rock composition dolomite stoichiometry and rock water reactions in dolomitic carbonate rocks dolomitic carbonate rock units from a variety of phanerozoic localities and depositional environments throughout north america were evaluated with respect to their dolomite content and dolomite composition by x ray diffraction analysis dolomite compositions range from 48 to 57 mole caco 3 with well defined modes at 51 nearly stoichiometric dolomite and 55 distinctly calcian dolomite caco 3 values of percentage of dolomite also exhibit a pronounced albeit less well defined bimodal distribution with modes at 97 dolostones and 20 dolomitic limestones dolomite similar distributions characterize most other published data on phanerozoic dolomitic carbonates these data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite 1 dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution calcite and dolomite precipitation reactions and 2 dolostones originate in diagenetically open systems during th

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

volatile emission during the eruption of baitoushan volcano china north korea ca 969 ad magma volume dre 24 5 km3 the main phase ca 95 vol is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites the eruption column is estimated to have reached ca 25 km and thus entered the stratosphere a late phase 5 vol is represented by trachyte emplaced chiefly as moderately welded ignimbrites the comendites contain 3 and the trachytes 10 20 vol phenocrysts mainly anorthoclase hedenbergite and fayalite primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra whereas those in phenocrysts in the trachytes are devitrified the comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra the mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method the average h2o concentration of the comenditic matrix glass is 1 5 wt probably largely secondary and of the corresponding melt inclusions 5 2 wt melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm the comenditic matrix glasses are represented by a fluorine rich 3992 ppm f and fluorine poor group 2431 ppm f averaging 3853 ppm for chlorine only 20 of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of 250 ppm s the difference between pre and post eruptive concentration of h2o is at least 3 7 0 6 wt h2o taking into consideration re hydration of the matrix glass and possible leakage of melt inclusions the difference between pre and post eruptive concentrations of the halogens amounts to 909 90 ppm cl and 1863 280 ppm and 302 40 ppm f the difference for s was estimated based on the average of the maximum s concentrations in the melt inclusions 455 ppm s and the detection limit resulting in 205 40 ppm s the calculated mass of volatiles injected into the atmosphere based on the erupted magma volume and volatile data is 1796 453 megatons for h2o 45 10 megatons for chlorine 42 11 megatons for fluorine and 2 0 6 megatons for sulfur the 969 20 ad eruption of baitoushan volcano one of the largest eruptions of the past 2000 years is thought to have had a substantial but possibly short lived effect on climate

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

latest pleistocene and holocene river network evolution in the ethiopian lakes region abstract river network geomorphologic paleohydrologic stratigraphic and sedimentologic analyses document a dramatic reorganization of the drainage pattern in the northern part of the main ethiopian rift mer during latest pleistocene and early holocene the river network modification was induced by tectonic deformation volcanic activity and by the arid conditions connected with the last glacial maximum lgm this arid phase triggered the shrinking of a pleistocene megalake that formerly flooded large part of the main ethiopian rift the northern tributaries paleo awash and paleo mojo rivers extended following the lake shore retreat and incised the fesesa koye and cheleleka sulula hafa paleovalleys through the pleistocene deposits at the beginning of the holocene humid conditions induced a water level rise in the lacustrine basin ziway shala basin supplied from the north by the large awash mojo meki fluvial system a well exposed cross section of the cheleleka paleovalley at the confluence with the meki river and the use of paleohydrological methods allowed to infer the bankfull paleo discharge of the larger awash mojo river system tectonic events allowed the awash and mojo rivers to divert their courses to the east toward the afar depression depriving the ziway shala lacustrine basin of large volumes of water supply this and the further increase in aridity during the late holocene led to the separation of the ziway shala paleolake into the present four lakes ziway langano abjiata shala this study indicates that in the main ethiopian rift climatic changes cannot be inferred from lake level variations alone because changes in water supply are also influenced by the tectonic induced rearrangement of the fluvial drainage networks

interfacial turbulent mixing in stratified magma reservoirs abstract we analyse the interfacial turbulent mixing of molten magmas in a two layer stratified convecting magma chamber we examine situations where convection is driven by the heat and or compositional flux associated with the melting of a solid boundary the model applies to the case in which the rayleigh number is sufficiently large that the effect of a viscosity difference on turbulent entrainment is negligible and so the model predictions represent an upper bound for interfacial mixing our results suggest that the extent of mixing induced by the thermal buoyancy flux is relatively small however the compositional buoyancy fluxes associated with phase change may be much larger and the mixing they induce may significantly affect the evolution of the reservoir our analysis of the melting of the floor of a two layer stratified magma chamber shows that turbulent mixing at the interface between the two magmas may suppress the melting and decrease the final amount of melt produced when the system reaches thermal equilibrium it is also shown that although floor melting tends to decrease the density of the lower layer of magma somewhat counterintuitively turbulent interfacial mixing with the overlying layer of smaller density may inhibit large scale overturn and preserve the stratified state of the chamber in the case in which both the floor and roof of a chamber melt turbulent mixing at the interface between the newly formed light melt at the roof and the lower layer of dense magma induces a shift of the melting process from the floor to the roof for typical parameter values in a basaltic magma chamber we estimate that under conditions of very large rayleigh number the amount of crustal melt produced at the roof may be as high as 50 of that produced at the floor

paleoenvironmental change in the late middle pleistocene holocene kibish formation southern ethiopia evidence from ungulate isotopic ecology abstract east africa has produced the earliest record of homo sapiens 200 ka and a punctuated record of middle stone age and later stone age behaviors we lack however a detailed late quaternary paleoenvironmental record for the region particularly during humid periods without a regional record hypotheses about the evolution and ecology of early homo sapiens in east africa remain vague and untestable the kibish formation of southern ethiopia presents a long albeit punctuated record of late middle pleistocene to holocene faunal change in east africa which was deposited during humid periods here we present oxygen and carbon stable isotope data of the kibish ungulates and test whether there are environmental changes within the kibish formation significant differences in 18o enamel isotopes are consistent with more humid conditions during the holocene age member iv 13 4 ka than either pleistocene age member i 196 ka or member iii 104 ka mesowear data document a shift toward more attritional wear among grazers in member iv and are correlated with more depleted 18o enamel values suggesting that the wear pattern shift is linked to the onset of more humid conditions during the holocene 13c enamel values show subtle variations through time but do not suggest any major changes in diets we propose that the paleoenvironmental differences evident in member iv based on 18o enamel values mesowear and bovid abundances may be explained by cooler and wetter conditions at the beginning of the holocene in the lower omo valley the evidence suggesting that the holocene humid phase is more pronounced than earlier humid phases may explain why arid adapted grassland ungulates became extinct in east africa by the pleistocene holocene transition but persisted through previous humid phases of the late quaternary

geochemistry of spanish sepiolite palygorskite deposits genetic considerations based on trace elements and isotopes sepiolite palygorskite deposits in spanish tertiary basins were formed in lacustrine environments the mineral associations present in the mineralised intervals are made up of neoformed phyllosilicates sepiolite palygorskite stevensite detrital silicates quartz feldspars illite interstratified smectite illite al smectite and kaolinite and carbonates calcite dolomite opal a gypsum and halite may also appear sporadically two groups of chemical elements and minerals can be distinguished according to their origin al ti fe mn k ree and transition trace elements are almost exclusively included in the detrital al silicates mg ca cl f and li are concentrated in the minerals formed in the depositional basins si na sr and ba are contained in both detrital and neoformed minerals ree transition trace elements f and li contents can be used to distinguish between phyllosilicates formed by chemical precipitation in the depositional basins detrital phyllosilicates and those formed by transformation of the latter during early diagenetic processes these data together with those on 18o isotopic fractionation indicate the formation of sepiolite and mg smectite as chemical precipitates whereas palygorskite would derive from diagenetic transformation of other inherited clay minerals the values calculated for the sepiolite water and palygorskite water fractionation factors at 20 c are 1 031 and 1 027 respectively

abrupt return of the summer monsoon 15 000 years ago new supporting evidence from the lower white nile valley and lake albert abstract the last glacial period ended with an abrupt return to warmer and wetter conditions at a number of sites in intertropical africa asia australia and south america 15 000 years 15 ka ago similar abrupt warming is also evident at this time in high latitudes and in air trapped in greenland ice we here report new supporting evidence of this event from a climatically sensitive region in africa the white nile valley of the central sudan and lake albert in the uganda headwaters during the last glacial maximum the white nile valley was even more arid than it is today with desert dunes active as far south as latitude 12 s the sudden overflow of lake victoria in the ugandan headwaters of the white nile 14 5 ka confirmed here by sr isotope statigraphy caused extensive flooding in the lower white nile valley and severe flooding in egypt the volume of fresh water flowing into the eastern mediterranean at this time ultimately resulted in accumulation of highly organic sediments sapropel 1 on the floor of the mediterranean sea flooding in the central sudan attained an elevation of 382 m along a north south distance of 400 km the recessional shoreline of the 382 m white nile is indicated by concentrations of now buried freshwater gastropod shells and beach sands dated respectively by calibrated ams radiocarbon and osl to 15 16 ka this mega flood event marks the abrupt return of the summer monsoon and may reflect a globally synchronous event

