incremental caldera collapses of fernandina 1968 miyakejima 2000 and piton de la fournaise 2007 are analyzed in order to understand the collapse dynamics in basaltic setting and the associated edifice deformation for each caldera the collapse dynamics is assessed through the evolution of the 1 time interval t between two successive collapse increments 2 amount of vertical displacement during each collapse increment and 3 magma outflow rate during the whole collapse caldera process we show from the evolution of t that piton de la fournaise and fernandina were characterized by a similar collapse dynamics despite large differences in the caldera geometry and the duration of the whole collapse caldera process this evolution significantly differs from that of miyakejima where t strongly fluctuated throughout the whole collapse process quantification of the piston vertical displacements enables us to determine the magma outflow rates between each collapse increment displacement data tiltmeter and or gps for piton de la fournaise and miyakejima are used to constrain the edifice overall deformation and the edifice deformation rates these data reveal that both volcanoes experienced edifice inflation once the piston collapsed into the magma chamber such a deformation which lasts during the first collapse increments only is interpreted as the result of larger volume of piston intruded in the magma chamber than magma withdrawn before each collapse increment once the effect of the collapsing rock column vanishes edifice deflates we also determine for each caldera the critical amount of magma evacuated before collapse initiation and compare it to analog models the significant differences between models and nature are explained by the occurrence of preexisting weak zones in nature i e the ring faults that are not taken into account in analog models finally we show that t at piton de la fournaise and fernandina was equally controlled by the frictional resistance along the ring faults and the magma outflow rate in addition to these two parameters the collapse dynamics of miyakejima was also influenced by variations of the magma bulk modulus which changed after the influx of deep gas rich magma into the collapse related magma chamber altogether our results show that the dynamics of caldera collapse in basaltic volcanoes proceeds in two phases phase 1 starting with the first collapse is characterized by the largest collapse amplitude an incremental edifice inflation and a step by step increase of the rate of magma outflow phase 2 shows a rapid decrease of the magma discharge rate to a low level concomitant with the continuous edifice deflation if deep magma is injected into the magma chamber as at miyakejima an additional phase occurs phase 3

the northern apennines palynological record as a contribute for the reconstruction of the messinian palaeoenvironments abstract the messinian stage has long been associated with an overall warm and dry climate whereas recent researches indicate either a warm and humid or a cool and dry climate the integrated stratigraphic record of vegetation and climatic changes from northern apennines sites provides the solution to this apparent contradiction its integration with the updated geological and sedimentological studies provides additional data for the reconstruction of the depositional palaeoenvironments in both marginal and deeper sub basins of the apennines foredeep the onset of the mediterranean salinity crisis msc is recorded in the gessoso solfifera of the vena del gesso marginal sub basin cyclical humid conditions corresponding to precession minima developed during the deposition of the shales interbedded with the gypsum 5 9 to 5 6 ma some cooler events took also place under the effects of global glacial stadials and regional factors apennines uplift at present no major changes from moist to dry conditions are attested to just before the salinity crisis as well as in sicily so climate did not play a major role in the onset of the msc despite the favourable context provided by inferred thermo xeric conditions in southern italy a drier episode indicated by the expansion of the open vegetation including the northward migration of lygeum postdates the onset of the salinity crisis of about 400 kyr in the lower post evaporitic deposits of maccarone deeper sub basin it falls within a period of global warming whereas at a regional scale it could correlate p p to the evaporite deposition in deeper basins and to hiatuses in the marginal basins of sicily and of the western sector of northern apennines its sudden end about 100 kyr later in coincidence with a significant increase of pinaceae indicates a turnover in the terrestrial setting not linked to major climate changes but possibly to a complex interaction between other palaeoenvironmental factors e g tectonics and eustatism in contrast organic walled dinoflagellate cysts exclude significant modifications in aquatic settings insaturation of either open marine or brackish conditions in the latter a later change is marked by the arrival of impagidinium sp 1 a species here referred instead to caspidinium rugosum about 7 m below the first colombaccio this occurrence together with the spread of pediastrum indicates a freshwater dilution i e the lago mare event during wetter climatic conditions on the adjacent landmass increase of tsuga and cedrus the successive arrival and or dominance of other paratethyan taxa such as i sp 2 i sp 3 and galeacysta etrusca indicate highly variable water environments marine vs continental water inputs during the deposition of the uppermost post evaporitic deposits the lago mare is stratigraphically sandwiched between an ash layer 130 m below dated at 5 5 ma and the beginning of the pliocene where a peak of impagidinium patulum marks the onset of open marine conditions the dominant humid subtropical to warm temperate climate indicates differences in both temperature and moisture values with respect to the coeval southern sections revealing climatic gradients within the mediterranean at least from the messinian no dramatic vegetation and climate changes have been recorded during the msc major changes occurred later as indicated by the palynological record from 2 6 ma this palynostratigraphic record is a good reference for more recent models of the development of the msc and for establishing time relationships between the apennine and sicilian successions

how summit calderas collapse on basaltic volcanoes new insights from the april 2007 caldera collapse of piton de la fournaise volcano in april 2007 piton de la fournaise volcano experienced a caldera collapse during its largest historical eruption we present here a structural analysis both of the caldera and the surrounding area and precise gps data recorded with a dense gps network specifically dedicated to the analysis of deformation related to the summit collapse structures despite a collapse of more than 300 m in the central zone the geometry of the new caldera is similar in map view to that of the pre existing collapsed structure which was formed from the coalescence of several pit craters the caldera shows an asymmetric inner geometry with sub vertical walls in the nw quadrant and steep scarps composed of inward tilted blocks in the southern half the presence of preserved polished surfaces on the lower part of the sub vertical scarp indicates that it corresponds to the caldera north western ring fault the april 2007 caldera collapse led to the development and the reactivation of concentric fractures on the caldera rim mostly along the southern limit of the caldera gps data show that fractures result from radial extensional stresses that are restricted within the first tens of meters of the caldera edge gps data also reveal that the caldera collapse was coeval with a centripetal deflation whose magnitude is largest along the southern half of the caldera the displacements recorded by gps result from both a general deflation due to magma withdrawal from piton de la fournaise s summit magma chamber and additional local effects related to the caldera collapse comparison of the caldera collapses at piton de la fournaise miyakejima and fernandina reveals striking similarities with cyclic seismic signals accompanying small scale deflation inflation cycles this strongly suggests a common mode of collapse hence we propose a unifying model of caldera collapse in basaltic setting in which the inward deflation due to magma withdrawal from the magma chamber prevents the collapse of the caldera roof until the gravitational stress acting on the rock column above the magma chamber exceeds the shear strength along pre existing ring faults the downward displacement stops when the pressure increase into the magma chamber is able to again sustain the rock column the succession of 1 inward deflation that prevents the collapse 2 collapse due to gravitational stress and 3 stopping of the downward motion is repeated many times the frequency of the cycles is influenced by the rate of magma withdrawal and by the amount of intrusion of magma along the ring faults

the caldera forming eruption of volcan ceboruco mexico of magma erupted 95 of which was deposited as fall layers during most of the deposition of p1 eruptive intensity mass flux was almost constant at 4 8 107 kg s 1 producing a plinian column 25 30 km in height size grading at the top of p1 indicates however that mass flux waned dramatically and possibly that there was a brief pause in the eruption during the post p1 phase of the eruption a much smaller volume of magma erupted although mass flux varied by more than an order of magnitude we suggest that caldera collapse began at the end of the p1 phase of the eruption because along with the large differences in mass flux behavior between p1 and post p1 layers there were also dramatic changes in lithic content p1 contains 8 lithics post p1 layers contain 30 60 and magma composition p1 is 98 rhyodacite post p1 layers are 60 90 rhyodacite however the total volume of magma erupted during the jala pumice event is close to that estimated for the caldera these observations appear to conflict with models which envision that after an eruption is initiated by overpressure in the magma chamber caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma the conflict arises because firstly the p1 layer makes up too large a proportion 75 of the total volume erupted to correspond to an overpressurized phase and secondly the caldera volume exceeds the post p1 volume of magma by at least a factor of three the mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption but no record of such early collapse is evident in the tephra sequence the apparent inability to place the jala pumice eruptive sequence into existing models of caldera collapse which were constructed to explain the formation of calderas much greater in volume than that at ceboruco may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed

the pliocene pleistocene stratigraphic and tectonic evolution of the central sector of the western periadriatic basin of italy abstract the central portion of the western periadriatic basin is a foreland basins systems associated to the central apennine outer orogenic wedge that stretches along the marche and abruzzi regions of italy an area that has been studied since long time and where a plenty of surface and subsurface data are available the present study is aimed to review and synthetize these data add new surface and subsurface data in order to define a firm stratigraphic and tectonic framework that is base for the reconstruction of the pliocene pleistocene tectonic stratigraphic evolution of the central portion of the western periadriatic basin the study integrates field mapping published geological maps seismic lines well logs and an updated chronostratigraphic scheme six major nne ssw thrust stacks and eight unconformity bounded stratigraphic units have been defined then a ne sw oriented zone separates a northern from a southern sector which have differences in structural styles amount of shortening basins widths and thicknesses of stratigraphic units despite these differences the deformation timing the depositional history and the migration of basins system are common in both sectors and eight tectonic stratigraphic evolutionary stages have been reconstructed with sequential restoration of two representative cross sections the results highlight that the pliocene pleistocene stratigraphic and tectonic evolution of the central western periadriatic basin preserves the records of supra regional and global controls which drive thrust propagation orogen growth basins migration and basins sedimentary infill in particular the stratigraphy preserves global third order eustatic climatic changes that have a strict feed back and concomitance with tectonic pulses the latter are regulated 1 by cyclic growth of an orogenic wedge undergoing coeval erosion and sedimentation and 2 by eastward retreat of a westward subducting and flexing continental lithosphere

