the potential acid producing capacity and factors controlling oxidation tailings in the guryong mine korea a detailed characterization of the site is crucial to designing an efficient method of managing the risks associated with tailings from abandoned mines therefore samples collected from various depths within tailings in guryong mine korea were analyzed for their chemical physical and mineralogical characteristics all samples of the guryong tailings had acid generating potential however in the oxidation zone the net acid generation nag was low 30 kg h2so4 t 1 although the acid neutralization potential anp was less than zero the anp values in the unoxidation zone were higher 56 0 kg caco3 t 1 than in the other zones as a result the amount of alkali ions that are needed to neutralize the acid needs to be considered in this experiment g3 g4 and g6 drill cores containing fine tailings particles near the unoxidation zone were observed to contain calcite caco3 with acid neutralizing capacity a low ph 2 4 in the oxidation zone of the tailings changed to a neutral ph in the unoxidation zone of the tailings these results suggest that the acid neutralizing capacity of the tailings was controlled by particle and mineral composition of tailings

acid generating potential of waste rock and coal ash in new zealand coal mines abstract acid producing potential static tests have been undertaken on sample lithologies from new zealand coal mines to rank the lithologies into acid producing waste rock and stable non acid producing waste rock results indicated that the main acid producing waste rocks in the waikato coal region are the marine whaingaroa siltstone and the glen massey formations and the main alkaline producing lithologies were the tauranga group volcanic derived sediments a d and the mesozoic basement coal ash analysis of 28 samples from 28 mines has also been analysed for reactive sulphur percentage and their base potential has been quantified by the chemical index of alteration cia the results provide mine management in the waikato coal region with a classification of problematic and stable waste rock types new zealand coal ash has the potential to be a problematic acid generator if the cia value is above 20 cnk proportion in an a cnk fm ternary plot is between 30 and 56 reactive sulphur per tonne of coal is 0 5 kg and dry ash weight percent in the coal is 5 xrf and cia values determined from wash drill cuttings would provide a method of determining waste rock volumes and magnitude of palaeoweathering underground

the interaction of acid mine drainage with a carbonate terrane evidence from the obey river north central tennessee abstract the production of acidic drainage from active or abandoned coal mines is controlled by a complex series of reactions triggered by the oxidation of sulphide minerals limestone is frequently used as a neutralizing agent neutralization occurs both through dissolution of the limestone and exsolution of carbon dioxide from the water demonstration of these processes in a natural system is provided by the east fork of the obey river where acidic mine waters travel through conduits in limestone bedrock for several kilometers application of discharge concentration relationships allows tracking of the evolution of these waters within the karst aquifer several trends are evident virtually all of the iron in the acid mine drainage is precipitated along the flow path within the limestone the h released during this reaction together with the h generated by pyrite oxidation is largely neutralized by both exsolution of co 2 and dissolution of calcite dolomite in the carbonate bedrock releasing calcium and magnesium nevertheless the ph is still sufficiently low that all bicarbonate is converted to carbonic acid leaching of clay minerals by the acid waters releases small amounts of al and si the species cl f and no 3 are more or less inert the sulphate concentration substantially increases during evolution of the acid drainage this seems to be related to an input of sulphate enriched waters that have dissolved gypsum anhydrite as they follow deep flow paths within the limestone calculations of the relative proportions of h removed by reaction with calcite dolomite and co 2 degassing show that the latter is important in acidic waters that have reacted significantly with limestone and can readily lose co 2 to the atmosphere these results indicate that acid neutralization through carbonic acid dissociation in addition to neutralization through dissolution of carbonates is a useful method for the commercial a batement of acid mine drainage