a 25 000 year record of climate induced changes in lowland vegetation of eastern equatorial africa revealed by the stable carbon isotopic composition of fossil plant leaf waxes the debate of climate versus co2 in controlling the long term dynamics of tropical african vegetation has focused on events at the upper tree line since the relevant paleodata tend to be from mid elevation sites 2000 3000 m less well known is the relative importance of co2 in regulating the dynamics of tropical lowland 1500 m vegetation particularly that of the dry open woodlands bush and grasslands covering much of eastern equatorial africa here we examine the stable carbon isotopic composition of n alkanes in the sediment record of lake challa a lowland crater lake near mt kilimanjaro covering the last 25 000 years the distributions of the n alkanes with dominance of the long chain odd carbon numbered components and their isotopic composition reveal a mixed origin the c23 and c25 n alkanes are depleted in 13c with 13c values between 30 and 50 in shallow lakes these n alkanes are thought to derive from non emergent aquatic plants but this is unlikely in this steep sided crater lake as it lacks a significant littoral habitat the c27 n alkanes are predominantly derived from leaf wax lipids of terrestrial plants brought into the lake predominantly by local soil run off their 13c values in particular that of the n c31 alkane reveal a marked transition in local lowland vegetation from being dominated 70 100 by c4 plants during the glacial period until 16 5 cal kyr bp to a more mixed c3 c4 composition 30 60 c4 during the holocene the start of the late glacial trend towards a greater proportion of c3 plants coincided with the start of increasing monsoon rainfall 1500 years after the onset of the rise in atmospheric co2 and 3500 years after the onset of post glacial warming the transition was interrupted during the dry younger dryas period 13 0 11 7 cal kyr bp when c4 plants again became much more prevalent almost reaching their glacial period abundance notably the principal trend in leaf wax 13c values infers c4 dominance during both wet and dry phases of the glacial period and a mixed c3 c4 vegetation during both wet and dry phases of the holocene our results indicate that long term variation in pco2 exerted important control on the composition of drought adapted savanna vegetation in this currently semi arid lowland region of equatorial east africa

abrupt early eocene global climatic change as a control of carbonate facies and diagenesis a new record of the palaeocene eocene thermal maximum in the umm er radhuma formation saudi arabia and kuwait terra nova 24 487 498 2012 abstract records of the palaeocene eocene thermal maximum petm and associated carbon isotope excursion cie are relatively rare in shallow marine carbonates when compared with those from other regimes and it remains unclear exactly what effect this global climatic event had upon the widespread epeiric carbonate platforms of the palaeogene in this study we demonstrate that the umm er radhuma formation in the wafra field kuwait and saudi arabia shows a carbon isotope profile consistent with the cie that can be correlated globally and further that global climate change during and subsequent to the petm exerted a fundamental control on primary depositional facies and subsequent diagenesis the palaeocene eocene thermal maximum was a brief hyper thermal event marking the onset of sustained high global temperatures during the early eocene this warm period increased aridity in epeiric carbonate platform interiors of what is now the middle east affecting composition of depositional facies and the precipitation of early dolomite this is demonstrated through the use of new data from the umm er radhuma formation wafra field partitioned zone saudi arabia and kuwait biostratigraphy and carbon isotope profiles have been used to constrain the position of the petm and carbon isotope excursion a distinct 2 3 negative spike the observed excursion is demonstrated to correlate globally and provides age dating of this regionally important succession core and thin section data show marked restriction of facies across the petm evidence of increasing aridity reflected in proportion of evaporite precipitates and a stratigraphically restricted non mimetic dolomite coeval to the cie that is interpreted to relate to a change in the rate of dolomite precipitation this work provides an example of how a global environmental event is manifest in depositional and early diagenetic changes in marine carbonates

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

nile behaviour and late palaeolithic humans in upper egypt during the late pleistocene abstract the reconstruction of the environment and the human population history of the nile valley during the late pleistocene have received a lot of attention in the literature thus far there seems to be a consensus that during mis2 extreme dry conditions prevailed over north eastern africa which was apparently not occupied by humans the nile valley seems to be an exception numerous field data have been collected suggesting an important population density in upper egypt during mis2 the occupation remains are often stratified in or at least related to aeolian and nile deposits at some elevation above the present day floodplain they are rich in lithics and animal bones mainly fish illustrating the exploitation of the nile valley by the late palaeolithic inhabitants the fluvial processes active during that period have traditionally been interpreted as a continuously rising highly braided river in this paper we summarize the evidence thus far available for the late pleistocene on the population densities in the nile valley and on the models of nilotic behaviour in the discussion we include data on the environmental conditions in eastern africa on the aeolian processes in the western desert of egypt derived from satellite images 14 c and osl dates in order to formulate a new model that explains the observed high remnants of aeolian and nilotic deposits and the related late palaeolithic sites this model hypothesizes that during the late pleistocene and especially the lgm dunes from the western desert invaded the nile valley at several places in upper egypt the much reduced activity of the white nile and the blue nile was unable to evacuate incoming aeolian sand and as a consequence several dams were created in the upper egyptian nile valley behind such dams the created lakes offered ideal conditions for human subsistence this model explains the occurrence of late palaeolithic hunter fisher gatherers in a very arid environment with very low nile flows even in late summer

magnetite scavenging and the buoyancy of bubbles in magmas part 1 discovery of a pre eruptive bubble in bishop rhyolite little is known about the presence distribution and size of bubbles in rhyolitic magmas prior to eruption using x ray tomography to study pumice from early erupted bishop rhyolite we discovered a large vesicle with abundant magnetite crystals attached to its walls attachment of magnetite crystals to bubble walls under pre eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals alternatively bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it we argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression the inferred presence of pre eruptive bubbles in the bishop magma confirms the interpretation that the magma was volatile saturated prior to eruption the pre eruptive size of this bubble is estimated based on three independent approaches 1 the current size of the vesicle 2 the total cross sectional area of the magnetite crystals and 3 the bubble size required for the aggregate to be neutrally buoyant these approaches suggest a pre eruptive bubble 300 850 m in diameter with a preferred value of 600 750 m

the hargeisan revisited lithic industries from shelter 7 of laas geel somaliland and the transition between the middle and late stone age in the horn of africa abstract this paper presents the lithic analysis from the stratified deposits of one of the laas geel rock shelters in somaliland shelter 7 this set of sites identified in 2002 is composed of around 20 rock shelters most featuring polychromic paintings attributed to the holocene moreover shelter 7 yielded numerous remains that are divided into two major chono cultural phases microlithic industries from the upper part of the deposit are ascribed to the late stone age lsa in contrast the industries from the lower part of the deposit feature an association of lsa characteristics but quite different from the ones from the upper part of the deposit with technical traits that belong to the msa and especially the presence of a levallois technology this association reintroduces the question of the existence of an industry or facies once labeled hargeisan and characterized by mixed features from the msa and the lsa in this part of africa at the end of the pleistocene our analysis of these industries culminates in a reconsideration of the definitions proposed by clark in his seminal book the prehistory of the horn of africa 1954 this study provides new typo technological data from the upper part of the laas geel shelter 7 deposit that contribute to refining clark s definition of the somaliland wilton industry in addition the association of blade bladelet production with typical levallois production in the lower part of this site s deposits supports a comparison with the hargeisan industry the industries recovered from laas geel shelter 7 reintroduce debates on the existence of the hargeisan industry and provide new data on the technological transition between the msa and the lsa in the horn of africa

cooling rates of plinian fall and pyroclastic flow deposits in the bishop tuff inferences from water speciation in quartz hosted glass inclusions cooling rates inferred from water speciation in rhyolitic glass inclusions in quartz phenocrysts from the bishop tuff california vary over more than 5 orders of magnitude from about 10 to 10 4 c s 1 many of the cooling rates are within the previously published experimentally calibrated range of this technique but the slowest rates 10 4 c s 1 are extrapolated such extrapolation suggests cooling rates as low as 10 8 c s 1 0 3 c year 1 for the interiors of thick pyroclastic flow deposits the temperatures at which oh and molecular h2o species would be in equilibrium range from 400 c for glass inclusions from plinian fall deposits to 250 c for the most slowly cooled glassy inclusions from pyroclastic flow deposits notwithstanding appreciable uncertainties cooling rates correlate with depositional process fall vs flow deposition and distance above the cold ground or below overlying hot pyroclastic flow deposits plinian inclusions within 2 5 m of the cold pre bishop ground cooled at 1 10 c s 1 consistent with thermal modeling of small pumice clasts falling through cold air very slow inferred cooling rates for pyroclastic flow deposits 10 6 to 10 8 c s 1 also agree with thermal conduction models faster cooling rates 10 1 to 10 4 c s 1 for inclusions in the lowermost 0 5 m of pyroclastic flow deposits reflect the effect of quenching against the cold ground at blind spring hill east of long valley caldera intermediate cooling rates 10 3 to 10 6 c s 1 are observed in plinian deposits overlain by and partially interbedded with pyroclastic flow deposits indicating that reheating of rapidly cooled plinian inclusions can reset water speciation melt inclusions in pyroclastic flow deposits are quenched to glass only if the deposit is relatively thin or has a relatively low emplacement temperature in relatively thick or hot deposits inclusions may be quenched to glass at the base of the deposit where it is more rapidly cooled by contact with the pre eruption ground

