calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

amphibole as an archivist of magmatic crystallization conditions problems potential and implications for inferring magma storage prior to the paroxysmal 2010 eruption of mount merapi indonesia amphibole is widely employed to calculate crystallization temperature and pressure although its potential as a geobarometer has always been debated recently ridolfi et al contrib mineral petrol 160 45 66 2010 and ridolfi and renzulli contrib mineral petrol 163 877 895 2012 have presented calibrations for calculating temperature pressure fo2 melt h2o and melt major and minor oxide composition from amphibole with a large compositional range using their calibrations we have i calculated crystallization conditions for amphibole from eleven published experimental studies to examine the problems and the potential of the new calibrations and ii calculated crystallization conditions for amphibole from basaltic andesitic pyroclasts erupted during the paroxysmal 2010 eruption of mount merapi in java indonesia to infer pre eruptive conditions our comparison of experimental and calculated values shows that calculated crystallization temperatures are reasonable estimates calculated fo2 and melt sio2 content yields potentially useful estimates at moderately reduced to moderately oxidized conditions and intermediate to felsic melt compositions however calculated crystallization pressure and melt h2o content are untenable estimates that largely reflect compositional variation in the crystallizing magmas and crystallization temperature and not the calculated parameters amphibole from merapi s pyroclasts yields calculated conditions of 200 800 mpa 900 1 050 c nno 0 3 nno 1 1 3 7 7 2 wt melt h2o and 58 71 wt melt sio2 we interpret the variations in calculated temperature fo2 and melt sio2 content as reasonable estimates but conclude that the large calculated pressure variation for amphibole from merapi and many other arc volcanoes is evidence for thorough mixing of mafic to felsic magmas and not necessarily evidence for crystallization over a large depth range in contrast bimodal pressure estimates obtained for other arc magmas reflect amphibole crystallization from mafic and more evolved magmas respectively and should not necessarily be taken as evidence for crystallization in two reservoirs at variable depth

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

a thermodynamic model for the plagioclase liquid hygrometer thermometer a new thermodynamic model for the plagioclase liquid exchange reaction between the albite naalsi 3 o 8 and anorthite caal 2 si 2 o 8 components is presented which can be used as a plagioclaseliquid hygrometer or thermometer the model incorporates calorimetric and volumetric data for the pure liquid and crystalline components which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition this allows a more accurate assessment of the effect of melt composition including dissolved water concentration on the exchange reaction from plagioclase liquid equilibrium experiments activity composition relations for the plagioclase solid solution are taken from holland and powell 1992 the new hygrometer is calibrated on 71 plagioclase liquid experiments of which 45 are hydrous and 26 are anhydrous three filters were applied to the phase equilibrium data 1 crystallanities 30 2 pure h 2 o fluidsaturated and 3 compositional totals including h 2 o component of 97 101 for hydrous quenched glasses the final data set spans a wide range of liquid compositions 46 74 wt sio 2 plagioclase compositions an 93 an 37 temperatures 825 1230 c pressures 0 300 mpa and dissolved melt water concentrations 0 7 wt h 2 o the standard error of estimate see for the model is 0 32 wt h 2 o and all liquid compositions are fitted equally well when the model is used as a thermometer all measured temperatures are recovered equally well within 14 c on average the model is only recommended for applications that fall within the compositional bounds of the calibration data set i e metaluminous basalts through rhyolites in equilibrium with an 95 an 35 it is not yet calibrated for rhyolites crystallizing plagioclase more sodic than an 30 owing to an absence of phase equilibrium experiments on rhyolites that pass the required filters the new plagioclase liquid hygrometer thermometer is available as a visual basic program that runs on excel 2004