evolution of the aqueous geochemistry of mine pit lakes blanzy montceau les mines coal basin massif central france origin of sulfate contents effects of stratification on water quality abstract the flooding of two mine pit lakes lake st louis and lake fouthiaux situated in the blanzy montceau les mines coal basin massif central france has been under close scrutiny for 5 a the aim was to understand i what sources can influence the lake water geochemistry ii what is the role of lake stratification on water chemistry iii how the lake water chemistry evolves during flooding to answer these questions t conductivity ph and eh depth profiles were measured with a specific probe seabird in the lakes seven surveys between july 1998 and february 2002 at the same time water samples were collected every 5 m along the profiles and were analysed for na ca2 k mg2 so 4 2 no 3 f cl hco 3 fe cu ni zn co cd and mn the geological and hydrogeological contexts play a major role on the water chemistry of the lakes and three sources of influence have been distinguished i the contribution of underlying mine groundwater enriched in so 4 2 due to the leaching of sulfide oxidation products from old flooded underground works ii the seepage through coal mine spoils also containing sulfides iii to a lesser extent triassic formations containing gypsum present in the fouthiaux area which lead to a so 4 2 enrichment of lake waters all these influences result in an enrichment in so 4 2 showing essentially a neutralised acid mine drainage process that is characterised by neutral to slightly alkaline waters and an absence of high metal contents except for fe and mn which are non soluble at the ph and eh conditions present in the lakes the two studied lakes were initially meromictic and ions such as so 4 2 or mn can show high concentrations at depth in the lake layers which are not subject to winter mixing during the studied period lake st louis remained meromictic and non removed elements continue to have high concentrations in deep zones in contrast lake fouthiaux initially meromictic became holomictic with time and the bottom layer did not maintain the enrichment in so 4 2 mn and other elements shown at the beginning

soil temperature and co 2 degassing so 2 fluxes and field observations before and after the february 29 2016 new vent inside nyiragongo crater nyiragongo volcano threatens 1 5 million inhabitants of goma dr congo and gisenyi rwanda cities and people living in the surrounding villages in 2002 the volcano produced lava flows which invaded goma and destroyed the economic district of the city forced a mass exodus of the population and caused the loss of several lives nyiragongo volcanic activity is therefore closely followed by the inhabitants and any news related to increased activity agitates people in the area especially those in goma here we report a short time series of soil temperature and carbon dioxide degassing for four locations and plume sulphur dioxide fluxes preceding and following the opening of a new vent inside the main nyiragongo crater on february 29 2016 the observed sudden and unexpected changes in nyiragongo activity raised the fear of a new volcanic eruption and led to panic in goma and the surroundings inducing some people to leaving the city we use the dataset and field observations before and after the opening of the new vent in conjunction with published information about nyiragongo s eruptive mechanism and of the volcano s plumbing system geometry mainly the crater to show that the new vent was fed by magma intruded from the lava lake or the upper conduit

mt niragongo renewed activity of the lava lake abstract on the 21st of june 1982 mt niragongo ended a period of dormancy that had begun on january 11 1977 and fresh lava began to flow into the 800 m deep crater on october 3 a huge lava lake wider and deeper than any previously observed 500 m across and close to 400 m deep rose to within 440 m of the crater rim the observed activity consisted of a large central upwelling fountain of very fluid lava from which concentric lava waves expanded radially numerous small relatively viscous lava flows creeped over the surrounding thin solidified crust that covered about 95 of the lake area these observed features seem to characterize the upper part of a large convective system the persistence of such an extraordinarily large steady state lava lake may be due to the equally exceptional fluidity of the magma rising at the intersection of four different tectonic trends of fractures in the subvolcanic basement