mechanism of palygorskite formation in the red clay formation on the chinese loess plateau northwest china abstract the morphological characteristics and microstructures of the authigenic palygorskite occurring with smectite and carbonate minerals in the late miocene red clay formation on the chinese loess plateau have been investigated with the powder x ray diffraction xrd scanning electron microscopy sem and high resolution transmission electron microscopy tem the xrd results indicate that the red clay sediments contain quartz feldspar calcite dolomite illite kaolinite chlorite palygorskite and smectite and sem analyses show that palygorskite is a common component delicate palygorskite fibers radiate from the margins of platy smectite suggesting a pedogenic origin during the weathering of pre existing smectite a process that involved intense dissolution of the smectite and an adjustment in its structure moreover palygorskite occurs as coatings pore fillings and pore bridging cements among dolomite rhombs suggesting that the fibrous palygorskite was formed by a direct dolomite precipitation from pore fluids mostly after dolomite crystallization there is evidence to show that these two mechanisms took place synchronously in the red clay formation and that the two processes are related under alkaline conditions with high ph 8 0 fluids rich in magnesium magnesium ions occupy the interlayer positions in the smectite and this results in a misfit between the mg octahedral sheet and the smectite layer thereby allowing interaction between the magnesium ions and smectite layers and a reorganization of the structure to form palygorskite then with this kind of palygorskite as a crystal nucleus more palygorskite grows through a direct chemical deposition process according to previous studies the red clay sediments experienced a steady dry warm climate with periods of high rainfall and evaporation the warm climate would have increased the rate of weathering of minerals and subsequently enhanced the ions content and ph in the soil solutions during dry periods this soil environment is favorable for smectite transformation and palygorskite deposition this study has improved our understanding of the environmental and paleoclimatic changes during the time that palygorskite formed in the arid soils of the chinese loess plateau

kinetics of dolomitization dolomitization of caco3 in the laboratory at 175 c demonstrates that the transformation has two distinct stages an induction stage during which no detectable products form and a nucleation growth stage during which dolomite forms at the expense of the reactant caco3 experiments at 218 c demonstrate that as the mg ca ratio of the solution increases the length of the induction stage decreases addition of co2 to the solution decreases the induction stage and also changes the overall transformation mechanism previously published data show that the induction stage is affected by the temperature of the reaction and the surface area and mineralogy of the reactant the induction stage is often the slowest part of the overall transformation characteristics of natural dolomites that are consistent with the experimental results include the following 1 dolomite selectively replaces fine crystalline caco3 2 most carbonate rocks are 100 dolomite or 100 calcite 3 limestone dolomite contacts are usually sharp and involve a decrease in the number of dolomite rhombs but the size of the rhombs remains constant 4 dolomite in completely dolomitized rocks is more stoichiometric than dolomite in partially dolomitized rocks 5 aragonite is selectively dolomitized in modern sabkha sediments and 6 modern dolomite is most abundant in areas of elevated mg ca ratio or hco3

magma evolution recorded in plagioclase zoning in 1991 pinatubo eruption products plagioclase the most abundant phenocryst at mount pinatubo displays varying textures and compositions within the 1991 eruption products in june 7 14 dome forming andesite plagioclase phenocrysts show prominent rims with higher mgo 0 04 0 06 wt jo fe20j 0 6 0 8 wt o and k20 at given an than the interiors the compositions of the rims are identical to those of microlites which are abundant in the groundmass glass white dacitic pumice the most voluminous product of the june 15 eruption contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites the interiors of phenocrysts from the dacitic pumice and the dome forming andesite are remarkably similar in terms of textures and compositions they show oscillatory zoning mostly an35 6o low mgo 0 02 wtojo and fe20j 0 10 0 30 wt jo and similar k20 at given an this similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt the oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids a portion of the felsic magma 800 c mixed with a mafic melt 1000 1100 c to become an andesitic magma that extruded to form the june 7 14 dome all plagioclase phenocrysts in the andesite were derived from the felsic magma the mixing caused destabilization of phenocrysts forming sieve textures dusty zones and partial resorption extrusion of the mixed magma resulted in overgrowths on once resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass in the unmixed remaining portion of the felsic magma some plagioclase underwent partial resorption but did not develop overgrowths this lack of overgrowths and the absence of micro lites in the groundmass glass of the june 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption supporting the linkage between the two

fragmentation phenomena in populations of magmatic crystals fragmentation of crystals is an important mechanism and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention recent advances in textural analysis extraction techniques digital imaging and computer assisted measurements enable rapid accumulation of 3d data on particle shapes and size distributions this paper reviews fragment size distributions fsd that result from fragmentation lognormal fractal loggamma and weibull discusses their genesis mechanisms and presents relevant examples of fragmentation from experimental physics next the paper considers fsds of feldspars on digitized images of thin sections and on published images and quartz extracted from vesicular pumice by acid from eight well known large volume eruptive units the acid solution of pumice enables examination of volume abundance 3d shapes proportions of fragmented crystals and measurements of their csds and fsds fsds were also measured in samples of welded tuff and a granite disaggregated by electric pulse products of syneruptive shock wave fragmentation and fragmentation by an electric pulse are found to be fractal with large breakage probabilities branching ratios and fractal dimensions of 2 to 3 in contrast most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism which results in low breakage probability and small number 2 3 of fragments per breakage cycle these results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal fsd measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions and low survival rate for large inclusions and inclusions with large radius crystal size ratio the modeling of periodic fragmentation of crystals with melt inclusions due to overheating and or decompression which may occur many times during the lifetime of a long lived magma body may explain concave down lognormal csds abundant in igneous rocks the genesis of lognormality can be explained by the fragmentation algorithm of kolmogorov 1941 other algorithms may generate lognormal like loggamma distributions fragmentation serves as an important size limiting factor a nucleation aid and it facilitates isotopic and trace elemental exchange

dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling we have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity volatile concentration and diffusivity on the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems we conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2 4 orders of magnitude our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions an analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics r t1 2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship r log t is more appropriate during active bubble growth under certain conditions a second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression model results for degassing of initially water saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent on the basis of decompression history explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates 1 5 m s 1

pedogenic palygorskite and dolomite in a late neogene sedimentary succession northwestern transvaal south africa abstract a detailed field survey identified a late neogene sedimentary succession which accumulated in ephemeral palaeolakes on the edge of the kalahari basin in the northwest of transvaal province south africa palygorskite and dolomite formed along lake margins which fluctuated between lacustrine and dry land conditions the sedimentary sequence presented an opportunity to investigate the interrelationships between these minerals and study their possible modes of formation either by lacustrine sedimentation or pedogenic processes clay mineralogy transmission and scanning electron microscopy and geochemical analysis revealed an increase in magnesium enriched minerals palygorskite and dolomite within the pedogenically altered marginal lacustrine sediments field relationships showed that the dolomite is a replacive phase within the decomposing palygorskite although the scanning electron micrographs do not display such unequivocal evidence for a direct replacive relationship they suggest rather that the dolomite forms by neoformation from soil solutions which included decomposition products released by the breakdown of the palygorskite seen in a regional context subaerial pedogenic processes acted on marginal lacustrine sediments and resulted in the decomposition of smectite and neoformation of palygorskite a change in climate to drier conditions encouraged the formation of calcrete in surrounding areas which increased the magnesium calcium ratio in groundwaters this increase was supplemented by evaporative concentration during the desiccation of the lacustrine environment the high magnesium concentration led to the formation of the dolomite rich dolocrete this interpretive model is a means by which existing genetic models for calcareous pedocretes and those models that define the concentration of magnesium in groundwater and saline lacustrine environments can be combined the pedogenic mineral relationships described are suggested as useful palaeoenvironmental indicators for the late neogene early pleistocene of southern africa

late quaternary dynamics of southern africa s winter rainfall zone abstract variations in the nature and extent of southern africa s winter rainfall zone wrz have the potential to provide important information concerning the nature of long term climate change at both regional and hemispheric scales positioned at the interface between tropical and temperate systems southern africa s climate is influenced by shifts in the intertropical convergence zone the westerlies and the development and position of continental and oceanic anticyclones over the last glacial interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems central to this discussion has been the extent to which the region s glacial climates would have been affected by expansions of antarctic sea ice equatorward migrations of the westerlies more frequent intense winter storms and an expanded wrz this paper reviews the developing body of evidence pertaining to shifts in the wrz and the evolution of ideas that have been presented to explain the patterns observed dividing the region into three separate axes along the western and southern margins of the continent and across the interior into the karoo and the kalahari a range of evidence from both terrestrial sites and marine cores is considered and potential expansions of the wrz expansions are explored despite the limitations of many of the region s proxy records a coherent pattern has begun to develop of a significantly expanded wrz during phases of the last glacial period with the best documented being between 32 17 ka while more detailed inferences will require the recovery and analysis of longer and better dated records this synthesis provides a new baseline for further research in this key region

lithostratigraphy and environment of deposition of the top of the wafra member radhuma formation wafra field kuwait abstract a sedimentologic study of a core section 321 406 m 1030 1303 ft from the top of the wafra member radhuma formation in the northwest wafra field neutral zone shows variations in depositional environment with depth the lower part of the interval is characterized by supratidal deposits which grade upwards to intertidal deposits the wafra member is capped by the rus formation of supratidal anhydrite marking a return to a shallow water environment this is consistent with the location of the wafra field within the carbonate evaporite belt of murris 1980 during paleocene lower eocene times the fauna is consistent with a paleocene lower eocene age assignment a more precise age cannot be given because of the degree of alteration dolomitization currently available data on the radhuma formation are summarized and its subdivision in the wafra field is discussed

ion microprobe studies of water in silicate melts concentration dependent water diffusion in obsidian abstract the temperature dependence of water diffusivity in rhyolite melts over the range 650 950 c and pt h2o 700 bars is evaluated from water concentration distance profiles measured in glass with an ion microprobe diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10 8 cm2 s at 650 c to about 10 7 cm2 s at 950 c at 2 wt water water solubility also varies with temperature at a rate of 0 14 wt per 100 c increase the avtivation energy ea appears to be constant at 19 1kal mole for 1 2 and 3 wt h2o comparison of these data with results for cation diffusion indicates that this value is a minimum ea for diffusion of any species in a rhyolite melt compensation plots of log10d0 the frequency factor versus ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts d0 increases for h2o and ca2 1 as ea decreases this suggests that these molecules may diffuse by different mechanisms than do monovalent cations and that hydration of the melt affects diffusion of ca2 and h2o differently than it does monovalent cation diffusion the results imply that dramatic increases in cation diffusivities by hydration 1 may occur with additions of less than 1 wt h2o