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

magmatic conditions and magma ascent as indicated by hornblende phase equilibria and reactions in the 1995 2002 soufriere hills magma a study of amphiboles and associated minerals in samples of soufri ere hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts a separate set of small pargasitic crystals in the groundmass and two types of reaction rims on the phenocrysts the composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma as many as seven zones 5200 mm wide in the hornblende phenocrysts begin with a sharp increase in mg and si and then change gradually to a more feand al rich hornblende a transition that is consistent with a temperature rise analyses of the hornblende phenocrysts and associated fe ti oxides verify previous conclusions that the pre eruption magma was at 130mpa and 830 10 c but was variably heated before eruption the heating occurred within 30 days of eruption for all magmas erupted based on the width of ti rich rims on titanomagnetite phenocrysts experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855 c at a ph2o of 130mpa and would be even more al rich if crystallized at higher pressure pargasite is not stable in the andesite and its presence along with high an plagioclase microphenocrysts requires mafic magma mingling and hybridization with pre existing andesite experimental melts of the andesite at 130mpa and 830 and 860 c compare well with melt inclusions in quartz and plagioclase respectively reaction rims on a few hornblende crystals in each andesite sample are rich in high ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende decompression induced breakdown rims occur in some samples and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830 860 c the average ascent rate of magma during much of the 1995 2002 eruption has been40 02 m s the rate that allows hornblende to erupt free of decompression induced reaction rims

environmental changes and the rise and fall of civilizations in the northern horn of africa an approach combining δd analyses of land plant derived fatty acids with multiple proxies in soil the domains of the ancient polities d mt and aksum in the horn of africa s highlands are a superior natural system for evaluating roles of environmental change on the rise and fall of civilizations to compare environmental changes of the times of the two polities we analyzed stable hydrogen isotopic ratios d of land plant derived fatty acids n c26 30 and other proxies from soil sequences spanning the holocene from the region three results suggest that trends in d values unambiguously reflect changes in rainfall first increases in d coincide with dry periods inferred from studies of eastern african lakes second changes in d values were parallel among sections during overlapping time intervals third consideration of vegetation history did not alter directions of trends in d values over time by unambiguously recording precipitation the d values also enhanced interpretations of proxies that are affected by both climate and land clearing both d mt ca 2750 2350 cal y bp and the aksumite ca 2100 1250 cal y bp rose during wetter intervals of the drier part of the holocene after ca 6000 cal y bp analyses of charred matter indicated that fire had been a common agent of land clearing in all sites the influence of climate on fire varied however prior to the emergence of d mt d values were correlated with c4 c3 plant ratios estimated from 13c values there are no c4 trees and precipitation may have been the main influence on canopy openness after ca 4300 cal y bp there was no significant relationship between d and c4 c3 plant ratios suggesting that factors such as fire influenced canopy openness regardless of climate furthermore the impact of land clearance differed between sites and between d mt and the aksumite s times in one site the interval from 3550 cal y bp to the decline of d mt had several anomalies that suggested dramatic increases in thermal severity of fire and human impact among these were a large contribution of charred matter to a high total organic carbon that low hydrogen and oxygen indices suggest was severely altered by other factors than humification these results support hypotheses about the rise of civilizations being favored by specific climatic conditions but suggest that patterns of land clearing differed during the declines of d mt and the aksumite

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

geochronologic and stratigraphic constraints on canyon incision and miocene uplift of the central andes in peru the deepest valleys of the andes have been cut in southern peru by the rios cotahuasi ocona and colca majes these canyons are late miocene landforms based on a new ignimbrite stratigraphy supported by 42 new 40ar 39ar age determinations obtained on plateau forming and valley filling ignimbrites between 19 and 13 ma a gently sloping surface bevelling the clastic wedge southeast of the developing mountain front was mantled by widespread ignimbrites after 13 ma this paleosurface was tilted up from 2 2 km a s l at the mountain front to 4 3 km a s l at the base of the pliocene and pleistocene volcanoes that crown the southwestern edge of the altiplano the canyons incised this topography after 9 ma while the dated base of younger ignimbrite valley fills suggests that these canyons had been cut down to near their present depths as early as 3 8 ma by 1 4 ma however the canyons had been almost completely refilled by 1 3 km thick unwelded pyroclastic deposits which were subsequently eroded valley incision since 9 ma at an average rate of 0 2 mm yr 1 is the response to topographic uplift after 13 ma combined with increasing runoff due to a wetter climate recorded after 7 ma although long term aridity generated an imbalance between high long term uplift rates and low plateau denudation rates the combination of aridity and volcanism still promoted canyon incision because episodic volcanic fills maintained a cycle of catastrophic debris avalanches and subsequent dam breakouts