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

mobility of trace and potentially harmful elements in the environment from high sulfur indian coal mines abstract coal mine rejects and sulfide bearing coals are prone to acid mine drainage amd formation due to aqueous weathering these acidic effluents contain dissolved trace and potentially harmful elements phes that have considerable impact on the environment the behavior of these elements in amd is mainly controlled by ph the focus of the present study is to investigate aqueous leaching of mine rejects for prediction of acid producing potential rates of weathering and release of phes in mine drainage mine reject mr and coal samples from the active mine sites of meghalaya india typically have high s contents 1 8 5 7 in mr and 1 7 4 7 in coals with 75 90 of the s in organic form and enrichment of most of the phes in rejects aqueous kinetic leaching experiments on mine rejects showed high acid producing potential and release of trace and potentially harmful elements the elements sb as cd cr co cu pb mn ni v and zn in mine sample leachates are compared with those in mine waters the concentrations of al si p k ti mn fe co ni cu zn and pb are found to increase with leaching time and are negatively correlated with ph of the solution the processes controlling the release of these elements are acid leaching precipitation and adsorption the critical loads of phes in water affected by amd are calculated by comparing their concentrations with those of regulatory levels the enrichment factors efs and soil pollution indices spis for the elements have shown that phes from coal and mine reject samples are mobilized into the nearby environment and are enriched in the associated soil and sediment

mineralogy and petrology of the angrite northwest africa 1296 we report on a new angrite northwest africa nwa 1296 a fine grained rock with a magmatic texture of rapid cooling dendritic olivine fo50 crystallized first in association with anorthite microcrysts an98100 forming composite chains separated from one another by intergrown al fe diopside hedenbergite pyroxenes in addition some olivines with lower mg and increased cao up to 12 are found between the chains as equant microphenocrysts pyroxenes and olivines are both normally zoned from mg 0 52 to less than 0 01 in the rims ca rich olivines are surrounded by intergrown with or replaced by subcalcic kirschsteinite they appear after plagioclase crystallization stopped at the end of the crystallization sequence minor phases are pyrrhotite fapatite and titanomagnetite pyroxene is the last silicate phase to grow interstitial to idiomorphic olivine kirschsteinite numerous small vesicles and some channels are filled with microcristalline carbonate the mode vol is about 28 olivine 3 kirschsteinite 32 anorthite 34 pyroxene and 3 of the minor phases close to that reported previously for d orbigny and sahara sah 99555 the bulk chemical composition of nwa 1296 is similar to d orbigny and sah 99555 nwa 1296 differs by its texture and mineralogy which are interpreted as resulting from rapid crystallization an evidence of impact melting angrites cannot be produced by partial melting of a cv source because segregation of a planetary core is necessary to explain the low feo mgo ratio of magnesian olivines neither the odd ca al ratio nor the very low sio2 content can be explained by conventional partial melting scenarios we suggest that carbonate is the key to angrite genesis this is supported by the striking similarities with terrestrial melilitites low sio2 superchondritic ca al ratio presence of carbonate the lack of alkalies could be the result of either loss after impact melting or absence of alkalies in the source

petrology and geochemistry of the nyamuragira volcano zaire abstract nyamuragira volcano in the western rift of the east africa rift valley is characterized by violent volcanic eruptions the most recent of which occurred in 1967 1971 1976 1977 1980 and 1981 1982 its extrusives consist of a highly potassic suite of olivine basanite tephritic phonolites among which phonolitic tephrite is the commonest the petrographic features of the whole suite such as phenocryst and groundmass mineral assemblages and modal variations are rather simple major elements vary smoothly from olivine basanite to phonolitic interstitial glass without any abrupt change with increasing k 2 o in addition the typical incompatible trace elements ba ce f nb rb and sr increase continuously with the increase of k 2 o these suggest that chemical variations were controlled mainly by the effective fractionation of olivine and clinopyroxene associated with plagioclase and titanomagnetite furthermore in the k 2 o incompatible trace element diagrams trend lines extrapolated to zero converge on the coordinate axes of the vertical and horizontal lines this suggests that these incompatible elements were preferentially moved toward the primitive basaltic magma during the partial melting of metasomatized upper mantle peridotites furthermore these elements are concentrated almost completely in residual liquids during fractional crystallization