half precessional dynamics of monsoon rainfall near the east african equator extensive records exist with which to assess the relationship between external climate forcings such as changes in insolation and climate variability for middle and high latitudes but records from equatorial regions are relatively few especially from regions experiencing the passage of the intertropical convergence zone a continuous and well resolved climate proxy record of hydrological variability during the past 25 000 years from equatorial east africa is now presented and analysed

sulfur diffusion in rhyolite melts diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800 1100 c water contents of 0 7 3 wt and oxygen fugacities from the quartz fayalite magnetite buffer to air experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses regression of the diffusion data in dry rhyolite melt gives dsulfur 0 05 exp 221 80rt which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as h2o co2 and cl diffusion of sulfur in melt with 7 wt dissolved water is 1 5 to 2 orders of magnitude faster than diffusion in the anhydrous melt depending on temperature sulfur is known to dissolve in silicate melts as at least two different species s2 and s6 the proportions of which vary with oxygen fugacity despite this oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions this result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity the most likely candidate for the diffusing species is the sulfide ion s2 re equilibration between s2 and s6 in oxidized melts must generally be slow compared to s2 diffusion in order to explain the observed results in a silicic melt undergoing degassing sulfur will tend to be fractionated from other volatile species which diffuse more rapidly this is consistent with analyses of tephra from the 1991 eruption of mount pinatubo philippines and from other high silica volcanic eruptions

petrography and significance of zoned dolomite cements from the cayman formation miocene of cayman brac british west indies abstract cavities in dolostones from the cayman formation miocene from cayman brac commonly contain complex cement successions that record three main phases of diagenesis phase i cements formed of low ca calcian dolomite lcd and high ca calcian dolomite hcd are divided into monomineralic lcd crystals oscillatory zoned lcd hcd crystals and step zoned hcd lcd crystals dissolution during phase ii produced dolomite crystals with hollow cores hollow growth zones and hollow crosscutting zones calcite precipitated during phase iii filled many of the cavities that had not been filled by the dolomite cements etching shows that the lcd is formed of blocky subcrystals that have their long axes perpendicular to the growth surfaces whereas hcd is characterized by a modulated structure that is evident as growth parallel slightly sinuous strands 10 20 nm wide the contrast in crystallographic microstructures supports the notion that lcd and hcd are distinct types of dolomite two levels of variability that operated on different time scales controlled diagenesis in the cayman formation of cayman brac long term variability controlled the evolution from dolomite precipitation to dolomite dissolution to calcite precipitation short term variability controlled variations in the physiochemical environments that characterized each phase of diagenesis thus during phase i diagenesis short term fluctuations in water chemistry and or crystal growth patterns controlled the mol caco3 in the dolomite cement crystals similarly short term variations in pore water compositions controlled the style of dolomite dissolution during the second phase of diagenesis

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

melt inclusion hosted excess 40ar in quartz crystals of the bishop and bandelier magma systems 40 ar 39 ar experiments on melt inclusion bearing quartz mibq from the bishop and bandelier tuff plinian deposits indicate high concentrations of excess 40 ar in melt inclusions two rhyolite glass melt inclusion populations are present in quartz exposed melt inclusions and trapped melt inclusions air abrasion mill grinding and hydrofluoric acid treatments progressively remove exposed melt inclusions while leaving trapped melt inclusions unaffected laser step heating of mibq yields increasing apparent ages as a function of exposed melt inclusion removal reflecting the higher nonatmospheric 40 ar concentrations hosted in trapped melt inclusions exposed melt inclusion free mibq from the bishop upper bandelier and lower bandelier tuffs yield total gas ages of 3 70 1 00 ma 11 54 0 87 ma and 14 60 1 50 ma respectively we interpret these old apparent ages as compelling evidence for the presence of excess 40 ar in mibq trapped melt inclusions in sanidine phenocrysts may contain excess 40 ar concentrations similar to those in mibq this excess 40 ar has the potential to increase single crystal laser fusion ages of sanidine by tens of thousands of years relative to the actual eruption age

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

the effect of nonlinear decompression history on h2o co2 vesiculation in rhyolitic magmas magma ascent rate is one of the key parameters that controls volcanic eruption style tephra dispersion and volcanic atmospheric impact many methods have been employed to investigate the magma ascent rate in volcanic eruptions and most rely on equilibrium thermodynamics combining the mixed h2o co2 solubility model liu et al 2005 with the diffusivities of both h2o and co2 for normal rhyolitic melt zhang et al 2007 behrens and zhang 2009 we model the kinetics of h2o and co2 in rhyolitic eruptions that involve non linear decompression rates our study focuses on the effects of the total magma ascent time the non linearity of decompression paths and the influence of different initial co2 h2o content on the post eruptive h2o and co2 concentration profiles around bubbles within the melt our results show that under most circumstances volatile diffusion profiles do not constrain a unique solution for the decompression rate of magmas during an eruption but instead provide a family of decompression paths with a well defined trade off between ascent time and non linearity an important consequence of our analysis is that the common assumption of a constant decompression rate averaged value tends to overestimate the actual magma ascent time

on the rise of turbulent plumes quantitative effects of variable entrainment for submarine hydrothermal vents terrestrial and extra terrestrial explosive volcanism 1 the maximum height reached by a turbulent plume rising in a stratified environment is a key tool to estimate the flux released at its source particularly for large scale flows because flux can often be very hard to measure directly this height is known to be mainly controlled by the stratification of the ambient fluid source buoyancy flux and the efficiency of turbulent mixing between the plume and the external fluid the latter effect has been only superficially explored in spite of its fundamental control on the dynamics here we show that commonly used one dimensional models incorporating a constant entrainment coefficient do not provide satisfying predictions we propose a new model allowing for variable entrainment which gives excellent predictions of maximum heights reached by laboratory plumes in stratified environments we then apply our formalism to natural plumes produced by explosive volcanic eruptions under terrestrial paleo martian and venusian conditions and by submarine hydrothermal activity at mid ocean ridges source mass discharge rates deduced from maximum volcanic column heights for terrestrial eruptions are found to be greater than those estimated with the commonly used constant entrainment parameterization by a factor of 2 in the paleo martian atmosphere existing models overestimate plume heights by 14 27 in the current atmosphere of venus the maximum height reached by a volcanic plume is also found to be smaller than previously estimated for large eruption rates the source heat flux released by the tag field atlantic ocean deduced from several submarine hydrothermal plumes is found greater by a factor 3 with our model

origin of broken phenocrysts in ash flow tuffs surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff although many geologists have briefly alluded to broken phenocrysts none have addressed their origin in any detail petrographic study of 117 cooling units in the middle tertiary ash flow province of the great basin united states provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash flow tuffs although not proven to be wholly ineffective breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons first the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes e g only 1 probability for 10 phenocrysts collision is less likely if pyroclasts move by laminar rather than turbulent flow second the coating of glass and or melt on the phenocrysts provides a cushion that absorbs the impact force third plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in great basin tuffs fourth euhedral phenocrysts of feldspar are commonplace in many great basin tuffs and in some they constitute a significant proportion of the phenocrysts indicating that mutual impact does not modify all intratelluric crystals during explosive eruption the two most populated categories of phenocryst shape in great basin tuffs probably correspond to what has been previously called broken phenocrysts somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral these are similar in shape size and composition to grains in polycrystalline aggregates within the same thin section kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption more than half of the plagioclase and all of the quartz phenocrysts in great basin tuffs consist of irregularly shaped fragments with cuspate embayed outlines resembling pieces of a jigsaw puzzle which we call phenoclasts inclusions of glass are common and are especially evident in larger more or less whole crystals textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma as the erupting magma decompressed vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart forming the phenoclasts shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement euhedral phenocrysts are common in ash flow tuffs as well as lava flows phenoclasts however are diagnostic of ash flow tuffs because they do not occur in plinian ash fall deposits and are rare in lava flows these textural contrasts are useful for interpretation of generally older but in any case altered and recrystallized volcanic rocks in such rocks critical groundmass features and field relations that could provide clues to their origin have been obscured but the shapes of relict phenocrysts are commonly well preserved

direct physical evidence of dolomite recrystallization the possibility of recrystallization is a long standing barrier to deciphering the genetic origin of dolomites there is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event results from 65 new high temperature dolomite synthesis experiments 1 m 1 0 mg ca ratio solutions at 218 c demonstrate dolomite recrystallization affecting stoichiometry cation ordering and nanometre scale surface texture the data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization which occur by various dissolution precipitation reactions during the first phase induction period no dolomite forms despite favourable conditions the second phase replacement period occurs when ca rich dolomite products with a low degree of cation ordering rapidly replace calcite reactants during the replacement period dolomite stoichiometry and the degree of cation ordering remain constant and all dolomite crystal surfaces are covered by nanometre scale growth mounds the third phase primary recrystallization period which occurs in the experiments between 97 and 100 dolomite is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering the end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat laterally extensive layers the fourth phase of the reaction secondary recrystallization period occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time inferences of recrystallization in natural dolomite based on cation order or stoichiometry of dolomite usually depend on assumptions about the precursor dolomite subjected to recrystallization if it is assumed that the experimental evidence presented here is applicable to natural low temperature dolomites then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization

cooling cycles heinrich event 1 and the desiccation of lake victoria abstract east africa s enormous lake victoria dried out at the close of the last glacial but the precise timing and origin of that drying event have not been clarified largely because of uncertainty regarding core stratigraphy and ancient carbon effects on 14 c dates from the lake new dates and re examination of evidence from widely distributed cores shows that desiccation occurred some time between 15 900 and 14 200 calendar years bp and perhaps also ca 18 17 kyr bp these lake level minima were briefer than has been previously suggested and were synchronous with pronounced global climate disruptions including north atlantic ice rafting heinrich event 1 less severe declines occurred during the older and younger dryas intervals dansgaard oeschger type cooling cycles registered in the gisp2 ice core record tracked major lake level regressions in east africa and weakenings of afro asian monsoons during the late quaternary possibly linking the desiccation of lake victoria to century scale reductions in solar radiation output

anhydrite in the 1989 1990 lavas and xenoliths from redoubt volcano alaska abstract the eruption of redoubt volcano in alaska produced a moderate sulfur emission estimated at 1 10 tons so 2 but relatively small volume of lava 0 11 km with pre eruption estimates of 840 950 c and f o 2 1 5 to 2 0 log units above nno swanson s e nye c j miller t p avery v f 1994 magma mixing in the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry journal of volcanology and geothermal research 62 453 468 petrologic estimates of sulfur production sigurdsson h devine j d davis a n 1985 the petrologic estimation of volcanic degassing jokull 35 1 8 from this eruption gerlach t westrich h r casadevall t j finnegan d l 1994 vapor saturation and accumulation in magmas of the 1989 1990 eruption of redoubt volcano alaska journal of volcanology and geothermal research 62 317 337 are considerably less than the measured sulfur emission leading workers to propose the existence of a pre eruption vapor phase to explain the excess sulfur initial examination of the 1989 1990 redoubt eruptive products reported anhydrite nye c j swanson s e avery v f miller t p 1994 geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry journal of volcanology and geothermal research 62 429 452 in interstitial glass from some cognate gabbroic xenoliths but anhydrite was not noted in any of the andesites a boeing 747 encountered the ash plume from the initial eruptive phase on december 15 1989 and provided ash samples that reportedly contained gypsum bayhurst g k wohletz k h mason a s 1994 a method for characterizing volcanic ash from the december 15 1989 eruption of redoubt volcano alaska u s geological survey bulletin 2047 13 17 however the identification was based on eds analyses on a sem and the mineral could have been anhydrite reexamination of the 1989 1990 redoubt lavas and xenoliths revealed the presence of rare fine grained 10s of microns anhydrite crystals in the lavas and larger 10s to 100s of microns crystals in the xenoliths the anhydrite forms elongate crystals with sharp contacts against the groundmass glass grains show rounded terminations suggesting breakdown of the anhydrite perhaps during eruption the anhydrite is a sparse phase in the lavas but occurs in lavas and cognate xenoliths erupted throughout the eruption the presence and breakdown of anhydrite may explain the excess sulfur associated with the redoubt eruption a careful search of other freshly erupted andesitic lavas solidified from hydrous oxidizing magmas should reveal other examples of anhydrite and cause a rethinking of the source of sulfur in volcanic emissions

sulfur evolution of the 1991 pinatubo magmas based on apatite abstract using electron microprobe emp and laser ablation inductively coupled plasma mass spectrometry la icp ms to collect major and rare earth elements ree respectively from apatites from the 1991 mt pinatubo juvenile eruption products we have determined that two statistically distinct populations of apatite exist one population crystallized from the juvenile basaltic melt basalt apatites and the other population crystallized from the main dacitic magma body silicic apatites both populations contain high s apatites 0 7 wt so3 apatite has previously been shown to be a potential monitor for magmatic sulfur contents via numerous proposed coupled substitutions of p5 for s6 however simple apatite melt partitioning cannot account for high s silicic apatites which grew from a silicic melt with an apparent maximum s concentration of 80 ppm disparate apatite morphology i e skeletal and acicular for basalt apatites and euhedral for silicic apatites as well as compositional evidence reveal that high s silicic apatites were not inherited from the juvenile basalt during mingling mixing prior to eruption sulfur gain from neighboring anhydrite phenocrysts can also be ruled out as a source of high sulfur emp sulfur mapping of silicic apatites shows highly irregular patterns of sulfur enrichment that do not correspond with adjacent anhydrite and can be found within apatites hosted by other minerals e g hornblende and fe ti oxides with these data in mind we propose high s silicic apatites from pinatubo and other sulfur rich systems achieved elevated sulfur concentrations during high sulfur fluxing events that originated from underplated basalt during degassing of a so2 rich fluid phase that basalts were indeed sulfur rich and oxidized is here indicated by high s contents of apatites growing in basalt the predominant location of s rich areas of silicic apatite is crystal interiors of apatite inclusions in other mineral phases while large apatite microphenocrysts surrounded by groundmass glass have low sulfur contents this is evidence for placing sulfur fluxing periods long before events that ultimately led to the 1991 eruption

the geochemical and petrological characteristics of prenatal caldera volcano a case of the newly formed small dacitic caldera hijiori northeast japan evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards to understand pre eruptive conditions of a magma chamber before its first appearance at the surface this paper describes the case of hijiori caldera volcano in northeastern japan which emerged approximately 12 000 years ago at a place where no volcano ever existed we estimated the depth density bulk modulus vesicularity crystal content and bulk h 2 o content of the magma chamber using petrographic interpretations bulk and microchemical compositions and thermodynamic calculations the chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the hijiori magmatic plumbing system was located at depths between 2 and 4 km and had the following characteristics 1 pre eruptive temperature about 780 circ c 2 bulk magma composition 66 1 5 wt sio 2 3 bulk magmatic h 2 o approximately 2 5 wt and variable characteristics that depend on depth 4 crystal content le 57 vol 5 bulk modulus of magma 0 1 0 8 gpa 6 magma density 1 8 2 3 g cm3 and 7 amount of excess magmatic h 2 o 11 32 vol or 48 81 mol the range of melt water contents found in quartz hosted melt inclusions 2 9 wt suggests the range of depth phenocrysts growth to be wide 2 sim 13 km our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated this chamber has probably grown and re mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth

eruption mechanisms and short duration of large rhyolitic lava flows of yellowstone abstract large volume effusive rhyolite lava flows are a common but poorly understood occurrence from silicic volcanic centers we integrate characterization of lava flow topographic morphology and petrographic textures and zoning of crystals with physical models of viscous fluid flow in order to interpret the eruption durations and discharge rates for the most recent effusive volcanic eruptions from yellowstone these large volume 10 70 km3 crystal poor rhyolite lavas erupted within the yellowstone caldera as 100 200 m thick flows and have a cumulative erupted volume of 650 km3 that is similar to less frequent caldera forming events but occur as individual eruptions spread over 100 ka most of this work is focused on the axisymmetric 124 ka 50 km3 summit lake flow we examined crystallinity major and trace element concentrations oxygen and hydrogen isotopic values and quartz morphology and zoning in samples from the center to margin of this flow water contents down to 0 1 wt and d values of 110 are low and require closed system degassing until near surface lithostatic pressure while major elements are consistent with water undersaturated pre eruptive storage and crystallization at 4 8 km depth we found some evidence for subtle km scale zoning within the lavas but describe significant microscopic scale compositional diversity including sharp boundaries between high ti cores and 200 m thick rims on quartz phenocrysts embayed quartz external morphology and rim growth may be the result of undercooling during coalescence of magma bodies during shallow transport between dikes and sills modeling the emplacement of the lava flow as a simple viscous fluid suggests that emplacement of rhyolite lava at 800 c occurred over 2 to 5 yrs with high discharge rates 100 m3 s such high magma discharge rates are accommodated through 6 km long fissures that allow for slower magma ascent velocities of 10 yr flow durations and significant cooling of the flow front that should result in a more complex compound flow morphology than observed higher temperatures require unrealistically wide 50 m dike widths to accommodate large discharge rates petrographic and isotopic evidence from crystals suggests recharge and merger of individual magma batches occurs on a similar timescale to the eruption duration and may directly cause overpressure and emplacement of these rhyolite lava flows from a shallow ephemeral magma chamber large volume rhyolitic lavas are able to erupt effusively through elongate fissures that utilize preexisting zones of crustal weaknesses such as ring fractures less common explosive eruptions at yellowstone may result when ascending magmas are forced through narrower conduits or when recharge rates are especially high the results of this study provide a unique top down constraint on effusive eruption rates make new interpretations of common petrographic textures and presents a comprehensive model for eruption control

magma systems of the kutcharo and mashu volcanoes ne hokkaido japan petrogenesis of the medium k trend and the excess volatile problem abstract despite the existence of a growing database of the amount of h 2 o in melt inclusions it is difficult to calculate the bulk h 2 o content for vapor saturated magma because the observable h 2 o concentration in melt is limited by its solubility to overcome this problem we focus on k 2 o which shows similar geochemical behavior to h 2 o but is largely retained within melt in the case of volatile saturation and magma degassing as an initial trial of this approach we document the evolution of k 2 o content for the kutcharo and mashu volcanoes in northeastern hokkaido japan which are located close to each other and have similar timing of eruptive activity but are chemically distinct with kutcharo being medium k in composition and mashu being low k despite the contrasting bulk rock compositions of the two volcanoes the melt inclusions show a smooth compositional trend that is continuous from low k basalt to andesite rhyolite medium k rhyolite and finally high k rhyolitic melt the observed medium k series probably is made by mixing of low k basalt and medium k rhyolite which derive from solidified low k basalt thermodynamic computations using melts show that the observed chemical variation in melt inclusions can be explained by fractional crystallization and subsequent re melting of low k basalt in addition the similar sr nd and pb isotopic compositions of the basalt and rhyolite combined with the progressive enrichment of trace elements from basalt to rhyolite support the interpretation that the two rock types are genetically related the observed evolution of k 2 o contents suggests that the total volume of basalt supplied intermittently beneath the volcanoes was approximately 10 times the volume of erupted rhyolite magma the solidified basalt which contains h 2 o 5 wt s 0 3 wt and cl 0 01 wt supplied a much larger amount of magmatic volatiles to the overlying felsic magma than could be dissolved