the caldera forming eruption of the quaternary payun matru volcano andean back arc of the southern volcanic zone abstract the payun matru caldera is a quaternary andean back arc caldera 36 25 s 69 12 30 w and represents the main volcanic edifice of the homonym payun matru volcanic field located within the payenia basaltic province the payun matru volcanic field is also characterized by the payun liso volcano and 220 monogenetic basaltic cones and associated lava flows the pleistocene holocene payun matru caldera presents a long lived pre caldera stage which built a multi vent and shield like volcanic edifice mainly trachytic in composition the syn caldera stage is represented by the trachytic portezuelo ignimbrite the caldera forming eruption deposits constrained between 148 and 82 ka and the resulting caldera depression of 8 5 km in diameter afterwards the caldera event volcanism continued mostly along the caldera rim stratigraphic sections of the portezuelo ignimbrite and its correlations are presented in addition to mineral compositions and previous whole rock chemical analyses only the extra caldera deposits can be observed while the intra caldera ignimbrite is hidden by post caldera volcanic products and unconsolidated modern sediments different facies within the ignimbrite were recognized being most of them massive or eutaxitic and massive lapilli tuff facies deposited by dense pyroclastic density currents fall deposits are found only proximally and restrained to the caldera margin the absence of a widespread fall deposit suggests the lack of a sustained eruptive column at the onset of eruption massive tuff facies at the topmost deposits indicates the development of a co ignimbritic plume as the pyroclastic density currents wanned on the basis of color size and shape vesicularity and crystallinity four distinct juvenile clasts types were defined gray fiamme black juvenile dark gray and light gray pumice available chemical analysis of gray fiamme and black juvenile clasts indicate a similar trachytic composition although their mineralogical composition presents some differences several eruptive units for the northern and southern outer slopes of the caldera were defined on the basis of the juvenile types present and discontinuity surfaces the onset of caldera collapse is not marked by the presence of a lithic breccia but it is suggested by the appearance of dark gray and light gray pumice along with black juveniles in the topmost eruptive units together with an increase in lithic content relative to previous units the sequential appearance of different juvenile clasts types suggests a zoned magma chamber in terms of crystal content and composition

vertical and lateral collapses on tenerife canary islands and other volcanic ocean islands recent studies demonstrate that tenerife has undergone large lateral collapses this has led to the suggestion that the las canadas caldera one of the best exposed calderas in the world is the result of lateral collapse we have tested this idea using the available structural stratigraphic volcanological and geochronological data we conclude that the las canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of tenerife our results indicate however that vertical collapses may have played a major role in triggering lateral collapses we propose that the association of vertical and lateral collapse events such as inferred for tenerife can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands

stress fields controlling the formation of nested and overlapping calderas implications for the understanding of caldera unrest abstract commonly the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area in many cases successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures occasionally subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures during the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium the numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas our models take into account the effect of the collapse structure considering it as a mechanical discontinuity we also investigate the mechanical influence of the intra and extra caldera deposits on the formation of new calderas all the numerical models are two dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material the computational domain corresponds to a cross section of the upper crust 50 25 km and magma chambers are modelled as sill like cavities located at a certain depth below the earth s surface the existing collapse caldera depression is 8 km wide and 2 75 km deep however we consider the caldera infill i e intra caldera material to be 1 25 or 1 75 km thick we assume as loading conditions an underpressure of 10 mpa imposed at the chamber walls that is negative excess pressure in the chamber in some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins we find that the stress field around shallow level magma chambers favouring the formation of either nested or overlapping caldera structures depend on 1 the dimensions of the previous caldera collapse structure 2 the size of the new magma chamber susceptible to generate the subsequent caldera forming event and 3 the mechanical properties of the surrounding host rock and the syn and post collapse materials moreover results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems our numerical results confirm that the formation of multiple calderas nested overlapping or separate imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera in consequence interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera

easternmost mediterranean evidence of the zanclean flooding event and subsequent surface uplift adana basin southern turkey according to the literature the adana basin at the easternmost part of the mediterranean basin in southern turkey records the pliocene stage with shallow marine to fluvial deposits our micropalaeontological analysis of samples from the adana basin reveal late lago mare biofacies with paratethyan ostracod assemblages pertaining to the loxocorniculina djafarovi zone grey clays rich in planktonic foraminifera lie above the lago mare deposits within the grey clays the continuous occurrence of the calcareous nannofossil reticulofenestra zancleana and the base of the reticulofenestra pseudoumbilicus paracme points to an early zanclean age 5 332 5 199 ma both ostracod and benthic foraminifera indicate epibathyal and bathyal environments 87 sr 86 sr measurements on planktonic and benthic foraminifera fall below the mean global ocean value for the early zanclean indicating potentially insufficient mixing of low 87 sr 86 sr mediterranean brackish lago mare water with the global ocean in the earliest pliocene we utilize the ages and palaeodepths of the marine sediments together with their modern elevations to determine uplift rates of the adana basin of 0 06 to 0 13 mm a 1 since 5 2 5 3 ma total uplift of 350 650 m from surface data and 0 02 0 13 mm a 1 since c 1 8 ma total uplift of 30 230 m from subsurface data

the mineralogy of arsenic in uranium mine tailings at the rabbit lake in pit facility northern saskatchewan canada a detailed investigation of the mineralogy of as in the tailings of the rabbit lake uranium ore processing facility was conducted the milling ore extraction process was sampled at three different locations to obtain information about when where and under what condition secondary as phases form these samples were compared with four samples of varying as content from the rabbit lake in pit tailings management facility tmf up to 20 as in the tailings are present in primary minerals that reach the tailings directly because they are not dissolved during the uranium extraction the remaining 80 constitute as that was dissolved during ore extraction and then re precipitated before being discharged into the tailings pond it was not possible to conclusively identify any individual re precipitated secondary as minerals in the rabbit lake tmf indirect evidence from sequential extraction analyses suggests the presence of an amorphous ca as phase and a possible but unlikely minor amount of an amorphous fe as phase however the close association between hydrous ferric oxide hfo and as could be clearly demonstrated hfo was identified to be 2 line ferrihydrite and its xrd pattern geometry indicates a substantial amount of adsorbed as this is in good agreement with sem tem and sequential extraction analyses that all showed the close association of hfo and as

geochemistry of arsenic in uranium mine mill tailings saskatchewan canada abstract the rabbit lake u mine in pit tailings management facility tmf 425 m long 300 m wide 91 m deep is located in northern saskatchewan canada the objectives of this study were to quantify the distribution of as phases in the tailings and evaluate the present day geochemical controls on dissolved as these objectives were met by analyzing pore fluid samples collected from the tailings body for dissolved constituents measuring eh ph and temperature of tailings core and pore fluid samples conducting sequential extractions on solid samples conducting geochemical modeling of pore fluid chemistry using available thermodynamic data and by reviewing historical chemical mill process records dissolved as concentrations in 5 monitoring wells installed within the tailings body ranged from 9 6 to 71 mg l pore fluid in the wells had a ph between 9 3 and 10 3 and eh between 58 and 213 mv sequential extraction analyses of tailings samples showed that the composition of the solid phase as changed at a depth of 34 m the as above 34 m was primarily associated with amorphous fe and metal hydroxides while the as below 34 m was associated with ca likely as amorphous poorly ordered calcium arsenate precipitates the change in the dominant as solid phases at this depth was attributed to the differences in the molar ratio of fe to as in the mill tailings below 34 m it was 4 the high ca as ratio during tailings neutralization would likely precipitate ca4 oh 2 aso4 2 4h2o type ca arsenate minerals geochemical modeling suggested that if the pore fluids were brought to equilibrium with this ca arsenate the long term dissolved as concentrations would range between 13 and 126 mg l

evolution of the caldera forming eruption at crater lake oregon indicated by component analysis of lithic fragments crater lake caldera 8 10 km formed 6845 years b p 14c age during the climactic eruption of the volcanic edifice known as mount mazama is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of mount mazama initial activity was from a single vent that produced a rhyodacite pumice fall from a plinian column altered preexisting volcanic rocks are the predominant lithic type in the plinian deposit and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin the wineglass welded tuff lies atop the plinian deposit and contains a higher proportion of fresh volcanic rocks suggesting enlargement of the single vent by slumping of its walls this same vent enlargement caused the plinian eruption column to collapse and feed valley hugging pyroclastic flows that deposited the wineglass welded tuff when enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported mount mazama collapsed to form the caldera while highly energetic pyroclastic flows produced the climactic ignimbrite a lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the wineglass welded tuff and contains predominantly altered volcanic rocks of deeper origin accompanied by minor granitoids from the magma chamber walls azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence such as crater lake nevertheless may form by syneruptive subsidence along ring fractures