pre eruptive magmatic processes associated with the historical 218 14 abp explosive eruption of tutupaca volcano southern peru magma recharge into a differentiated reservoir is one of the main triggering mechanisms for explosive eruptions here we describe the petrology of the eruptive products of the last explosive eruption of tutupaca volcano southern peru in order to constrain the pre eruptive physical conditions p t xh2o of the tutupaca dacitic reservoir we demonstrate that prior to the paroxysm magma in the tutupaca dacitic reservoir was at low temperature and high viscosity 735 23 c with a mineral assemblage of plagioclase low al amphibole biotite titanite and fe ti oxides located at 8 8 1 6 km depth 233 43 mpa the phenocrysts of the tutupaca dacites show frequent disequilibrium textures such as reverse zonation resorption zones and overgrowth rims these disequilibrium textures suggest a heating process induced by the recharge of a hotter magma into the dacitic reservoir as a result high al amphibole and relatively high ca plagioclase phenocryst rims and microlites were formed and record high temperatures from just before the eruption 840 45 c based on these data we propose that the recent eruption of tutupaca was triggered by the recharge of a hotter magma into a highly crystallized dacitic magma reservoir as a result the resident dacitic magma was reheated and remobilized by a self mixing process these magmatic processes induced an enhanced phase of dome growth that provoked destabilization of the ne flank producing a debris avalanche and its accompanying pyroclastic density currents

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

amphibole perspective to unravel pre eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes a case study from ciomadul volcano se carpathians ciomadul is the youngest volcano in the carpathian pannonian region produced crystal rich high k dacites that contain abundant amphibole phenocrysts the amphiboles in the studied dacites are characterized by large variety of zoning patterns textures and a wide range of compositions e g 6 4 15 wt al2o3 79 821 ppm sr often in thin section scale and even in single crystals two amphibole populations were observed in the dacite low al hornblendes represent a cold 900 c mafic magma amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage 8 12 km this was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple zoned and composite amphiboles this includes reheating by 200 c and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population including pargasites breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes suggesting that pre eruptive heating and mixing could take place within days or weeks before the eruption the decompression rim of pargasites suggests around 12 days of magma ascent in the conduit several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the ciomadul but in our dacite the two amphibole population can be found even in a single crystal composite amphiboles our study indicates that high al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone thus they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes the simple zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the al tschermak substitution can be recognized suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation

zircon u pb ages of super units in the coastal batholith peru implications for magmatic and tectonic processes zircon u pb ages on 50 plutonic samples from super units in the lima arequipa and toquepala segments of the peruvian coastal batholith range from 188 to 37 m y revealing previously unrecognized jurassic elements the jurassic plutons represent a distinct time defined continental arc separated from the cretaceous arc by a quiescent period of 50 m y only 4 of the 50 samples have discordant zircons and these are all from the arequipa and toquepala segments this regionally limited discordance interpreted to be due to inherited radiogenic pb from the precam brian basement suggests that there are fundamental differences in the nature of the crust beneath the lima and the arequipa toquepala segments on a grand scale magmatism in the coastal batholith generally evolved through time from mafic to siliceous melts but zircon u pb ages show that the occurrence of small bodies of old siliceous and young mafic rocks was also important close age association between all rock types raises the possibility that mantle derived mafic magmas provided the heat that produced siliceous magmas in either the underplated wedge of the subduction complex or the lower crust throughout the period of batholith emplacement in general age results confirm that super units have short time spans for the santa rosa paccho and linga super units however mapping generalizations were made in the absence of thorough isotopic age and trace element data remapping of these super units with supporting geochemical and geochronological studies will be required before the emplacement history of the batholith is fully understood dated plutons in the paccho super unit yield ages between 64 0 and 39 0 m y they are thus considerably younger than has been previously recognized and cannot belong to one super unit their young ages also verify the eastward migration of the locus of magmatism through time a migration rate of 1 3 mm yr computed for the rocks along latitude 11 08 1 s on the basis of the 105 m y old patap super unit and a 61 m y old pluton in the paccho super unit suggests that the geometry of the andean subduction zone in peru was very stable during batholith emplacement