origin and petrophysics of dolostone pore space abstract the common claim that dolomitization creates 12 porosity is based on the mole for mole replacement equation however in the past 50 years data have been collected demonstrating that dolomitization does not create porosity instead dolostones inherit the porosity and fabric of the precursor limestone and porosity is reduced by overdolomitization the porosity of the precursor limestone depends on the diagenetic history up to the time of dolomitization data show that 1 carbonates are born with high porosity and lose porosity gradually over a long period and 2 mud dominated fabrics compact more readily than grain dominated fabrics the problem of estimating the time of dolomitization is minimized by confining observations to young limestones and associated dolostones limited data from holocene dolomitic sediments suggest no change in porosity with dolomitization study of plio pleistocene carbonates in bonaire netherlands antilles demonstrates that precursor limestones are more porous than dolostones limestones average 25 porosity whereas dolostones average 11 porosity data from the neogene of the great bahama bank show that dolostones and adjacent limestones both have 40 porosity porosity studies of the jurassic arab d formation show that dolostones and associated grain dominated limestones have similar porosity ranges and that the decrease in porosity with increasing dolomitization results from compaction of the mud dominated fabrics these data suggest that porosity in dolostone is not created by a mole for mole replacement mechanism instead dolostone porosity is 1 inherited from the precursor limestone and 2 occluded by the process of overdolomitization palaeozoic dolostones however are commonly more porous than juxtaposed limestones the explanation for this observation is that limestones lose porosity through compaction and cementation whereas dolostones resist compaction and retain much of their porosity permeability studies have demonstrated that dolomitization of grain dominated limestones usually does not change porosity permeability relationships instead precursor limestone fabric controls pore size distribution the dolomite crystal size of a muddominated dolostone may however be larger than the carbonate mud size improving the porosity permeability relationship substantially hence there is a predictable relationship between interparticle grains or crystals porosity permeability precursor grain size and dolomite crystal size

lithostratigraphy of the middle eocene dammam formation in qatar arabian gulf effects of sea level fluctuations along a tidal environment abstract the middle eocene dammam formation dominates the surface exposure of qatar it consists of shales followed by limestone and is topped by dolomitic limestone it is subdivided from oldest to youngest into the midra shale dukhan umm bab and abaruq members three patterns of facies dominate the formation shale marl facies in the lower part mudstone wackestone packstone facies in the middle part and argillaceous dolomitic wackestone packstone facies in the upper part the main fauna of the dammam formation include nummulites alveolines linderines shark teeth molluscs and echinoderms the nummulites species are common particularly within the middle members dukhan and umm bab members coarsening upward parasequences reflect the cyclicity of the eocene dammam formation in qatar at least three short lived transgressive regressive cyclothems can be recognized in sediments of the formation these small scale cycles resulted from fluctuations in sea level a local hiatus is believed to separate these cycles in northeastern qatar a warm shallow marine environment was predominant as indicated by extensive dolomitization and the presence of alveolines linderines and nummulites with globular tests

the origin of common dolomite fabrics clues from the pliocene abstract characteristic features common to dolomites of all ages are found in pliocene dolomites from the netherlands antilles these features include 1 pseudomorphic replacement 2 selective replacement 3 fossil moldic porosity 4 cloudy centered clear rimmed dolomite 5 sucrosic dolomite and 6 dolomite cement the development of a particular feature is controlled by 1 mineralogy of the material being replaced 2 the degree of saturation of the bulk solution with respect to the mineral being replaced and 3 the availability of dolomite nucleation sites low mg calcite lmc cement and fossils were often neither replaced by dolomite nor dissolved but aragonite and high mg calcite hmc were both replaced and dissolved the resistence of lmc to dolomitization was due to solutions saturated with respect to lmc and the lack of nucleation sites cryptocrystalline lmc was more readily replaced by dolomite because it provides more nucleation sites cloudy centered clear rimmed dolomite and sucrosic dolomite formed in sediments with abundant lmc and few nucleation sites fossil moldic porosity developed concurrent with pseudomorphic replacement suggesting that the dolomitizing solution was undersaturated with respect to the mineral being replaced dolomite cement commonly lines pores and fills intraparticle pore space dolomite cement occasionally has inclusions of a precursor lmc cement dolomite cement crystals are larger than replacement crystals because there are fewer nucleation sites for the cement crystals

salinity reflux and dolomitization of southern australian slope sediments the importance of low carbonate saturation levels anomalously saline waters in ocean drilling program holes 1127 1129 1130 1131 and 1132 which penetrate southern australian slope sediments and isotopic analyses of large benthic foraminifera from southern australian continental shelf sediments indicate that pleistocene holocene meso haline salinity reflux is occurring along the southern australian margin ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50 salinity a well flushed zone at the top of all holes contains pore waters with normal marine trace element contents alkalinities and ph values dolomite precipitation occurs directly below the well flushed zone in two phases phase 1 is a nucleation stage associated with waters of relatively low ph ca 7 caused by oxidation of h2s diffusing upward from below this dolomite precipitates in sediments 80 m below the sea floor phase 2 dolomite grows on dolomite cores precipitated during phase 1 phase 2 dolomite has d 13 c values similar to those of the surrounding bulk carbonate and high sr values relative to phase 1 dolomite consistent with having formed in waters affected by aragonite and calcite dissolution the nucleation stage in this model phase 1 challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water

melt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation 1 a new method is presented to estimate the decompression rate of silicic magma prior to fragmentation and is applicable to rates of 1 100 kpa s this method uses the remaining concentrations of h2o and co2 in bubble free cylindrical melt now glass pockets in phenocrysts that are connected to the surrounding bubbly melt these melt pockets are common in volcanic rocks during magma ascent bubbles grow in the surrounding melt and the concentrations of dissolved h2o and co2 decrease establishing a boundary condition of low volatile concentration at the outlet of the melt pocket in turn volatiles diffuse from the melt pocket into the surrounding bubbly magma and form a gradient that is time dependent the volatile concentration at the inner limit of the melt pocket likewise diminishes with time knowing the diffusivity and solubility of h2o and co2 permits the remaining h2o and co2 contents to be estimated for different decompression rates using diffusion equations we apply this approach to the measured h2o and co2 contents in quartz hosted melt pockets in pumice from the phreatomagmatic 26 5 ka oruanui eruption taupo new zealand a decompression rate of 1 7 kpa s 5 35 cm s was obtained for oruanui magma

late stone age variability in the main ethiopian rift new data from the bulbula river ziway shala basin abstract research conducted along the bulbula river ziway shala basin main ethiopian rift has yielded archaeological assemblages that document various phases of the upper pleistocene and the early holocene the oldest period documented is dated to 34 33 000 cal bp dw1 and the second is dated to the pleistocene holocene transition between 14 000 and 11 000 cal bp b1s1 dw2s2 and dw2s1 the analysis of these assemblages and their comparison offer new perspectives on early lsa industries and on the development and diversity of partially microlithic industries at the end the pleistocene and the beginning of the holocene through the characterization of techno economic changes in the context of shifting environments and their exploitation by humans our results intend to provide new insights into human behavioral evolution on a local scale and for a time period that is currently insufficiently known this study highlights the existence of major breaks in the archaeological record of the area during mis 3 1

evidence for a copper bearing fluid in magma erupted at the valley of ten thousand smokes alaska several lines of evidence point to the existence of a pre eruptive fluid phase in the magmas erupted in 1912 at the valley of ten thousand smokes alaska the discovery of rare cu rich bubbles in some melt inclusions is best explained by the random entrapment of a low density fluid phase within growing phenocrysts the h2o s and cl concentrations of the high silica rhyolite are also consistent with fluid saturation in the rhyolite water contents as determined by infrared spectroscopy on doubly polished melt inclusions cluster between 3 5 and 4 5 wt h2o without any apparent differences between magmas vented explosively and effusively these concentrations would be sufficient for h2o saturation at a pressure of 100 mpa though the presence of other volatiles such as co2 and so2 could allow saturation with respect to a fluid phase at higher pressures the s cu and cl contents of the phenocryst assemblages as determined by xrf analyses are too low to account for the decrease in the concentrations of s and cu in the melt and only modest increase in cl with differentiation therefore the behavior of these volatile elements was controlled either by eryptic fractionation if phenocryst phasese or more likely by partitioning into a coexisting fluid phase the presence of this low density phase apparently provided metals such as cu with a volatile phase into which they could partition the concentration of cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0 05 wt cu the distribution of cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite which cyrstallized directly from the silicate melt

coherent changes of southeastern equatorial and northern african rainfall during the last deglaciation during the last deglaciation wetter conditions developed abruptly 14 700 years ago in southeastern equatorial and northern africa and continued into the holocene explaining the abrupt onset and hemispheric coherence of this early african humid period is challenging due to opposing seasonal insolation patterns in this work we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical african precipitation changes our results show that meltwater induced reduction in the atlantic meridional overturning circulation amoc during the early deglaciation suppressed precipitation in both regions once the amoc reestablished wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas ghg concentrations whereas wetter conditions south of the equator were a response primarily to the ghg increase