exhumed conduit records magma ascent and drain back during a strombolian eruption at tongariro volcano new zealand field evidence from a basaltic andesite dyke preserved in the eroded wall of a scoria cone at red crater tongariro volcano new zealand records a history of up conduit magma flow during a strombolian eruption subsequent drain back and final cessation of flow the dyke intrudes pre strombolian andesite lavas and the overlying proximal basaltic andesite scoria deposits associated with contemporaneous lavas which are in turn overlain by laminated lapilli tuff and large blocks textural and kinematic evidence of ductile shear recorded in basaltic andesite at the dyke margins records magma deformation imposed by bypassing movement of magma up the centre of the conduit during the eruption whereas the basaltic andesite occupying the central part of the lowermost exposures of the dyke preserves ductile flow folds with the opposite down flow shear sense the evidence indicates that the downward magma flow followed the eruption and this draining left the central part of the dyke empty unfilled at uppermost levels we discuss the kinematic constraints in the context of the criteria for up flow of mafic magma and present the factors most likely to result in a final drain back event with reference to experimental and numerical work we propose a draining model for the end of this eruption and that magmatic drain back may feature commonly during closing stages of strombolian eruptions at mafic volcanoes drain back which leaves large cavities in a volcanic edifice could result in hazardous structural instabilities

edifice growth deformation and rift zone development in basaltic setting insights from piton de la fournaise shield volcano reunion island abstract the overall morphology of basaltic volcanoes mainly depends on their eruptive activity effusive vs explosive the geometry of the rift zones and the characteristics of both endogenous and exogenous growth processes the origin of the steep geometry of the central cone of piton de la fournaise volcano which is unusual for a basaltic effusive volcano and its deformation are examined with a combination of a detailed morphological analysis field observations gps data from the piton de la fournaise volcano observatory and numerical models the new caldera walls formed during the april 2007 summit collapse reveal that the steep cone is composed of a pyroclastic core inherited from an earlier explosive phase overlapped by a pile of thin lava flows this suggests that exogenous processes played a major role in the building of the steep central cone magma injections into the cone which mainly occur along the n25 30 and n120 rift zones lead to an asymmetric outward inflation concentrated in the cone s eastern half this endogenous growth progressively tilts the southeast and east flanks of the cone and induces the development of a dense network of flank fractures finally it is proposed that intrusions along the n120 rift zone are encouraged by stresses induced by magma injections along the n25 30 rift zone

extraterrestrial lava lakes abstract active lava lakes are rare on earth with only ten documented examples all formed by lavas of basaltic composition and housed inside summit craters or calderas the existence of lava lakes on other planetary bodies may imply similarities in either composition or volcano tectonic settings and so has important implications for understanding the link between melt production and volcanism we review lava lakes on earth and other planets particularly active caldera like features interpreted to be currently containing active lava lakes on jupiter s moon io and features interpreted as remnants of lava lakes on venus and mars mercury and the moon do not boast the major calderas or shield volcanoes of their larger rocky neighbors partly due to a horizontally compressive tectonic regime arising from global contraction that makes it difficult to move melts through the upper crust we discuss the evidence for active lava lakes on io and show how modeling based on terrestrial lava lakes can reveal how these phenomena differ on both bodies the superficial similarities do not necessarily imply that the plumbing is similar observations of the largest lava lake on io loki patera provide insight into the nature of ionian lava lakes in general which may be more similar to eruptive episodes on the east pacific rise on earth which lead to temporary lava lakes although temporal data for io s lava lake activity are scarce studies of temporal variability of lava lakes on earth are useful for providing ground truth for comparisons future studies of earth by remote sensing and field observations and of io by both ground based observations and future missions are needed to answer many questions including why lava lakes rare on earth appear to be common on io