microscale mineralogical characterization of as fe and ni in uranium mine tailings abstract uranium u ores can contain high concentrations of elements of concern eocs such as arsenic as and nickel ni present in sulfide and arsenide minerals the u in these ores is often solubilized by adding h2so4 to attain a ph 1 under oxic conditions this process releases some eocs from the primary minerals into solution the barren raffinate solution remaining after u extraction is subsequently neutralized with ca oh 2 to a terminal ph of 10 5 resulting in a reduction in the aqueous concentrations of the eocs these neutralized raffinates are mixed with the non reacted primary minerals and discharged as tailing into tailings management facilities tmfs to aid in the accurate characterization and quantification of the mineralogical controls on the concentrations of eocs in the tailings porewater their spatial distribution and speciation were studied at the micron scale in tailings samples collected from the deilmann u tailings management facility dtmf northern saskatchewan canada backscattered electron images of the tailings samples generated using an electron microprobe show the presence of nodules 10 200 m size surrounded by bright rims wavelength dispersive spectrometric wds and synchrotron based micro x ray fluorescence xrf elemental mapping show that the nodules are dominated by ca and s as gypsum and the bright rims are dominated by fe as and ni micro x ray absorption near edge structure xanes spectra collected within and near the rims indicate that the fe and ni are present mainly in the 3 and 2 oxidation states respectively for as the 5 oxidation state dominates but significant amounts of the 3 oxidation state are present in some areas linear combination fit analyses of the k edges for the fe as and ni xanes spectra to reference compounds suggest the fe in the rims is present as ferrihydrite with as and ni are adsorbed to it energy dispersive spectrometric eds data indicate that isolated highly reflective particles distributed throughout the tailings matrix are primary as cu fe and ni bearing minerals geochemical modeling of the neutralization process shows that the nodules gypsum formed at ph 1 and acted as a substrate for the precipitation of ferrihydrite at ph 3 4 the as and ni subsequently adsorb to the ferrihydrite overall the microscale data suggest that the as and ni adsorbed onto the ferrihydrite should remain stable for many years and continue to the control the fe as and ni concentrations in the tailings porewater

an integrated model of magma chamber conduit and column for the analysis of sustained explosive eruptions abstract explosive volcanic eruptions comprise a complex series of processes involving withdrawal from the magma chamber magma ascent along the conduit and eruption column dynamics numerous studies have modeled the different sub domains of a volcanic system but their interplay has seldom been analyzed to this end we developed c 3 c cubed that stands for chamber conduit and column a new integrated model that describes the dynamics of an explosive eruption as a series of steady state regimes and as a function of geometry and initial conditions of the magma reservoir we used global sensitivity analysis to quantify the role of the relevant model parameters and describe the interplay between the different volcanic sub domains in particular we analyzed the evolution of a sustained explosive eruption in order to identify the conditions for buoyant super buoyant and collapsing columns input data were based on field reconstructions of quaternary explosive eruptions in the vulsini volcanic district roman province central italy model results show that 1 the column regime although affected by complex interactions among several factors mostly depends on the conduit radius the volatile content i e supersaturation concentration at the top of the chamber and length of the conduit in decreasing level of importance 2 the amount of mass erupted is independent of the conduit radius and depends mostly on volatile supersaturation the radius of the magma chamber the length of the conduit and the overpressure at the conduit inlet 3 the mass flow rate column height and duration of the eruption are largely controlled by the conduit radius 4 the flow pressure and density at the conduit exit are mostly controlled by the conduit inlet overpressure at the onset of the eruption and by the length of the conduit at the end of the eruption 5 the exit velocity from the conduit is mostly controlled by the volatile content the length of the conduit and the inlet overpressure in this model framework and with specific reference to selected plinian events of the vulsini volcanic district simulation results show that column collapse is not achieved for reasonable eruption durations order of hours and conduit widths tens of meters this is consistent with field reconstructions suggesting that column collapse did not likely occur and that pyroclastic flows were therefore generated by independent mechanisms from ring fissures and or multiple vents concomitant to caldera collapse

on the origin of the strait of gibraltar most interpretations of the early pliocene opening of the strait of gibraltar involve a tectonic process however no tectonic structure of this age has been unequivocally documented that could account for such a hypothesis on the other hand the sea level drop of the mediterranean during the messinian salinity crisis has dramatically enhanced continental erosion and in particular regressive fluvial erosion we show that such erosional process inevitably developed in the gibraltar area we finally propose that regressive fluvial erosion was at the origin of the opening of the strait of gibraltar

formation and development of normal fault calderas and the initiation of large explosive eruptions the ring fractures that form most collapse calderas are steeply inward dipping shear fractures i e normal faults at the surface of the volcano within which the caldera fault forms the tensile and shear stresses that generate the normal fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano numerical results indicate that normal fault calderas may initiate as a result of doming of an area containing a shallow sill like magma chamber provided that the area of doming is much larger than the cross sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming this model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera when the caldera fault does not slip eruptions from calderas are normally small nearly all large explosive eruptions however are associated with slip on caldera faults during dip slip on and doming of a normal fault caldera the vertical stress on part of the underlying chamber suddenly decreases this may lead to explosive bubble growth in this part of the magma chamber provided its magma is gas rich this bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption

inferring magma ascent timescales and reconstructing conduit processes in explosive rhyolitic eruptions using diffusive losses of hydrogen from melt inclusions abstract we integrate field observations with geochemical data from quartz hosted melt inclusions to develop a hydrogen diffusion based method to shed light on timescales and conduit processes during the opening stages of two supereruptions we focus on the initial explosive phases first 5 of the total magma volume erupted from 1 the episodic oruanui eruption taupo nz 530 km3 25 4 ka and 2 the continuous bishop tuff eruption long valley 650 km3 0 765 ma quartz hosted melt inclusions for both eruptions exhibit coherent trends for major and trace elements e g al2o3 sio2 nb u rb however in the first three phases of ten in the oruanui eruption there is significant variability in measured h2o values 2 6 5 8 wt particularly in phases 1 and 3 each of which reflects the opening stages of two respective inferred vent sources phase 3 inclusions also record greater scatter in li concentrations in contrast melt inclusions from the first two bishop tuff fall layers yield a narrower range of h2o concentrations 4 0 5 8 with 90 between 4 5 and 5 8 wt we interpret the scatter in measured h2o concentrations to represent changes during slow magma ascent or partial stalling within the conduit at lower pressures using a diffusion model we estimate that 61 of the oruanui melt inclusions experienced modification of original h2o concentrations due to slow ascent in the conduit system prior to eruptive quenching with the longest timescales 5 days mostly in phase 3 inclusions in contrast only 33 of melt inclusions from the initial bishop fall layer f1 experienced diffusive losses from their starting concentrations implying that most magma spent less than a day ascending from the storage region at the eruption onset similarities between the oruanui and the previously studied huckleberry ridge tuff include comparable extents and distribution of diffusive losses seen as h and li scatter and field evidence for episodic activity our observations imply that in these two systems the first erupted magmas rose from reservoirs with low degrees of overpressure and that magma mobilization was controlled by external factors e g tectonics rather than by strong volatile over pressures in contrast the bishop eruption may well have been initiated by overpressure leading to continuous escalating activity

messinian paleogeography and erosional surfaces in italy an overview abstract the messinian deposits of italy display an extreme variability both lateral and vertical including strong changes in salinity and in sediment accumulation rates this variability strongly contrasts with the much more uniform nature of both the older i e tortonian and younger i e zanclean strata changes in salinity range from fresh water to hypersaline conditions leading to evaporite precipitation salinity crisis the chloride facies it recorded in calabria facing the deep ionian sea in the central sicilian basin and in a single satellite basin of southern tuscany fed by the ligurian sea erosional surfaces are widespread at the southern edge of the alps and extend to the subsurface of the po basin rapidly subsiding and outward moving foredeeps deep seated brine pools uplifted in post messinian times post tectonic extensional basins provide a different stratigraphic record of the same paleogeographic event a dramatic drop of sea level which lasted overall less than one million years

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

climatically versus tectonically forced erosion in the alps thermochronometric constraints from the adamello complex southern alps italy abstract we apply u th sm he dating and fission track thermochronometry of apatites to the adamello complex located in the southern european alps our goal is to constrain exhumation rates since the miocene in particular to determine whether the increase in sedimentation documented in the foreland basins since 5 ma is recorded in cooling rates of the southern alps thermochronometry provides a cooling rate for rocks which can be converted to an exhumation rate through thermal modeling thereby quantifying the amount of rock exhumed at different time scales apatite fission track and u th sm he analyses all record miocene ages we find no evidence for an increase in exhumation rate during the plio pleistocene the results of our exhumation rate estimates and tectono geomorphic modeling suggest that unroofing during thrust propagation of the southern alps in the late miocene drove most of the exhumation while changes in relief mainly associated with plio pleistocene climate change likely did not affect the adamello complex the findings from this study combined with other studies highlight that there appears to be a gradient across the alps with more intensive plio pleistocene erosion in the western and central alps compared to the eastern and southern alps

the timing of caldera collapse at mount katmai in response to magma withdrawal toward novarupta several thin layers of nonjuvenile lithic rich tephra from mount katmai are intercalated with 1912 plinian pumice fall deposits that originated at novarupta 10 km away demonstrating that caldera collapse at katmai and eruption at novarupta were synchronous emplaced by phreatic explosions and avalanche generated dustclouds each katmai derived layer is thickest and coarsest at the caldera rim and pinches out within 5 8 km in contrast thicknesses and clast sizes of plinian layers a g decrease radially away from novarupta the first katmai derived layer lies between novarupta derived layers b2 and b3 indicating that collapse began after release of 6 9 km of magma at novarupta the first major earthquake ms 6 5 at 2356 h on 6 june 11 hours after the eruption began may have marked the onset of collapse the katmai river landslide and several debris flow deposits are also sandwiched within the plinian deposits indicating that they too were emplaced during the eruption