h2o co2 s fluid triggering the 1991 mount pinatubo climactic eruption philippines the factors that trigger explosive eruptions often remain elusive because of the lack of direct data from representative samples here we report the first micro raman spectroscopy measurements of fluid and multiphase inclusions trapped in quartz xenocrysts and microlites from andesitic lavas and basaltic enclaves of the 1991 mount pinatubo eruption our analyses reveal two phase h2o co2 s inclusions containing a co2 dominated phase and an aqueous sulfate bearing liquid phase and less commonly anhydrite caso4 solid the two fluid phases are low temperature products of a supercritical h2o co2 s fluid which was associated with a hydrous silicate melt prior to eruption the average density of the co2 phase is 0 4 0 2 g cm3 at room temperature corresponding to a supercritical fluid density of 0 6 0 1 g cm3 at the conditions of entrapment at 760 1000 c and up to 260 mpa for the first time a dense co2 bearing fluid is reported in mount pinatubo volcanic samples we suggest that this hybrid h2o co2 s fluid originated from mixing between sulfur rich basaltic and hydrous dacitic magmas as the former was intruded into and interacted with the pre eruptive mount pinatubo dacite magma reservoir at depths of at least 10 km thermodynamic modeling demonstrates that part of the so2 liberated from the intruded basaltic magma was consumed via interaction with the aqueous fluid saturated dacitic magma according to the reaction 4so2basalt 4h2odacite 3hso4 h2s 3h yielding early cu rich sulfides late abundant anhydrite and so4 rich apatites which are commonly found in the mount pinatubo dacites we suggest that this hybrid h2o co2 s fluid played an important role in triggering the 1991 climactic eruption

inside out dolomite abstract inside out dolomite crystals are unusual because their cores are younger than their encasing cortices in the cayman formation middle miocene of grand cayman they developed diagenetically from zoned dolomite crystals that have cores formed of high ca calcian dolomite hcd 55 mol caco3 and outer cortical zones formed of low ca calcian dolomite lcd 3 or oscillatory zoned lcd hcd the cores of these crystals labeled as type i have a euhedral rhombic outline with no embayments or angular reentrants and commonly contain calcite inclusions and or micropores hollow dolomite crystals evolved from these crystals as preferential dissolution removed all or part of the hcd core later calcite or dolomite cement was precipitated inside the hollow crystals the former produced dolomite crystals with a calcitic core i e dedolomite whereas the latter produced inside out dolomite crystals that have a dolomite core lcd or hcd encased by a dolomite cortex the cores of the inside out dolomite crystals labeled as type ii have an irregular outline characterized by angular reentrants and or embayments micropores that are usually larger than those in the type i cores but few calcite inclusions the cortical zones have a euhedral rhombic motif irrespective of the type of core development of insideout dolomite crystals may significantly alter the average ca of the dolostones as hcd is lost to dissolution and lcd or hcd cement is precipitated in its place the 18o may also change inasmuch as it is thought to covary with ca in some tertiary dolostones the genesis of inside out dolomite crystal may offer an alternative explanation for the textures e g clotted cathodoluminescence patterns in some ancient dolomite crystals that have been attributed to replacement and or recrystallization

highly explosive 2010 merapi eruption evidence for shallow level crustal assimilation and hybrid fluid abstract the processes responsible for the highly explosive events at merapi central java indonesia have been investigated through a petrological mineralogical and geochemical study of the first stage tephra and pyroclastic flows sampled in october and november 2010 and second stage ash sampled shortly after the 5 6th november 2010 paroxysmal subplinian eruption several chemical and physical parameters suggest that the magma assimilated calc silicate xenoliths derived from the surrounding carbonate bearing crust javanese limestone the bulk volcanic samples have highly radiogenic 87 sr 86 sr 0 70571 0 70598 ratios that approach the compositional field of material similar to the calc silicate xenoliths the 2010 plagioclase phenocrysts from the pyroclastic flow and tephra reveal anorthite cores up to an 94 97 with low feo contents 0 8 wt and 18 o enrichment 6 5 18 o the major and trace elements of the silicic glasses and phenocrysts plagioclase low al augite and titanomagnetite the sr isotopic compositions of the bulk samples and plagioclases erupted in 2010 can be explained by complete digestion of the 1998 and 2006 calc silicate xenoliths the bulk assimilation proceeded through binary mixing between a calcic melt representing crustal assimilant cao up to 10 5 wt and cao al 2 o 3 up to 1 2 and the deep source hydrous k rich melt similarly to the 1998 and 2006 calc silicate xenolith composition the 2010 crustal assimilant is enriched in mn mno up to 0 5 wt zn v and sc contents in contrast the hydrous k rich melt is enriched in volatiles cl up to 0 37 wt and bulk h 2 o co 2 up to 5 1 wt al 2 o 3 tio 2 and ree contents consistent with its derivation from deep source this hydrous k rich melt may have been saturated with an aqueous cl rich fluid at about 200 mpa a pressure consistent with the level of the crustal assimilation we estimated that the pre eruptive basaltic andesite magma assimilated from 15 to 40 wt of the calc silicate crustal material corresponding to introduction of additional 0 19 to 2 1 mt of co 2 to the magma experimental leaching of the ash samples documents the release of an aqueous fluid enriched in cl na ca cd sb and zn during the paroxysmal subplinian eruption the paroxysmal eruption may have been produced by saturation of the pre eruptive basaltic andesite magma with hybrid aqueous carbonic nacl hcl rich fluid due to bulk assimilation creating elevated partial pressure of co 2 at shallow crustal conditions of about 200 mpa in contrast mildly explosive block and ash flows typical merapi type may result from selective assimilation of the carbonate bearing xenoliths and lower co 2 partial pressure that may not lead to explosive degassing

petrologic and sulfur isotopic significance of highly oxidized and sulfur rich magma of mt pinatubo philippines dacitic pumices from pyroclastic flow deposits and air fall tephra of the june 14 15 1991 eruption of mt pinatubo philippines are characteristically rich in sulfur the presence of microphenocrystic anhydrite suggests that sulfur existed mostly as oxidized species in the magma this supposition is corroborated by unusually high sulfur contents up to 0 78 wt as so3 in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals and by the highly oxidized state of the magma near or slightly below the hematite magnetite buffer on the basis of extrapolation from biotite compositions and the two oxide method this highly oxidized state possibly caused the extraordinarily high sulfur content of mt pinatubo dacite through prohibition of sulfide fractionation and because of increasing solubility of sulfur as oxidized species with increasing oxygen fugacity hornblende geobarometry indicates a pressure of 2 kbar for phenocryst formation among the two pumice types cummingtoniterimmed hornblende phenocrysts are typically found in type 1 white and crystal rich pumice whereas rare hypersthene has been observed as discrete microphenocrysts in type 2 tan and crystal poor pumice these observations indicate conditions at least near if not at vapor saturation in type 1 magma and undersaturation in type 2 magma prior to eruption vapor saturated type 1 dacitic magma probably caused the explosive june 14 15 1991 eruption the highly oxidized condition and magmatic water saturation are similar to those of intrusions related to porphyry copper deposits

implications of late quaternary mammalian fauna from lukenya hill south central kenya for paleoenvironmental change and faunal extinctions abstract late quaternary paleoenvironmental data for east africa are derived primarily from montane sources and thus we know little about the changing composition of east african savannas four archaeological sites at lukenya hill in the savanna of the athi kapiti plains of kenya that date to the last 40 000 yr preserve a large mammalian fauna the prehistoric hunters concentrated on migratory ungulates and virtually ignored the resident inselberg ungulates throughout the occupation faunas of the last glacial maximum are dominated by an extinct small alcelaphine antelope arid adapted ungulates are present that are regionally absent historically and pelorovis is present as well the small alcelaphine and arid adapted ungulates are absent in the holocene deposits this suggests that there was an expansion of dry savannas during the last glacial maximum the last glacial maximum aridity combined with a lack of pastoral set fires would have resulted in a vegetative mosaic distinct from the present dry woody growth and dry and or tall grass all of which are poor forage for ungulates would have been common where seasonally moist short grasslands are presently extant these conditions favored the large bodied highly hypsodont species in africa that became extinct with the onset of wet conditions during the early holocene

specific surface area of a crushed welded tuff before and after aqueous dissolution specific surface areas were measured for several reference minerals anorthoclase labradorite and augite welded tuff and stream sediments from snowshoe mountain near creede colorado crushed and sieved tuff had an unexpectedly small variation in specific surface area over a range of size fractions replicate surface area measurements of the largest and smallest tuff particle size fractions examined 1 0 3mm and 0 212mm were 2 3 0 2m2 g for each size fraction reference minerals prepared in the same way as the tuff had smaller specific surface areas than that of the tuff of the same size fraction higher than expected tuff specific surface areas appear to be due to porous matrix tuff reacted in solutions with ph values from 2 to 6 had little change in specific surface area in comparison with unreacted tuff tuff reacted with solutions having high acid concentrations 0 1 m hydrochloric acid or sulfuric hydrofluoric acid exhibited a marked increase in specific surface area compared to unreacted tuff

climatic change recorded in the sediments of the chew bahir basin southern ethiopia during the last 45 000 years abstract east african paleoenvironments are highly variable marked by extreme fluctuations in moisture availability which has far reaching implications for the origin evolution and dispersal of homo sapiens in and beyond the region this paper presents results from a pilot core from the chew bahir basin in southern ethiopia that records the climatic history of the past 45 ka with emphasis on the african humid period ahp 15 5 ka calbp geochemical physical and biological indicators show that chew bahir responded to climatic fluctuations on millennial to centennial timescales and to the precessional cycle since the last glacial maximum potassium content of the sediment appears to be a reliable proxy for aridity showing that chew bahir reacted to the insolation controlled humidity increase of the ahp with a remarkably abrupt onset and a gradual termination framing a sharply defined arid phase 12 8 11 6 ka calbp corresponding to the younger dryas chronozone the chew bahir record correlates well with low and high latitude paleoclimate records demonstrating that the site responded to regional and global climate changes