computer simulations of lava flow paths in the town of goma nyiragongo volcano democratic republic of congo 1 the 17 january 2002 fissure eruption of mount nyiragongo democratic republic of congo produced lava flows which entered and devastated the densely inhabited town of goma the 2002 and previous 1977 eruptions demonstrate the high level of volcanic risk in the area we present an analysis of the susceptibility to lava flow invasion in goma by means of computer simulations based on the steepest descent path and probabilistic computed flow spreading and obstacle overcoming the dem is obtained from a topographic map of goma by using the dest algorithm the numerical results show the distribution of probable lava flow paths from possible vents in the investigated topographic domain numerical simulations are validated through comparison with the mapped paths of the 2002 lava flows in goma the subsequent investigation includes the analysis of 1 hypothetical lava flow paths from ephemeral vents on the edge of the 2002 lava flows 2 paths from venting along the hypothetical extension of the 2002 fissures 3 paths in case of lava flow arrival in town from the north and 4 changes induced by the presence of the solidified 2002 lava field the results show the susceptibility of the different parts of the town of goma to be invaded by future lava flows and reveal the existence of a large area in town with minimum susceptibility and which can be invaded only in case of venting within it the areas destroyed by the 2002 lava flows are predicted to be characterized by maximum susceptibility to lava flow invasion

detailed multidisciplinary monitoring reveals pre and co eruptive signals at nyamulagira volcano north kivu democratic republic of congo this paper presents a thorough description of nyamulagira s january 2010 volcanic eruption north kivu democratic republic of congo based on a combination of field observation and ground based and space borne data it is the first eruption in the virunga volcanic province that has been described by a combination of several modern monitoring techniques the 2010 eruption lasted 26 days and emitted 45 5 106 m3 of lava field observations divided the event into four eruptive stages delimited by major changes in effusive activity these stages are consistent with those described by pouclet 1976 for historical eruptions of nyamulagira co eruptive signals from ground deformation seismicity so2 emission and thermal flux correlate with the eruptive stages unambiguous pre eruptive ground deformation was observed 3 weeks before the lava outburst coinciding with a small but clear increase in the short period seismicity and so2 emission the 3 weeks of precursors contrasts with the only precursory signal previously recognized in the virunga volcanic province the short term increase of tremor and long period seismicity which for example were only detected less than 2 h prior to the 2010 eruption the present paper is the most detailed picture of a typical flank eruption of this volcano it provides valuable tools for re examining former mostly qualitative descriptions of historical nyamulagira eruptions that occurred during the colonial period

volcanological and petrological evolution of nyiragongo volcano virunga volcanic field zaire three major phases are distinguished during the growth of nyiragongo an active volcano at the western limit of the virunga range zaire lavas erupted during phase 1 are strongly undersaturated melilities characterized by the presence of kalsilite phenocrysts perovskite and the abundance of calcite in the matrix such lavas crop out mainly on the inner crater wall and progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano adventive vents lying at the base of the cone developed along radial fracture systems and erupted olivine and or clinopyroxene rich melilitites or nephelinites stage 2 lavas are melilite free nephelinites clinopyroxene is the main phenocryst and feldspathoids are abundant in the lavas exposed on the crater wall these flows result from periodic overflowing of a magma column from an open crater extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread draining out of magma which in turn induces the formation of the summit pit crater magmas erupted during stage 3 are relatively aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit crater until the 1977 eruption fluctuations of the level of the lava lake was responsible for the development of the inner terraces periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows petrography and major element geochemistry allow the determination of the principal petrogenetic processes melilitites and nephelinites erupted from the summit crater are lavas derived via clinopyroxene fractionation from a more primitive melt the abundance of feldspathoids in these lavas is in keeping with nepheline flotation aphyric melilite nephelinites covering the flanks and the extensive fissure flows have a homogencous chemical composition rocks from the historical lava lake are slightly more evolved all these lavas differentiated in a shallow reservoir lavas erupted from the parasitic vents are mainly olivine and or clinopyroxene phyric rocks rushayite and picrites from muja cone are peculiar high magnesium lavas resulting from the addition of olivine xenocrysts to melilitic or nephelinitic melts fluid and melt inclusions in olivine and clinopyroxene phenocrysts indicate a crystallization depth of 10 14 km a model involving two reservoirs located at different depths and periodically connected is proposed to explain the petrography of the lavas this hypothesis is in accordance with geophysical data