the historical 218 14 abp explosive eruption of tutupaca volcano southern peru the little known tutupaca volcano 17 01 s 70 21 w located at the southern end of the peruvian arc is a dacitic dome complex that experienced a large explosive eruption during historical times based on historic chronicles and our radiometric data this eruption occurred 218 14 abp probably between 1787 and 1802 ad this eruption was characterised by a large sector collapse that triggered a small debris avalanche 1 km3 and an associated pyroclastic eruption whose bulk volume was 6 5 7 5 107 m3 both units were emplaced synchronously and spread onto the plain situated to the northeast of tutupaca volcano the spatial and temporal relationship between the debris avalanche and the pyroclastic density current deposits coupled with the petrological similarity between the juvenile fragments in the debris avalanche the pyroclastic density current deposits and the pre avalanche domes indicates that juvenile magma was involved in the sector collapse large amounts of hydrothermally altered material are also found in the avalanche deposit thus the ascent of a dacitic magma coupled with the fact that the tutupaca dome complex was constructed on top of an older altered volcanic sequence probably induced the destabilisation of the hydrothermally active edifice producing the debris avalanche and its related pyroclastic density currents this eruption probably represents the youngest debris avalanche in the andes and was accompanied by one of the larger explosive events to have occurred in southern peru during historical times

pre eruptive reheating during magma mixing at quizapu volcano and the implications for the explosiveness of silicic arc volcanoes effusive explosive transitions are observed in volcanoes erupting water rich magmas that lack mineralogical evidence of degassing suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts here we show that pre eruptive heating of water rich silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive explosive transition volcan quizapu chile emitted a 5 km3 mingled dacite andesite lava flow in a d 1846 1847 that was 130 c hotter than the 5 km3 of the same dacite that lead to a plinian eruption in a d 1932 we explain the suppression of explosive fragmentation in the 1846 1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity which facilitated bubble nucleation growth and coalescence while also inhibiting brittle fragmentation thus recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold volatile rich silicic magmas

alkali diffusion in plagioclase feldspar tracer diffusion kinetics at 1 atm pressure under dry conditions using 6li 41k and 87rb in four plagioclase feldspars have been determined diffusion profiles were measured using an ion microprobe sims the arrhenius parameters and 2 uncertainties for li k and rb respectively in albite are pre exponential factor log do 3 8 1 0 3 4 1 4 and 5 8 3 8 in m2 s and activation energies 146 14 296 30 and 283 85 kj mol the diffusion rate of k in albite at 700 c and 100 mpa water pressure equals that at 1 atm dry suggesting that diffusion of k is not dependent on water fugacity between 0 004 and 100 mpa the k diffusion rate depends on plagioclase anorthite content and is approximately 20 times slower in labradorite than albite li diffusion rates in albite and anorthite are identical a linear relation between log d for the alkalis measured and their ionic radius squared permits an estimate of cs diffusion kinetics diffusion rates are isotropic or nearly so in albite