arsenic mobility in mildly alkaline drainage from an orogenic lode gold deposit bralorne mine british columbia abstract the historical 1932 1971 bralorne mine produced over 87 million grams of au from an archetypal orogenic lode gold deposit in southwest british columbia high concentrations of as in mine drainage however represent an on going environmental concern prompting a detailed study of effluent chemistry the discharge rate at the mine portal was monitored continuously over a fourteen month period during which effluent samples were collected on a quasi weekly basis water samples were also collected on synoptic surveys of the adit between the portal and the main source of flow in the flooded workings total concentrations of as in the mildly alkaline ph 8 7 portal drainage average 3034 g l whereas at the source they average 5898 g l as emergent waters from the flooded workings flow toward the portal their dissolved oxygen content and ph increase from 0 to 10 mg l and from 7 7 to 9 respectively near the emergence point dissolved fe precipitates rapidly sorbing both as iii and as v with increasing distance from the emergence point dissolved as iii concentrations drop to detection limits through sorption on hydrous ferric oxide and through oxidation to as v concentrations of dissolved as v on the other hand increase and stabilize reflecting lower sorption at higher ph and the lack of available sorbent nonetheless based on synoptic surveys approximately 35 of the source as load is sequestered in the adit resulting in as sediment concentrations averaging 8 5 wt the remaining average as load of 1 34 kg d is discharged from the portal partitioning of as v between dissolved and particulate phases in portal effluent is characterized by a sorption density of 0 37 mol as mol fe 1 and by a distribution coefficient k d of 130 l g hfo the relatively high sorption density may reflect co precipitation of as with fe oxyhydroxides rather than a purely adsorption controlled process results of this study show that the as self mitigating capacity of drainage from orogenic lode gold deposits may be poor in high ph and fe limited settings

thermomechanical behavior of large ash flow calderas the fundamental thermal and mechanical processes that occur within the ash flow caldera magma chamber systems remain largely enigmatic to date the only models of caldera collapse are simple mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse they cannot however predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle ductile transitions and fault related thermal anomalies we have constructed analytical and numerical therrnomechanical that account for both elastic plastic ductile rheology and physical properties of the caldera rocks the overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera brittle deformation faulting and subsequent collapse of the structure are reproduced the results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios i e caldera diameter to the depth of the magma chamber exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10 thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults in the presence of regional extension the border faulting can be attenuated or disappear and the faults tend to localize around the central part of the chamber roof coupled therrnomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids the geometry of the brittle ductile transition largely controls that of the fractured zones within and around the chamber roof thus justifying a new mechanical definition of magma chamber geometry

the slow boiling of magma chambers and the dynamics of explosive eruptions abstract explosive eruptions are fed by shallow magma chambers and are mainly characterized by a water saturated magma a temporal increasing eruption rate and deep seismicity following the later stages of eruption we develop two simple physical models based on the decompression of a water saturated magma and the non equilibrium growth of bubbles that drives the magma out of the chamber the eruption ends when sufficient decompression is attained to cause magma chamber collapse and restoration of the initial pressure conditions the first model considers a single step decompression the second one a multi step decompression the models predict that the duration of the eruptive phase is mainly controlled by the bubble number densities and the initial decompression pressure decrease of the order 0 1 1 mpa results in eruption lasting days hours the erupted volume magnitude and eruption rate intensity are mainly controlled by the volume of the magma chamber by the strength of the rocks surrounding the reservoir and the physical properties of the magma water diffusivity and bubble density we simulated the eruptions at mt st helens 1980 and vesuvius 79 ad the models correctly predict the duration and evolution of the eruptions with the initial phases characterized by a slow eruption rate with the formation of a plinian column and the subsequent increase resulting eventually in pyroclastic flow activity

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

uranium bearing phases in a u mill disposal site in northern canada products of the interaction between leachate raffinate and tailings material abstract the above ground tailings management facility agtmf is a licensed tailings receiving facility at the key lake mine site northern saskatchewan canada tailings within the agtmf were deposited between 1983 and 1995 and are derived from mining and milling of the gaertner and deilmann ore bodies at key lake the tailings are primarily composed of quartz phyllosilicate and clay gangue minerals minor amounts of ni co s arsenides iron oxide minerals and gypsum they typically contain u concentrations on the order of between 50 and 300 ug g u a small area of the agtmf receives occasional mill process upset solutions with an initial ph range of 2 6 the solutions contain elevated activities of u bearing aqueous species and are rapidly buffered to neutral and sub alkaline ph values during interaction with surroundings tailings solids having a ph of approximately 10 this buffering and neutralization process results in the precipitation of gypsum and minor amounts of uranyl minerals phases of the autunite group chernikovite h3o uo2 po4 h2o 3 and troegerite h3o uo2 aso4 h2o 3 with minor nh 4 occur predominantly in the gypsum matrix but can also occur as crusts on silicates the occurrence of troegerite is due to elevated concentrations of as within the deposited tailings solids surface alteration on phyllosilicates within tailings affected by the mill process solutions results in elevated concentrations of k at the phyllosilicate water interface which are ideal nucleation sites for the formation of single crystals of zippeite k3 h2o 3 uo2 4 so4 2o3 oh use of nh4 sulfates and ammonia in the extraction and yellow cake precipitation cycles and subsequent entrainment of nh4 species in the tailings results in the occurrence of nh4 bearing phases of the zippeite and autunite group literature data on dissolution features dissolution kinetics and mineral solubilities suggests that the minerals of the autunite group control the mobility of u in the contact zone between the mill process solutions and the tailings

alteration adsorption and nucleation processes on clay water interfaces mechanisms for the retention of uranium by altered clay surfaces on the nanometer scale abstract nano scale processes on the solid water interface of clay minerals control the mobility of metals in the environment these processes can occur in confined pore spaces of clay buffers and barriers as well as in contaminated sediments and involve a combination of alteration adsorption and nucleation processes of multiple species and phases this study characterizes nano scale processes on the interface between clay minerals and uranyl bearing solution near neutral ph samples of clay minerals with a contact ph of 6 7 are collected from a u mill and mine tailings at key lake saskatchewan canada the tailings material contains cu as co mo ni se bearing polymetallic phases and has been deposited with a surplus of ca oh 2 and na2co3 slaked lime small volumes of mill process solutions containing sulfuric acid and u are occasionally discharged onto the surface of the tailings and are neutralized after discharge by reactions with the slaked lime transmission electron microscopy tem in combination with the focused ion beam fib technique and other analytical methods sem xrd xrf and icp oes are used to characterize the chemical and mineralogical composition of phases within confined pore spaces of the clay minerals montmorillonite and kaolinite and in the surrounding tailings material alteration zones around the clay minerals are characterized by different generations of secondary silicates containing variable proportions of adsorbed uranyl and arsenate species and by the intergrowth of the silicates with the uranyl minerals cuprosklodowskite cu uo2 2 sio3oh 2 h2o 6 and metazeunerite cu uo2 aso4 2 h2o 8 the majority of alteration phases such as illite illite smectite kaolinite and vermiculite have been most likely formed in the sedimentary basin of the u ore deposit and contain low amounts of fe 10 at formed most likely in the limed tailings at high contact ph 10 5 and their structure is characterized by a low degree of long range order adsorption of u and nucleation of metazeunerite and cuprosklodowskite are strongly controlled by the presence of the adsorbed oxy anion species arsenate and silica on the fe enriched silicates heterogeneous nucleation of nano crystals of the uranyl minerals occurs most likely on adsorption sites of binary uranyl arsenate and silica complexes as well as on ternary uranyl arsenate or uranyl silicate complexes the uranyl minerals occur as aggregates of misoriented nano size crystals and are the result of supersaturated solutions and a high number of nucleation sites that prevented the formation of larger crystals through oswald ripening the results of this study provide an understanding of interfacial nano scale processes between uranyl species and altered clay buffers in a potential nuclear waste repository as similar alteration conditions of clays may occur in a multi barrier system

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

the arsenic source term for an in pit uranium mine tailings facility and its long term impact on the regional groundwater abstract detailed field sampling and analyses and laboratory based diffusion cell experiments were used in conjunction with 3 d reactive transport modeling modflow and mt3d99 to quantify the fate and long term 10 ka transport of as in the rabbit lake in pit tailings management facility rlitmf northern saskatchewan canada the rlitmf 300 m 425 m 90 m thick was engineered to ensure solute transport within the rlitmf is dominated by diffusion concentrations of as in the tailings pore fluids ranged from 0 24 to 140 mg l n 43 arsenic speciation analyses indicate 90 of this arsenic exists as as 5 this observation is supported by ph eh measurements of pore fluids n 135 geochemical analyses yielded a strong inverse correlation between the fe as molar ratio in the tailings solids and the corresponding concentration of dissolved as which is attributed to the adsorption of as to secondary 2 line ferrihydrite present in the tailings diffusion cell testing yielded values for the effective diffusion coefficient sorption coefficient and effective porosity of as in the tailings of 4 5 10 10 m 2 s 2 4 cm 3 g and 0 36 respectively reactive transport simulations using the field and laboratory data show adsorption of as to the tailings and diffusive transport of dissolved as in the tailings should reduce the source term concentration of as to between 40 and 70 of the initial concentrations over the 10 ka simulation period based on these simulations the as concentrations in the regional groundwater 50 m down gradient of the tailings facility should be maintained at background concentrations of 0 001 mg l over the 10 ka period these findings suggest the engineered in pit disposal of u mine tailings can provide long term protection for the local groundwater regime from as contamination

the generation of overpressure in felsic magma chambers by replenishment abstract evidence of magma mixing is common in the products of explosive felsic eruptions and it is generally accepted as a common mechanism for triggering such events in order to quantify the potential for magma mixing to trigger explosive eruptions we have developed a simple analytical model based on previous models of magma chamber replenishment which considers the injection of volatile rich mafic magma into a chamber occupied by a homogeneous volatile rich felsic magma we assume that the overpressure caused by the injection of new magma is not sufficient to trigger an eruption for which additional overpressure is required two mixing related mechanisms have traditionally been considered to have the potential to generate the additional overpressure 1 exsolution of volatiles from the felsic magma during forced convection and 2 exsolution of volatiles from the mafic magma by oversaturation during cooling and subsequent crystallization our calculations suggest that exsolution of volatiles from the felsic magma is not an effective mechanism to generate additional overpressure however significant overpressure can be achieved by volatile exsolution from the mafic magma during its cooling and crystallization the time scale between intrusion and eruption considered in our model is of the order of a few days to a few months which coincides with petrological and geophysical evidence obtained from magma mixing related eruptions we also suggest that replenishment of shallow felsic magma chambers by mafic magma in most cases does not lead to large scale mixing as eruption will occur before thermal equilibrium between the two magmas is reached so that density and viscosity contrasts between the two magmas will remain significant

ranking welding intensity in pyroclastic deposits welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature progressive welding is recorded by changes in the petrographic e g fabric and physical e g density properties of the deposits mapping the intensity of welding can be integral to studies of pyroclastic deposits but making systematic comparisons between deposits can be problematical here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations e g oblateness of pumice lapilli and micro fabric orientation and measurements of physical properties including density porosity point load strength and uniaxial compressive strength our dataset comprises measurements on 100 samples collected from a single cooling unit of the bandelier tuff and parallel measurements on 8 samples of more densely welded deposits the proposed classification comprises six ranks of welding intensity ranging from unconsolidated rank i to obsidian like vitrophyre rank vi and should allow for reproducible mapping of subtle variations in welding intensity between different deposits the application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre welding thicknesses