seismicity and outgassing dynamics of nyiragongo volcano abstract active lava lakes at volcanoes can be regarded as open windows to their magmatic systems the dynamics of such lakes may therefore provide decisive insights into deeper magmatic processes potentially leading to fundamental theoretical implications and volcano monitoring improvements among the rare volcanoes on earth hosting a persistent lava lake nyiragongo in d r congo directly threatens a massive population of roughly 1 million inhabitants here we analyze close range i e summit and distant around 17 km seismic measurements acquired at this african volcano between 2011 and 2018 in order to better understand the seismic signature of the lava lake activity and how it relates to the deeper volcanic processes both summit and distant seismic records contain a similar continuous tremor pattern attributable to the lava lake activity combining this information with time lapse camera images and lava lake level measurements confirms the mechanism of gas pistoning at nyiragongo which is characterized by short duration a few minutes long and meter scale level variations during the period of observation we also characterize the dominant periodicity of this shallow tremor signature of about a few tens of minutes because this marked periodic pattern varies during a significant one month fluctuation of so2 emissions estimated from space we suggest that this particular seismic periodicity corresponds to the convective lake movement driven by the persistent degassing typical of active open vent volcanoes finally new seismic evidence reveals the effect of deep magmatic intrusion and consequent major pressure changes in the plumbing system resulting in sudden and large drops of the lava lake level associated with strong degassing such transient episodes have similar characteristics to total lava lake drainage associated with flank eruptions already observed at this volcano in 1977 and 2002 or at k lauea volcano in 2018

the dilemma of disappearing diatoms incorporating diatom dissolution data into palaeoenvironmental modelling and reconstruction abstract taphonomic issues pose fundamental challenges for quaternary scientists to recover environmental signals from biological proxies and make accurate inferences of past environments the problem of microfossil preservation specifically diatom dissolution remains an important but often overlooked source of error in both qualitative and quantitative reconstructions of key variables from fossil samples especially those using relative abundance data a first step to tackling this complex issue is establishing an objective method of assessing preservation here diatom dissolution that can be applied by different analysts and incorporated into routine counting strategies here we establish a methodology for assessment of diatom dissolution under standard light microscopy lm illustrated with morphological criteria for a range of major diatom valve shapes dissolution data can be applied to numerical models transfer functions from contemporary samples and to fossil material to aid interpretation of stratigraphic profiles and taphonomic pathways of individual taxa using a surface sediment diatom salinity training set from the northern great plains ngp as an example we explore a variety of approaches to include dissolution data in salinity inference models indirectly and directly results show that dissolution data can improve models with apparent dissolution adjusted error rmse up to 15 lower than their unadjusted counterparts internal validation suggests improvements are more modest with bootstrapped prediction errors rmsep up to 10 lower when tested on a short core from devils lake north dakota which has a historical record of salinity dissolution adjusted models infer higher values compared to unadjusted models during peak salinity of the 1930s 1940s dust bowl but nonetheless significantly underestimate peak values site specific factors at devils lake associated with effects of lake level change on taphonomy preservation and re working implied by dissolution data may override model improvements incorporating dissolution dissolution adjusted salinity models are also applied to a 150 year sediment record from spiritwood lake north dakota which suggests that this lake has a damped and lagged response to major regional climate forcing of salinity during the dust bowl at this site dissolution data also suggest different taphonomic behaviour of taxa related to their seasonal patterns of growth and sedimentation thus dissolution data can improve models and aid interpretation of sedimentary profiles as records of limnological ecological and environmental change filtered by taphonomy