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

microlite transfer by disaggregation of mafic inclusions following magma mixing at soufriere hills volcano montserrat the soufriere hills volcano on montserrat has for the past 12 years been erupting andesite with basaltic to basaltic andesite inclusions the andesite contains a wide variety of phenocryst textures and strongly zoned microlites analysis of minor elements in both phenocrysts and microlites allows us to put detailed constraints on their origins compositions of clinopyroxene from overgrowth rims on quartz and orthopyroxene and coarse grained breakdown rims on hornblende are identical to those from the mafic inclusions indicating that these rims form during interaction with mafic magma in contrast resorbed quartz and reversely zoned orthopyroxenes form during heating microlites of plagioclase and orthopyroxene are chemically distinct from the phenocrysts being enriched in fe and mg and al and ca respectively however microlites of plagioclase orthopyroxene and clinopyroxene are indistinguishable from the compositions of these phases in the mafic inclusions we infer that the inclusions disaggregated under conditions of high shear stress during ascent in the conduit transferring mafic material into the andesite groundmass the mafic component of the system is therefore greater than previously thought the presence of mafic derived microlites in the andesite groundmass also means that care must be taken when using this as a starting material for phase equilibrium experiments

deposits of the 30 march 1956 directed blast at bezymianny volcano kamchatka russia on 30 march 1956 a catastrophic directed blast took place at bezymianny volcano it was caused by the failure of 0 5 km3 portion of the volcanic edifice the blast was generated by decompression of intra crater dome and cryptodome that had formed during the preclimactic stage of the eruption a violent pyroclastic surge formed as a result of the blast and spread in an easterly direction effecting an area of 500 km2 on the lower flank of the volcano the thickness of the deposits although variable decreases with distance from the volcano from 2 5 m to 4 cm the volume of the deposit is calculated to be 0 2 0 4 km3 on average the deposits are 84 juvenile material andesite of which 55 is dense andesite and 29 vesicular andesite on a plot of sorting vs median diameter inman coefficients the deposits occupy the area between the fall and flow fields in the proximal zone less than 19 km from the volcano three layers can be distinguished in the deposits the lower one layer a is distributed all over the proximal area is very poorly sorted enriched in fragments of dense juvenile andesite and contains an admixture of soil and uncharred plant remains the middle layer layer b is distributed in patches tens to hundreds of metres across on the surface of layer a layer b is relatively well sorted as a result of a very low content of fine fractions and it contains rare charred plant remains the uppermost layer layer c forms still smaller patches on the surface of layer b layer c is characterized by intermediate sorting is enriched in vesicular juvenile andesitic fragments and contains a high percentage of the fine fraction and very rare plant remains which are thoroughly charred maximum clast size decreases from layer a to layer c the absence of internal cross bedding is a characteristic of all three layers in the distal zone more than 19 km from the volcano stratigraphy changes abruptly deposit here consists of one layer 26 to 4 cm in thickness is composed of wavy laminated sand with a touch of gravel is well sorted and contains uncharred plant remains the bezymianny blast deposits are not analogous with known types of pyroclastic surges with the exception of the directed blast deposits of the mount st helens eruption of 18 may 1980 the peculiarities of deposits from these two eruptions allow them to be separated into a special type blast surge this type of surge is formed when failure of volcanic edifice relieves the pressure from an inter crater dome and or cryptodome a model is proposed to explain the peculiarities of the formation transportation and emplacement of the bezymianny blast surge deposits

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

minor and trace element zoning in plagioclase implications for magma chamber processes at parinacota volcano northern chile textural and compositional zoning in plagioclase phenocrysts in a sample from parinacota volcano chile was investigated using backscattered electron images and electron microprobe analysis of major and trace elements large 2 mm oscillatory zoned crystals type i with resorption surfaces of moderate an discontinuities 10 an and decreasing trace element contents sr mg ti towards the rim reflect melt differentiation and turbulent convection in the main magma body early recharge with a low sr mafic magma is seen in the core small scale sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years smaller crystals type ii with low trace element an ratio reflect the influence of an h2o rich melt probably from a differentiated boundary layer closed system in situ crystallisation mafic magma recharge and the role of a water rich differentiated boundary layer can be distinguished from the an trace element relationships crystals apparently move relatively freely between different parts and regimes in the magma chamber evidence for convective crystal dispersion high sr type ii crystals indicate an earlier input of sr rich mafic magma recharge of two distinct mafic magma types is thus identified high sr and low sr which must have been present at increasing recharge rates with time in the plumbing system throughout the volcano s history