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

temporal evolution of flow conditions in sustained magmatic explosive eruptions abstract the temporal evolution of fundamental flow conditions in the magma chamber plus conduit system such as pressure velocity mass flow rate erupted mass etc during sustained magmatic explosive eruptions was investigated to this aim simplified one dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together the chamber model assumed an homogeneous composition of magma and a vertical profile of water content the chamber could have a cylindrical elliptical or spherical rigid geometry inside the chamber magma was assumed to be in hydrostatic equilibrium both before and during the eruption since the time scale of pressure variations at the conduit inlet of the order of hours is much longer than the travel time of magma in the conduit of the order of a few minutes the flow in the conduit was assumed as at steady state the one dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked flow conditions at the exit bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law above the nucleation level bubbles and liquid magma were considered in mechanical equilibrium the same equilibrium assumption was made above the fragmentation level between gas and pyroclasts due to the hydrostatic hypothesis the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and through the conduit model as a function of time simulation results pertaining to rhyolitic and basaltic magmas defined at the volcanic eruption mechanism modeling workshops durham nh 2002 nice france 2003 are presented important flow variables such as pressure density velocity shear stress in the chamber and conduit are discussed as a function of time and magma chamber and conduit properties results indicate that vent variables react in different ways to the pressure variation of the chamber pressure density and mass flow rate show relative variations of the same order of magnitude as the conduit inlet pressure whereas velocity is more constant in time sill like chambers produce also significantly longer and more voluminous eruptions than dike like chambers water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption duration and volume maximum erupted mass fractions of about 0 2 are computed for small water saturated and shallow chambers

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

the record of the messinian salinity crisis in mobile belts insights from the molise allochthonous units southern apennines italy abstract mediterranean marginal basins recorded the messinian salinity crisis msc in various structural settings including syn rift thrust top foredeep and foreland basins during the msc the apennines were one of the mobile belts of the peri mediterranean chain starting from more hinterland areas allochthonous units of both the northern and southern apennines migrated toward the adriatic foreland developing allochthonous top sedimentary basins one of these basins on top of the molise allochthonous units southern apennines recorded almost all of the main steps of the msc in a gypsum quarry district the occurrence of conduits yielding flow mobilized sediments which cross cut the lower evaporites testifies to a fluid migration event responsible for the formation of a brecciated mudgrade limestone buildup this event can be connected with the mediterranean drawdown responsible for the messinian erosional surface mes the post evaporitic marly succession that unconformably overlies the lower evaporites and the brecciated limestones is characterized by the presence of paratethyan molluscs and ostracods loxoconcha muelleri and loxocorniculina djafarovi zones and small mammals among which is recorded the occurrence of stephanomys debruijni a disconformity within the post evaporitic succession divides it into lower p ev1 and upper p ev2 lago mare deposits a further erosional surface separates the messinian lago mare sediments from fully marine lower zanclean mpl2 zone deposits whilst the unconformity separating the lower evaporites and the lago mare deposits although enhanced by a tectonic event could be related to a late messinian base level drop mes1 as well as the disconformity between p ev1 and p ev2 mes2 the younger angular unconformities affecting both the lago mare deposits p ev2 and the fully marine pliocene sediments might be related to two different phases of orogenic transport those events affected the molise allochthonous units during their late miocene early pliocene forelandward migration

on the influence of magma chambers in controlling the evolution of explosive volcanic eruptions abstract the influence of magma chambers on the evolution of explosive volcanic eruptions is examined by coupling a model of an elastic chamber containing compressible magma bower s m woods a w 1997 the control of magma volatile content and chamber depth on the mass erupted during explosive volcanic eruptions j geophys res 102 10273 10290 with a model of conduit flow wilson l sparks r s j walker g p l 1980 explosive volcanic eruptions iv the control of magma properties and conduit geometry on eruption column behaviour geophys j r astr soc 63 117 148 it is found that for a given magma volatile content and conduit geometry variations in the chamber volume depth and aspect ratio have a significant impact on the mass erupted and the eruption duration shallow volatile saturated chambers of large cross sectional area tend to erupt a much larger fraction of the chamber volume 10 than deep volatile unsaturated chambers of the same magma 0 1 1 0 however in either case the steady decrease in chamber pressure over the course of an eruption only leads to a decrease in eruption rate of 10 50 therefore for a given chamber volume shallow chambers of large cross sectional area produce much more long lived and voluminous explosive eruptions in contrast we show that for a given chamber geometry variations in magma volatile content can lead to significant changes in both the eruption rate and the erupted volume in turn this also affects the eruption duration finally we note that as described by wilson et al increases in conduit radius lead to very large increases in eruption rate and hence a decrease in eruption duration

palaeoenvironments of the mediterranean basin at the messinian hypersaline hyposaline transition evidence from natural radioactivity and microfacies of post evaporitic successions of the adriatic sub basin at the end of the messinian salinity crisis changes in the palaeoceanography and palaeoclimate induced a transition from hypersaline to hyposaline conditions in the mediterranean water body detailed investigation of natural radioactivity and microfacies analyses of two early post evaporitic messinian sections show that the transition occurred in poorly oxygenated and well stratified water masses these transitional deposits which mainly consist of marls and caco3 rich horizons are generally well laminated and totally barren in benthic and planktonic fauna the presence of barite small 5 8 lm authigenic framboidal pyrite and high values of u indicate persistent anoxic conditions during the early postevaporitic messinian stage in the adriatic sub basin of the mediterranean sea the presence of post evaporitic welllaminated barren deposits from both mediterranean borderland sections and odp sites suggests that these anoxic conditions extended to other sub basins of the mediterranean region

the mediterranean messinian salinity crisis an apennine foredeep perspective abstract owing to its expanded stratigraphic sections the apennine thrust belt offers the opportunity to better understand the evaporitic and post evaporitic messinian events a physical stratigraphic framework of messinian deposits based on facies analysis and basin wide correlation of key surfaces and sedimentary cycles is presented it is shown that the messinian apennine foredeep had marginal basins with shallow water primary evaporites and deeper basins where resedimented evaporites accumulated under relatively deep water conditions like many other mediterranean examples primary shallow water evaporites of apenninic marginal basins show evidence for subaerial exposure and erosion however the development of such an erosional surface does not correspond to the deposition of primary evaporites in the deepest part of the basin s here the unconformity can be traced towards the base of resedimented evaporites or to a level within them implying that the deeper basins of the apennine foredeep never underwent desiccation during the messinian salinity crisis but rather received the eroded marginal evaporites this fact usually overlooked raises important questions about the deep desiccation model of the mediterranean

erosion processes in volcanic conduits and application to the ad 79 eruption of vesuvius abstract the flow of gas magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall erosion may be produced by the impact of pyroclasts on the conduit wall fluid shear stress at the wall conduit wall collapse and volcanic tremor using a two phase flow non equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process this abrasion process was found to be the largest near the conduit exit where the gas magma velocities are the largest the erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit depending on the wall roughness and yield strength of wall rocks this shear stress is the largest near the magma fragmentation level where the gas magma viscosity and velocity gradients are very large the collapse of conduit wall due to the difference between the gas magma and lithostatic pressures can occur below and above the magma fragmentation level causing the production of lithics directly when the wall collapses inward and indirectly when the wall collapses outward the effectiveness of different erosion mechanisms was tested with the magma characteristics conduit geometry and wall rock properties pertaining to the ad 79 eruption of vesuvius it was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit the conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

gas driven filter pressing in magmas insights into in situ melt segregation from crystal mushes gas driven filter pressing is the process of melt expulsion from a volatile saturated crystal mush induced by the buildup and subsequent release of gas pressure filter pressing is inferred to play a major role in magma fractionation at shallow depths 10 km by moving melt and gas relative to the solid crystalline framework however the magmatic conditions at which this process operates remain poorly constrained we present novel experimental data that illustrate how the crystal content of the mush affects the ability of gas driven filter pressing to segregate melt hydrous haplogranite 2 1 wt water in the melt and dacite 4 2 wt water in the melt crystal mushes with a wide range of crystallinities 34 80 vol crystals were investigated using in situ high temperature 500 800 c synchrotron x ray tomographic microscopy with high spatial 3 m pixel and temporal resolution 8 s per three dimensional data set our experimental results show that gas driven filter pressing operates only below the maximum packing of bubbles and crystals 74 vol above this threshold the mush tends to fracture and gas escapes via fractures therefore the efficiency of gas driven filter pressing is promoted close to the percolation threshold and in situations where a mush inflates slowly relative to build up of pressure and expulsion of melt such observations offer a likely explanation for the production of eruptible crystal poor magmas within earth s crust