petrologic indicators of preeruption magma dynamics new phase equilibria experiments on dacite from the june 1991 eruption of mount pinatubo philippines allow calibration of a geobarometer for matrix glass at pressures of 160 220 mpa comparison with natural glass compositions suggests equilibration at lower water fugacity mathit f h 2 o than inferred from al in hornblende geobarometry the disparity between geobarometric techniques may be explained by a decrease in mathit f h 2 o and consequent crystallization several weeks before eruption a time period too short for hornblende reequilibration reduced mathit f h 2 o could have resulted from processes associated with basalt magma injection including 1 late stage magma ascent under h 2 o saturated conditions or 2 influx of other volatiles co 2 so 2 into the dacite magma system

paleoenvironmental and biogeographic implications of terminal pleistocene large mammals from the ziway shala basin main ethiopian rift ethiopia abstract the terminal pleistocene is a period of profound climatic and environmental change in africa with paleoenvironmental records documenting tremendous regional variation environmental records needed to understand these dynamics are currently lacking from the horn of africa here we provide a new paleoenvironmental record based of new faunal data from archeological sites along the bulbula river ziway shala basin main ethiopian rift in ethiopia research conducted in this area has uncovered archeological assemblages that document various phases of the late pleistocene and early holocene two sites dated to the terminal pleistocene are considered here b1s1 and b1s4 the former includes a very large assemblage of more than 20 000 skeletal remains both faunal assemblages are dominated by bovids of the tribes alcelaphini and antilopini and include damaliscus hypsodon a small extinct alcelaphine the faunal remains from these sites like those from the contemporary site of lukenya hill in south central kenya are indicative of widespread arid grasslands several ungulates are found outside of their historic ranges including grant s gazelle nanger granti and dorcas gazelle gazella dorcas their absence from the region today likely reflects a combination of paleoenvironmental change since the end of the pleistocene coupled with more recent human impacts the combination of archeological and faunal data suggests that b1s1 could have been a residential site with seasonal occupation to acquire raw material from local obsidian sources and to hunt bovids in the arid plain including species that ultimately disappear or shift their range at the onset of the holocene with the return of warmer and moister conditions

investigations of subsurface flow constructed wetlands and associated geomaterial resources in the akumal and reforma regions quintana roo mexico subsurface flow constructed wetlands in the village of akumal quintana roo mexico were surveyed to determine the general status of the wetland systems and provide baseline information for long term monitoring and further study twenty subsurface flow wetlands were surveyed and common problems observed in the systems were overloading poor plant cover odor and no secondary containment bulk mineral composition of aggregate from two subsurface flow constructed wetlands was determined to consist solely of calcite using bulk powder x ray diffraction some soil structure is developed in the aggregate and aggregate levels in wetlands drop at an estimated rate between 3 and 10 cm year for overloaded wetlands owing to dissolution mineral composition from fresh aggregate samples commonly is a mixture of calcite and aragonite trace amounts of pb zn co and cr were observed in fresh aggregate coefficients of permeability k varied from 0 006 to 0 027 cm s with an average values being 0 016 cm s grain size analysis of fresh aggregate samples indicates there are unimodal and multimodal size distributions in the samples with modes in the coarse and fine sand being common investigations of other geologic media from the reforma region indicate that a dolomite with minor amounts of fe oxide and palygorskite is abundant and may be a better aggregate source that the current materials used a ca montmorillonite bed was identified in the reforma region as well and this unit is suitable to serve as a clay liner to prevent leaks for new and existing wetland systems these newly discovered geologic resources should aid in the improvement of subsurface flow constructed wetlands in the region although problems do exist in these wetlands with respect to design these systems represent a successful implementation of constructed wetlands at a community level in developing regions

constraints on dacite magma degassing and regime of the june 15 1991 climactic eruption of mount pinatubo philippines new data on melt and crystal inclusions in quartz abstract to better constrain processes that operated prior to and during the dacite magma eruption of mount pinatubo on june 15 1991 primary inclusions found in quartz phenocrysts of white dacite pumice have been studied by combining micro analytical techniques with homogenisation experiments inclusion morphology and composition before and after homogenisation experiments performed in gas pressure vessels at 760 780 c and 185 200 mpa distinguish two stages of magma degassing during the first stage which occurred prior to eruption the host quartz trapped a fluid saturated rhyolitic melt with an average of 78 6 0 7 wt sio2 12 4 0 5 wt al2o3 and 7 0 3 wt h2o and a water rich fluid microanalyses on the quartz hosted multiphase and crystal inclusions indicate that the dacite magma crystallised at 750 5 c an oxygen fugacity of 1 5 1 7 log fo2 units above the nno nickel nickel oxide buffer and a total pressure of 190 50 mpa corresponding to depths from 4 to 7 km in the second stage which corresponds to the magma eruption and decompression to 12 23 mpa volatile exsolution provoked decrepitation of unstable melt inclusions and the loss of initially high h2o contents from all 80 m glassy and hourglass inclusions to 1 7 0 3 wt h2o the exsolution of 5 wt h2o from the dacite magma might have occurred at depths of 300 700 m due to vent blockage caused by the sudden collapse of the conduit walls these conditions of the climactic eruption in combination with an extreme mass eruption rate reaching 2 109 kg s 1 could create oscillations between convecting and collapsing columns which explains the alternation of pyroclastic flow deposits with plinian pumice fall deposits reported in previous studies

human occupation of northwest africa a review of middle palaeolithic to epipalaeolithic sites in morocco abstract this paper provides a summary of all available numerical ages from contexts of the moroccan middle palaeolithic to epipalaeolithic and reviews some of the most important sites particular attention is paid to the so called aterian albeit those so labeled assemblages fail to show any geographical and chronological pattern for this reason this phenomenon should not be considered a distinct culture or techno complex and is referred to hereinafter as middle palaeolithic of aterian type whereas anatomical modern humans amh are present in northwest africa from about 160 ka onwards according to current research some middle palaeolithic inventories are more than 200 ka this confirms that for this period it is impossible to link human forms with artifact material perforated shell beads with traces of ochre documented from 80 ka onwards certainly suggest changes in human behavior the transition from middle to upper palaeolithic here termed early upper palaeolithic at between 30 and 20 ka remains the most enigmatic era however the still scarce data from this period requires careful and fundamental revision in the frame of any future research by integrating environmental data in reconstruction of population dynamics clear correlations become obvious high resolution data are lacking before 20 ka and at some sites this period is characterized by the occurrence of sterile layers between middle palaeolithic deposits possibly indicative of shifts in human population after heinrich event 1 there is an enormous increase of data due to the prominent late iberomaurusian deposits that contrast strongly from the foregoing accumulations in terms of sedimentological features fauna and artifact composition the younger dryas shows a remarkable decline of data marking the end of the palaeolithic environmental improvements in the holocene are associated with an extensive epipalaeolithic occupation

magma ascent rates volcanoes can erupt explosively generating high columns of ash and occasional pyroclastic flows or they can erupt slowly forming lava flows and domes this variation reflects significant differences in mass eruption rate e g wilson et al 1980 mass eruption rates in turn are controlled by the rates of magma ascent through the volcanic conduit and the conduit size the magma ascent rate itself is a function of the pressure in the magma storage region the physical properties of the magma such as its density viscosity and crystallinity and the resistance to flow in the conduit that connects the magma storage zone to the surface papale and dobran 1994 mastin and ghiorso 2001 pinkerton et al 2002 sparks et al 2006 a number of important characteristics of volcanic eruptions are affected or controlled by the rate at which magma ascends from depth for example the bubble content i e vesicularity and the degree of crystallization that develop in the melt phase can be significantly different in rapidly vs slowly ascended magma in fact the inability of rapidly ascending magma to effectively lose exsolved gas may be one factor causing an eruption to change from effusive to explosive behavior as recently documented for the soufriere hills eruption on montserrat in the west indies sparks et al 1998 therefore in order to better understand the processes involved and the changes that occur in volcanic eruptions it is important to quantify the rates at which different magmas rise to the surface the fact that the rate of magma ascent controls a number of reactions that occur in volatile rich magmas suggests several ways to study magma ascent rates using reaction data and theoretical flow models one reaction that is a direct result of magma ascent is the exsolution of volatiles from the melt

textures water content and degassing of silicic andesites from recent plinian and dome forming eruptions at mount pelee volcano martinique lesser antilles arc previous petrological and phase equilibrium experimental studies on recent silicic andesites from mount pelee volcano have evidenced comparable pre eruptive conditions for plinian and dome forming pelean herein eruptions implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the mount pelee magmas during eruption to further investigate the degassing conditions of plinian and pelean magmas of mount pelee we study here the most recent mount pelee s products p1 at 650 years b p 1902 and 1929 eruptions which cover a range of plinian and pelean lithologies for bulk rock vesicularities glass water contents glass inclusions in phenocrysts and matrix glasses and microtextures water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally deduced pre eruptive melt water content i e 5 3 6 3 wt whereas they are much lower in the dominant pelean lithologies crystalline poorly vesicular lithics and dome samples this indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre eruptive water contents the water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation determination of the water contents of both the pre eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn eruptive degassing we find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones typically 90 100 and 65 70 of the initial water content respectively the vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices consistent with a closed system near equilibrium degassing up to fragmentation by contrast the low residual water contents low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open system degassing microtextural features including silica bearing and silica free voids in the pelean lithologies may represent a two stage vesiculation