the effect of varying acquisition parameters on the interpretation of sir c radar data the virunga volcanic chain abstract in 1994 the spaceborne imaging radar experiment c sir c acquired five data takes with different acquisition parameters over the virunga volcanic chain zaire changes in incidence angle 20 0 48 1 look direction west and east looking and pulse bandwidth 10 mhz 20 mhz and 40 mhz affect the geologic interpretations drawn from these data we examine differences in spatial distortion of features near the summit of nyiragongo volcano finding that opposite look direction data are helpful in interpreting volcanic features such as the shape of small craters in areas of high relief the relative brightness of lava flows and vegetation change and actually reverse for different wavelengths and polarizations although l band data are best for mapping the outline of the flows c and x band cross polarization data show differences in backscatter intensity between and within flows in addition our ability to discriminate lava flows and vents from surrounding vegetation is strongly influenced by pulse bandwidth resolution and angle of incidence pulse bandwidth is the most important parameter in mapping small topographic features such as volcanic cones and a set of particularly thick flows on the flanks of karisimbi volcano these examples illustrate important considerations of the tradeoffs and priorities in acquisition parameters for future missions such as envisat as well as potential pitfalls for interpretation of existing data

petrology and mineralogy of the angrite northwest africa 1670 northwest africa nwa 1670 contains olivines of up to 5 mm in size representing about 30 of the studied section with subordinate clinopyroxene and chrome spinel microphenocrysts 0 2 0 5 mm they represent a xenocrystic association phenocrysts are surrounded by a groundmass predominantly comprising bundles of plagioclase and clinopyroxene typically 20 x 200 m crystals olivine and kirschsteinite are present in the groundmass in lesser amounts the olivine xenocrysts fo90 are significantly fractured and show mosaicism for their major part the remaining showing faint undulatory extinction they are surrounded with a rim of 100200 m zoned down to fo80 and overgrown with serrated olivine fo80 to fo60 about 100 m olivine in the groundmass is zoned from mg 0 55 to 0 15 its cao content ranges 2 0 to 8 4 subcalcic kirschsteinite is zoned from mg 0 13 to 0 03 cao increasing from 15 8 to 21 3 pyroxenes xenocrysts mg 0 77 are superseded in the groundmass by less magnesian pyroxenes mg 0 61 to 0 17 with an average feo mno of 98 their compositions range from en30fs22wo27al ts28ti ts2 to en2fs37wo22al ts40ti ts1 anorthite microcrysts an99 100 are restricted to the groundmass accessories are pyrrhotite kamacite ca phosphate titanomagnetite hercynite and ca carbonate the bulk chemical composition confirms that nwa 1670 corresponds to a normal angrite melt that incorporated olivine high mg olivine xenocrysts and the associated mineralogy are typical of angrites we suggest that it is an impact melt with relict phenocrysts the strong silica undersaturation the presence of fo90 olivine xenocrysts and carbonate support their derivation as melilite like melts in the presence of carbonate

the potassic series of karisimbi volcano virunga range rwanda volcanological and petrological aspects abstract the karisimbi volcano rwanda is one of the principal eruptive centers of the virunga volcanic range it is located at the southern tip of a ne sw volcano tectonic axis on the western branch of the east african rift the different morphological units caldera branca pit crater muntango domes and flow domes built up on the main edifice make karisimbi the most complex central volcano of the virunga range the products of karisimbi s activity are essentially fluid lavas of basic or intermediate chemical composition and viscous differentiated lavas ash falls and pumice flows are evidence of pyroclastic episodes which occurred during the last stages of volcanic activity around 30 000 years ago the upper pleistocene potassic series of karisimbi is composed of a diverse range of lavas closely associated in space and time these lavas show many common features with rocks from rift provinces such as high k2o tio2 rb and ba contents the general evolution of the series from potassic basanites to trachytes is qualitatively compatible with the successive separations of the mineral phases observed in the lavas olivine clinopyroxene leucite ferro titanic oxides plagioclase biotite alkali feldspar coexisting fe ti oxides indicate equilibration temperatures lying between 1100 and 750 c and fo2 between n no and qfm buffers felsic magmas are considered to have crystallized at a relatively shallow level in the crust under near water saturated conditions which is consistent with the presence of pyroclastics with trachytic composition and the presence of hydrous mineral phases