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

the behavior of arsenic and antimony at pezinok mining site southwestern part of the slovak republic arsenic and antimony contamination is found at the pezinok mining site in the southwest of the slovak republic investigation of this site included sampling and analysis of water mineralogical analyses sequential extraction in addition to flow and geochemical modeling the highest dissolved arsenic concentrations correspond to mine tailings up to 90 000 g l and the arsenic is present predominately as as v the primary source of the arsenic is the dissolution of arsenopyrite concentration of antimony reaches 7 500 g l and its primary source is the dissolution of stibnite pore water in mine tailings is well buffered by the dissolution of carbonates ph values between 6 6 and 7 0 and arsenopyrite grains are surrounded by reaction rims composed of ferric iron minerals based on sequential extraction results most solid phase arsenic is in the reducible fraction i e ferric oxyhydroxides sulfidic fraction and residual fraction distribution of antimony in the solid phase is similar but contents are lower the principal attenuation mechanism for as v is adsorption to ferric oxide and hydroxides but the adsorption seems to be limited by the competition with sb v produced by the oxidation of stibnite for adsorption sites water in mine tailings is at equilibrium with gypsum and calcite but far from equilibrium with any arsenic and antimony minerals the concentrations of arsenic and antimony in the surrounding aquifer are much lower with maximum values of 215 and 426 g l respectively arsenic and antimony are transported by ground water flow towards the blatina creek but their loading from ground water to the creek is much lower compared with the input from the mine adits in the blatina creek arsenic and antimony are attenuated by dilution and by adsorption on ferric iron minerals in stream sediments with resulting respective concentrations of 93 and 45 g l at the site boundary south of mine tailing ponds

asymmetric multiple block collapse at rotorua caldera taupo volcanic zone new zealand calderas worldwide have been classified according to their dominant collapse styles although there is a good deal of speculation about the processes involved recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials however many other factors also contribute to final caldera morphology rotorua caldera formed during the eruption of the mamaku ignimbrite collapse structure and evolution of rotorua caldera is interpreted based its geophysical response geology and geomorphology and the stratigraphy of the mamaku ignimbrite rotorua caldera is situated at the edge of the extensional taupo volcanic zone in which major faults strike ne sw a second less dominant fault set strikes nw se these two fault sets have a strong influence on the morphology of rotorua caldera no one style of collapse can be applied to rotorua caldera it was formed during a single eruption but subsided as many blocks and shows features of trapdoor piecemeal and downsag types of collapse here rotorua caldera is described according to its composition activity and geometry as a rhyolitic single event asymmetric multiple block single locus collapse structure the mamaku ignimbrite is the only ignimbrite to have erupted from rotorua caldera extracaldera thickness of the mamaku ignimbrite is up to 145 m whereas inside the caldera it may be greater than 1 km thick the mamaku ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence the main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower middle and upper parts lmi mmi umi respectively based on crystal content welding jointing devitrification and vapour phase alteration juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments only the most evolved pumice type occurs in the basal tephra sequence all three pumice types occur together throughout the main ignimbrite sequence whereas the andesitic fragments are only present in umi lithic lag breccias in umi indicate a late stage of caldera collapse concentration of lithic fragments increases towards the middle of the ignimbrite and may also reflect increased subsidence rate during an earlier stage collapse of rotorua caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the mamaku ignimbrite allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence

geometrical and mechanical constraints on the formation of ring fault calderas ash flow plate subsidence piston like calderas are bounded by a set of arcuated sub vertical collapse faults named ring faults experimental studies on caldera formation performed mostly using spherical or cylindrical magma chamber geometries find that the resulting ring faults correspond to steeply outward dipping reverse faults and show that pre existing fractures developed during pre eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism a situation that should be expected in small to medium sized ring fault calderas developed on top of composite volcanoes or volcanic clusters on the other hand some numerical experiments indicate that large sill like elongated magma chambers may induce collapse due to roof bending without fault reactivation as seems to occur in large plate subsidence calderas formed independently of pre existing volcanoes also numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring faults in contrast with most of the analogue models using a thermoelastic model we investigate the geometrical and mechanical conditions to form ring fault calderas in particular the largest ones without needing a previous crust fracturing results are given in terms of two dimensionless geometrical parameters namely and e the former is the chamber extension to chamber depth ratio whereas the latter stands for the chamber eccentricity we propose that the e pair determinates two different types of ring fault calderas with different associated collapse regimes ring fault region a is related to large plate subsidence calderas i e andean calderas or western us calderas for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof in contrast ring fault region b is related to small to moderate sized calderas i e composite volcano calderas for which much depressurisation is needed our opinion is that collapse requires in the latter case reactivation of pre existing fractures and it is therefore more complex and history dependent

dissolution origin and syn entrapment compositional change of melt inclusion in plagioclase abstract in the system di ab an h 2 o melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments and the compositional differences from the surrounding matrix melt were evaluated at 200 mpa and 1050 1150 c a seed crystal of an 59 was partially dissolved into the matrix melt which was in equilibrium with more calcic plagioclase than the seed the reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase the melt channels were choked necked off by the recrystallized plagioclase at the crystal surface to create melt inclusions the experimentally formed inclusions having square and zigzag shapes up to a few tens of micrometers in size are similar in texture to some common types of natural melt inclusion since the partial dissolution of phenocrysts can occur during various magma mixing processes it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions especially for those in hydrous arc magmas the chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm the molar ratio of ca ca na and the mgo concentration in the synthesized melt inclusion are up to 17 and 61 relative lower than those in the matrix melt respectively this compositional difference is caused by a combination of factors 1 supersaturation with dissolved plagioclase component 2 insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase and 3 the choking off effect on the basis of the experimental results a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt these results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin

a numerical model for temporal variations during explosive central vent eruptions an axisymmetrical numerical model has been developed in order to find the temporal evolution of pressure the position of the exsolution level the velocity field the eruption rate and the amount of erupted material of a shallow volatile rich felsic magma chamber during a plinian central vent eruption the overpressure necessary to trigger the eruption is assumed to result from crystallization driven volatile oversaturation we solve the resulting set of equations using a finite element method the results obtained show that the pressure at the conduit entrance decreases exponentially as the eruption proceeds this produces a shifting of the exsolution level so that deeper parts of the chamber become progressively volatile oversaturated during the eruption we assess the influence of chamber geometry and the physical properties of the magma on the computed parameters using several numerical examples the results are also compared with those predicted by previous models from the literature and are found to be in good agreement with documented eruptions the model constitutes a first attempt to numerically model the dynamics and the temporal evolution of the most relevant physical parameters during withdrawal from a closed magma chamber

the cerro aguas calientes caldera nw argentina an example of a tectonically controlled polygenetic collapse caldera and its regional significance article i nfo polygenetic silicic collapse calderas are common in the central andes here we describe in detail the cerro aguas calientes caldera in nw argentina which comprises two caldera forming episodes that occurred at 17 15 ma and 10 3 ma we analyse the significance of its structural setting composition size and the subsidence style of both caldera episodes we find that the caldera eruptions had a tectonic trigger in both cases an homogeneous dacitic crystal rich n60 vol of crystals reservoir of batholithic size became unstable due to the effect of increasing regional transpression which favoured local dilation through minor strike slip faults from which ring faults nucleated and permitted caldera collapse both calderas are similar in shape location and products the 17 15 ma caldera has an elliptical shape 17 14 km elongated in a n30 trend both intracaldera and extracaldera ignimbrites covered an area of around 620 km 2 with a minimum volume estimate of 140 km

arsenic and lead mobility from tailing materials to the aqueous compartment this study concerns the mineralogy of the tailings of a former ag pb mine auzelles district france and the contribution of the waste materials to the heavy metal dissemination in the environment accumulation of metals in fish flesh was reported and this pollution is attributed to past mining activities tailings were studied to establish the major transfer schemes of as and pb in order to understand their mobility that leads to contamination of a whole ecosystem mineralogical investigation solubility and compliance tests were performed to assess the stability of the metal bearing phases among the various metallic elements measured as and pb show the highest bulk concentrations up to 0 7 and 6 3 respectively especially for samples presenting near neutral ph values according to x ray diffraction xrd scanning electron microscopy sem edx electron probe micro analysis epma and micro raman spectrometry rs tailings mineralogy still contain primary minerals such as sulfides e g galena pyrite phosphates monazite apatite and or carbonates e g hydro cerussite dolomite siderite sulfates e g anglesite lanarkite plumbojarosite and beudantite are the main secondary metal bearing phases with other interesting phases accounting for metals mobility such as fe and or pb and or mn oxides e g lepidocrocite goethite up to 15 wt of pb was measured plumboferrite type phase mimetite the lowest pb solubilities were obtained at ph 8 9 and at a larger range than for as for which the lowest solubilities are reached around ph 6 7 at this minimum solubility ph value pb concentrations released still over exceed the national environmental quality standards neqs whatever the samples the highest solubility is reached at ph 2 for both elements whatever the considered sample this represents up to 51 of total pb and up to 46 of total as remobilized and concentrations exceeding the neqs as and pb released mainly depends on the fe mn oxides e g goethite lepidocrocite and carbonates cerussite which are the less stable phases compliance tests also show that pb concentrations released are higher than the upper limit for hazardous waste landfills determination of the mineralogy allows understanding both the solubility and leaching test experiments results as well as to forecast the impact of the residues on the water quality at a mid term scale

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

timing of magma extraction during the campanian ignimbrite eruption campi flegrei caldera a core drilled within the northern part of the city of napoli has offered the unique opportunity to observe in one single sequence the superposition of the four pyroclastic flow units emplaced during the campanian ignimbrite ci eruption such a stratigraphic succession has never been encountered before in natural or in man made exposures therefore the ci sequence was reconstructed only on the basis of stratigraphic correlations and compositional data in literature the occurrence of four superposed ci flows together with all the data available in literature allowed us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the ci magma chamber the ci magma chamber includes two cogenetic magma layers separated by a compositional gap the upper magma layer was contaminated by interaction with radiogenic fluids the two magma layers were extruded either individually or simultaneously during the course of the eruption in the latter case they produced a hybrid magma but no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the eruption has been found comparison with the exposed ci deposits has permitted reconstruction of variable eruption phases and related magma withdrawal and caldera collapse episodes the eruption was likely to have began with phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the simultaneous extraction from both magma layers towards the end of this phase the upward migration of the fragmentation surface and the decrease in magma eruption rate and or activation of fractures formed an unstable pulsating column that was fed only by the most evolved magma layer this plinian phase was followed by the collapse of the eruption column and the beginning of caldera formation at this stage expanded pyroclastic flows fed by the upper magma layer in the chamber generated during the following major caldera collapse episode the maximum mass discharge rate was reached and both magma layers were tapped generating expanded pyroclastic flows towards the end of the eruption only the deeper and less differentiated magma layer was tapped producing more concentrated pyroclastic flows that traveled short distances

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

onset of caldera collapse during ignimbrite eruptions the rapid depressurisation and discharge of volatile rich silicic magma chamber and the onset of caldera collapse associated with ignimbrite eruption is investigated using combined theoretical and experimental approaches we first present a scaling analysis of the force balance on the roof of a depressurising magma chamber this provides a failure criterion for the chamber roof and shows that the chamber underpressure required to trigger coherent piston collapse along a vertical or steeply outward dipping reverse ring fault increases with the roof aspect ratio r thickness width the failure criterion is validated by a series of experiments that reveal a transition with increasing roof aspect ratio from piston collapse to either 1 no collapse low reservoir underpressure or 2 non coherent collapse high reservoir underpressure then taking into account the failure criterion obtained with the scaling analysis a theoretical model allows calculation of the erupted chamber volume fraction required to trigger caldera collapse we consider a two stage model with an initially overpressured chamber that becomes progressively underpressured as the eruption proceeds the main input parameters are the chamber depth vertical extent and ellipticity in plan view the magma water content the cohesion and coefficient of internal friction of the roof and the ring fault dip in the case of a mechanically homogeneous roof and a single large and rapid eruption the erupted volume fraction required to trigger piston collapse increases with the roof aspect ratio until the chamber is totally emptied before initiation of any collapse at r 1 1 4 the exact value depending mainly on the ring fault dip and to a lesser extent on the coefficient of internal friction of the roof as shown by experiments collapse into reservoirs with r 1 1 4 probably cannot collapse as a piston and if a surface caldera forms at all collapse is more likely to occur non coherently data for the onset of caldera collapse during seven well documented major ignimbrite eruptions are in broad agreement with the model

caldera subsidence in areas of variable topographic relief results from analogue modeling calderas form in volcanic areas commonly associated with topographic relief pre existing topography plays an important role in the style of the caldera subsidence topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface the morphology internal structure and temporal evolution of calderas are therefore sensitive to the local topography we carried out scaled analogue experiments to investigate the effect of pre existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus of variable mass diameter and positions prior to collapse we induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large shallow reservoir deformation was first manifested by sagging of the sand at the surface followed by the upward propagation of a set of subsidence controlling faults from the bladder in experiments with no topography these faults usually reached the surface near the centre of the cylinder as evacuation and incremental growth progressed a second outer set of subsidence controlling faults developed this outer set dominated the subsidence by increasing the topographic load by 6 the subsidence efficiency increased producing calderas up to approximately 20 deeper the inner set of subsidence controlling faults steepened and controlled most of the collapse by contrast to the experiments without topography adding load also reduced outward growth of peripheral sagging and development of extensional faults leading to a diameter of sagging 20 smaller than with no topography a comparative analysis of our results with mount mazama at crater lake oregon and glass mountain at long valley california supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults

arsenic and iron speciation in uranium mine tailings using x ray absorption spectroscopy abstract in northern saskatchewan canada high grade u ores and the resulting tailings can contain high levels of as an environmental concern in the u mining industry is the long term stability of as within tailings management facilities tmfs and its potential transfer to the surrounding groundwater to mitigate this problem u mill effluents are neutralized with lime to reduce the aqueous concentration of as this results in the formation of predominantly fe 3 as 5 secondary mineral phases which act as solubility controls on the as in the tailings discharged to the tmf because the speciation of as in natural systems is critical for determining its long term environmental fate characterization of as bearing mineral phases and complexes within the deposited tailings is required to evaluate its potential transformation solubility and long term stability within the tailings mass in this study synchrotron based bulk x ray absorption spectroscopy xas was used to study the speciation of as and fe in mine tailings samples obtained from the deilmann tmf at key lake saskatchewan comparisons of k edge x ray absorption spectra of tailings samples and reference compounds indicate the dominant oxidation states of as and fe in the mine tailings samples are 5 and 3 respectively largely reflecting their generation in a highly oxic mill process deposition in an oxidized environment and complexation within stable oxic phases linear combination fit analyses of the k edges for the fe x ray absorption near edge spectra xanes to reference compounds suggest fe is predominantly present as ferrihydrite with some amount of the primary minerals pyrite 8 15 in some samples and chalcopyrite 5 15 in some samples extended x ray absorption fine structure exafs analysis of as k edge spectra indicates that as 5 arsenate present in tailings samples is adsorbed to the ferrihydrite though an inner sphere bidentate linkage

retroarc volcanism in the northern san rafael block 34 35 30 s southern central andes occurrence age and tectonic setting article i nfo one of the major retroarc volcanic provinces in the southern central andes 34 and 37 s is developed in the andean foothills of the san rafael region between the orogenic front and foreland basement uplifts of late miocene age here we present the first comprehensive geochronological study of the quaternary volcanism previously dated mainly on the basis of stratigraphy the new unspiked k ar radiometric and two radiocarbon determinations encompass many volcanic centers most of them monogenetic and of basaltic composition exposed between 34 and 35 30 s the data constrains the basaltic volcanism to between 1 8 ma and the holocene the spatiotemporal distribution of the ages indicates that eruption in the retroarc was episodic with some distinct patterns the orogenic front of the san rafael block is associated with 1 8 0 7 ma volcanic eruptions while the malargue fold and thrust belt front in the andean foothills is related to younger eruptions produced at 0 1 0 01 ma both areas are associated with late cenozoic normal faults that dismembered an uplifted a late miocene peneplain as indicated by younger over older fault relationships between paleozoic rocks and tertiary strata this linkage indicates a major relationship between pleistocene holocene retroarc eruptions of the basaltic centers and extensional collapse of the foreland region that shows a migration of the last volcanic activity towards the trench

geochemistry and habitat of natural gases in po basin northern italy three types of natural gases can be distinguished in the po basin 1 bacterial gases dgr13cch4 60 ppt dgrdch4 180 to 200 ppt c2 0 2 which are found predominantly in pliocene pleistocene reservoirs 2 mixed gases dgr13cch4 60 to 50 ppt dgrdch4 180 to 200 ppt c2 0 2 to 5 which are predominantly in reservoirs of messinian upper miocene age but also in a few places in older and younger reservoirs 3 thermogenic gases dgr13c 50 ppt dgrd 200 to 150 ppt c2 5 which commonly are associated with oil and are produced predominantly from miocene and mesozoic reservoirs a quantitative assessment of gas in place for these three gas types is 1 80 2 10 and 3 10 respectively the areal distribution and amount of the three gas types are controlled strictly by geologic factors synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area where 70 of the bacterial gas in place is produced here bacterially formed methane is produced from depths of up to 4 500 m 15 000 ft and is the deepest bacterial gas so far reported in the literature in the pedealpine homocline lower sedimentation rates and a weak deformation of pliocene pleistocene sediments prevented appreciable accumulations only 15 of the bacterial gas in the basin is produced in this area thermogenic gases are found in the vicinity of the apennines where strong tectonic movements have favored its migration from deeper mesozoic sources into tertiary reservoirs or into deep mesozoic carbonate reservoirs of the pedealpine homocline

did the mediterranean marine reflooding precede the mio pliocene boundary paleontological and geochemical evidence from upper messinian sequences of tuscany italy abstract the events related to the messinian salinity crisis are among the most deeply investigated of earth s history according to the current hypothesis of neogene paleogeographic evolution approximately 5 5 ma ago after evaporitic sedimentation and before the mio pliocene boundary the mediterranean was characterized by the widespread development of non marine environments inhabited by molluscs and ostracods of brackish affinity the messinian post evaporitic deposits that testify such a dramatic environmental change are commonly referred to as lago mare and have been reported from several outcrops and boreholes throughout the entire mediterranean basin the origin of lago mare conditions is commonly interpreted as the result of the synergistic effect of both the humid climatic conditions and change of the drainage patterns at the mediterranean scale with the capture of paratethyan brackish waters a few recent studies however suggest that such a scenario probably represents an oversimplification of the original context from both a paleogeographical and paleoenvironmental point of view unfortunately the results of these studies have never been commonly accepted and the proposed evidences have been considered questionable in this paper we describe the fish assemblages from the lago mare deposits of two localities cava serredi and podere torricella located in the neogene hinterland basins of tuscany central italy these assemblages consists of a mixture of marine euryhaline and stenohaline taxa suggesting that the depositional environments were characterized by permanently open connections with a marine basin filled with normal marine waters in order to better define the paleontological significance of these upper messinian fish assemblages the oxygen carbon and strontium isotopic composition of fish otoliths and other fossils has been measured these isotopic compositions are strongly indicative of the presence of normal marine conditions close to the depositional environments testified by the sedimentary sequences thereby implying that the interpretation of the geochemical results are consistent with those derived from the paleoecological analyses of the fish assemblages based on the integrated paleoichthyological geochemical approach discussed in this paper it is possible to unambiguously demonstrate that normal marine rather than fresh or brackish waters were present in the mediterranean at least during the upper part of the lago mare event providing an unquestionable evidence that the marine refilling of the basin preceded the mio pliocene boundary

the role of phreatomagmatism in a plio pleistocene high density scoria cone field llancanelo volcanic field mendoza argentina abstract the plio pleistocene llancanelo volcanic field together with the nearby payun matru field comprises at least 800 scoria cones and voluminous lava fields that cover an extensive area behind the andean volcanic arc beside the scoria cones in the llancanelo field at least six volcanoes show evidence for explosive eruptions involving magma water interaction these are unusual in the context of the semi arid climate of the eastern andean ranges the volcanic structures consist of phreatomagmatic derived tuff rings and tuff cones of olivine basalt composition malacara and jarilloso tuff cones were produced by fallout of a range of dry to wet tephra the malacara cone shows more evidence for a predominance of wet emplaced units with a steep slump slope characterized by many soft sediment deformation structures such as undulating bedding planes truncated beds and water escape features the piedras blancas and carapacho tuff rings resulted from explosive eruptions with deeper explosion loci these cones are hence dominated by lapilli tuff and tuff units emplaced mainly by wet and or dry pyroclastic surges carapacho is the only centre that appears to have started with phreatomagmatic eruptions with lowermost tephra being rich in non volcanic country rocks the presence of deformed beds with impact sags slumping textures asymmetrical ripples dunes cross and planar lamination syn volcanic faulting and accretionary lapilli beds indicate an eruption scenario dominated by excessive water in the transportational and depositional regime this subordinate phreatomagmatism in the llancanelo volcanic field suggests presence of ground and or shallow surface water during some of the eruptions each of the tuff rings and cones are underlain by thick fractured multiple older lava units these broken basalts are inferred to be the horizons where rising magma interacted with groundwater the strong palagonitization at each of the phreatomagmatic cones formed hard beds resistant to erosion and therefore the volcanic landforms are well preserved

the luingo caldera the south easternmost collapse caldera in the altiplano puna plateau nw argentina abstract this article identifies the pucarilla cerro tipillas volcanic complex and its major eruptive source the luingo caldera 26 10 s 66 40 w detailed geological mapping stratigraphic sections facies analysis and correlations including the identification of typical caldera components allow us to infer the position of a collapse caldera elongated at n65 and with a diameter of 19 km 13 km which is responsible for an eruption of 135 km 3 dre of magma the high crystal contents of the associated ignimbrites combined with its tectonic setting indicate that regional and local tectonic structures played a crucial role in the formation of the caldera the luingo caldera is located on the south eastern border of the puna and is the south easternmost recognised caldera of the altiplano puna plateau the age of the caldera and its products is 12 1 to 13 5 ma based on its location near the cerro galan complex 2 to 6 5 ma we can imply that volcanism existed in the area for about 10 ma the caldera morphology and product distribution account for a middle miocene paleao topography similar to the present one

distribution of arsenic and nickel in uranium mill tailings rabbit lake saskatchewan canada abstract the rabbit lake u mine in pit tailings management facility tmf is located in northern saskatchewan canada the tailings body is approximately 425 m long 300 m wide and 91 m thick at its centre an investigation of the tmf was performed to collect tailings samples from depth to quantify the distribution of as and ni in the tailings with respect to ore type and assess the distribution of as and ni with respect to tailings mineralogy the tailings body consists of alternating layers of ice frozen tailings and unfrozen tailings which varied in texture from a slurry to a firm silty sand the tailings solids are predominately composed of quartz 16 36 calcium sulphate 0 3 54 and illite 3 and 14 arsenic and ni concentrations in the tailings show similar patterns with depth which were strongly related to historical changes in as and ni concentrations in the mill feed mineralogy of the ore bodies indicated that as and ni in the mill feed occurred primarily as 1 1 molar ratio arsenides such as niccolite and gersdorffite arsenic and ni concentrations in the tailings were also measured at a near 1 1 molar ratio mill process records showed that an average of 71 of the as in the mill feed was solubilized during leaching sem analysis suggested that solubilized as is precipitated as ca2 fe3 and ni2 arsenates during the neutralization process mill records indicated that 17 000 tonnes of as were discharged to the tmf of which approximately 88 was as arsenates and 12 as primary arsenides

compositional layers in the zoned magma chamber of the grizzly peak tuff the 34 ma grizzly peak tuff in west central colorado preserves evidence for distinct compositional layers in a high level magma chamber intracaldera tuff consists of a single cooling unit at least 2 7 km thick zoned from a high silica rhyolite base to an eroded low silica rhyolite top two horizons of heterogeneous tuff beneath wedges of caldera collapse breccia contain fiamme of dacite to mafic latite along with the rhyolitic fiamme that make up the rest of the tuff compositional zoning defined by fiamme is a step function rather than a continuous gradient seven petrographic groups of fiamme each have distinct compositions separated by compositional gaps as the same clusters and gaps are found in collections of fiamme from widely separated stratigraphic levels step function zoning must be intrinsic to the chamber rather than a consequence of the tapping process we interpret the seven fiamme groups as seven separately converting layers in a density stratified magma reservoir tapped by eruption of the grizzly peak tuff excluding the thin heterogeneous tuff horizons 3 km of caldera fill is drawn from the three most silicic layers suggesting that the three rhyolitic layers were each at least 1 km thick in the magma chamber

deep water clastic evaporites deposition in the messinian adriatic foredeep northern apennines italy did the mediterranean ever dry out a new genetic facies model for deep water clastic evaporites is presented based on work carried out on the messinian gessoso solfifera formation of the northern apennines during the last 15 years this model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts the model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites representing the most common evaporite facies of the northern apennines and the definition of the genetic and stratigraphic relationship with primary shallow water evaporites formed and preserved in marginal settings due to the severe recrystallization processes usually affecting these deposits petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity driven deposits ranging from debrisflow to low density turbidites almost all the laminar balatino gypsum previously considered a deep water primary deposit is here reinterpreted as the fine grained product of high to low density gravity flows facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the apennine foredeep basin the messinian salinity crisis occurred during an intense phase of geodynamic reorganization of the mediterranean area that also produced the fragmentation of the former miocene apennine foredeep basin in this area primary shallow water evaporites equivalent to the mediterranean lower evaporites apparently only formed in semi closed thrust top basins like the vena del gesso basin the subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the messinian tectonic evolution of the apennine thrust belt and related exhumation erosional processes the facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts the importance of clastic evaporites deposits has been overlooked in the study of other mediterranean areas based on the apennine basins experience it is suggested here that evaporites diffused into the deeper portions of the mediterranean basin may consist mainly of deep water resedimented deposits rather than shallow water to supratidal primary evaporites indicative of a complete basin desiccation

the influence of variable topography on the depositional behaviour of pyroclastic density currents the examples of the upper pollara eruption salina island southern italy abstract stratigraphic reconstruction of the upper pollara eruption has allowed for the inference of eruptive mechanisms and the distillation of a sedimentological model for pyroclastic density currents pdcs moving across variable topography the pre eruptive topography in the study area was characterised by a tuff ring like morphology with both inward and outward dipping slopes highly viscous moderately porphyritic dacitic to rhyolitic magmas fed the eruption which was characterised by a vulcanian eruptive style the stratigraphic succession was divided into five eruption units eus which result from different phases of the eruption separated by stases sustained columns occurred only during eu1 while pdc generation dominates eu2 5 lithofacies analysis of the pdc deposits indicates the prevalence of massive coarse grained deposits on the inner slopes of the pollara crater which are interpreted as the deposits of a flow boundary zone dominated by granular flow or fluid escape regimes dune bedded massive to stratified lithofacies dominate the outer slopes of the pollara crater and are interpreted as the deposits of pdcs with flow boundary zones in which traction played a major role thin massive pdc deposits are exposed on the sub horizontal malfa terrace and are interpreted as representative of flow boundary zones dominated by a granular flow regime the occurrence of stacked deposits indicates that most of the pdcs were characterised by unsteady pulsatory behaviour with development of trains of pulses during their transport the downcurrent lithofacies transitions observed for the upper pollara deposits have finally been compared with other similar lithofacies associations which have been described for short lived pdcs at tuff rings in order to discuss the influence of pre eruptive topography on lithofacies association

vent distribution in the quaternary payun matru volcanic field western argentina its relation to tectonics and crustal structures abstract the payun matru volcanic field consists of two polygenetic and mostly trachytic volcanoes payun matru with a summit caldera and payun liso along with around 220 scoria cones and basaltic lava flows this volcanic field belongs to the payenia basaltic province 33 30 38 s a quaternary andean back arc basaltic province of the southern volcanic zone in western argentina the vent density distribution of the payun matru volcanic field is different from the other volcanic fields within payenia the payun matru volcano and the scoria cones are distributed in an e w oriented fringe about 15 km wide and 70 km long with the payun matru caldera in the middle of this fringe the structural framework in which the volcanic field is located allows to infer that this vent density distribution is strongly conditioned by pre existing crustal anisotropies the volcanic field is located in a transfer zone related to jurassic extensional structures of the neuquen basin which were inverted also as a transfer zone during the miocene compressive deformation that formed the malargue fold and thrust belt and in addition it is located in the southern margin of a neogene syn orogenic basin the analysis of vent center location and vent morphology is helpful to determine basaltic vent alignments within the payun matru volcanic field and to infer the syn eruptive stress field this analysis shows that vent alignments are compatible with the present day maximum horizontal stress as measured by break out of oil wells