the december 2002 july 2003 effusive event at stromboli volcano italy insights into the shallow plumbing system by petrochemical studies abstract on 28 december 2002 the persistent strombolian activity at stromboli was interrupted by the sudden onset of lava emission onto the sciara del fuoco slope a horseshoe shaped depression on the nw flank of the volcano the effusive episode went on until 22 july 2003 and produced a cumulative volume of lavas of 11 106 m3 ranking the event as the largest occurred in the past 30 years the eruptive vents were mainly located in the ne sector of the sciara del fuoco depression at an elevation of 550 600 m a s l on 30 december the eastern portion of the sciara del fuoco collapsed producing a tsunamigenic landslide on 5 april 2003 a paroxysmal eruption occurred at the summit craters during which crystal poor pumiceous products were emitted the paroxysm did not interrupt the lava emission the strombolian activity at the summit craters gradually resumed starting from march 2003 and fully recovered by the end of july periodic sampling of the lava at the active vents was carried out during the entire effusive event all the analysed samples are shoshonitic basalts sio2 48 5 50 4 wt k2o 2 1 2 4 wt in the range of composition observed in the products erupted during the past 20 years they bear about 50 vol zoned crystals of plagioclase an90 60 diopside augite and olivine fo70 73 in a compositionally homogeneous shoshonitic groundmass bulk rock major and trace element contents measured with different methodologies at different laboratories show only minor variations sr and nd isotope ratios are close to those of the crystal rich scoriae erupted in the previous years despite of small compositional variations related to the emptying of the zoned topmost portion of the conduit the data show that the switch from strombolian explosive to effusive activity is not associated with changes of the textures and composition of the erupted products slight but somehow systematic variation of trace elements and isotope ratios between products erupted before and after the 5 april eruption are likely accounted by limited mixing between the fresh volatile rich crystal poor magma erupted as pumice during the paroxysm and the volatile poor crystal rich magma feeding the lava flow the uniform composition of the erupted lava indicates the presence of a large volume of well mixed crystal rich homogeneous magma residing in the shallow plumbing system of the volcano two possible trigger mechanism of the effusive event are here proposed i a discrete input of fresh magma into the lower part of the shallow magmatic system occurred some months before the eruption and was followed by crystallisation degassing and mixing with the crystal rich shallow magma which re homogenised the system these processes eventually led to the rise of the magma at a higher level and failure of the conduit walls ii the onset of the effusion may represent the consequence of a gradual rise of the magma level in the conduits occurred in the past two decades the process of progressive refilling initiated after the 1985 effusive eruption ultimately culminated on 28 december 2002 with the failure of the conduit wall and the opening of vents 100 150 m below the summit craters

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

high resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images new constraints on oscillatory zoning oscillatory zoning in plagioclase is investigated at small scale 10 m using accumulated backscattered electron bse images as high resolution imaging method combined with electron microprobe quantitative analysis gray value profiles across these images can be calibrated for an content with a resolution of 0 5 mol an applied to oscillatory zoned crystals this new application of bse imaging allows better characterization of zoning patterns along a profile and quantification of wavelength amplitude and shape of the oscillations we also obtain high resolution information on the morphology of growth zones boundaries this approach allows us to better classify the different types of oscillations and concentric zoning dissolution is more frequent than usually recognized major resorption surfaces crosscut several growth zones irregular 5 10 m saw tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small scale oscillations 1 3 m with straight boundaries this suggests at least two mechanisms for the formation of these zoning patterns faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics

compositional and dynamic controls on mafic silicic magma interactions at continental arc volcanoes evidence from cordon el guadal tatara san pedro complex chile

influence of lithospheric and mantle stratification on co and post seismic deformation due to finite faults summary coseismic and post seismic deformation in the fully relaxed state following large earthquakes is modelled for finite faults by means of multilayered viscoelastic self gravitating hydrostatically pre stressed spherical earth models based on the preliminary reference earth model prem with a maxwell rheology for both finite dip slip and strike slip sources we find that the elastic layering of the lithosphere has a major impact on the modelling of co and post seismic deformation for large and deep earthquakes embedded at the base of the lithosphere in the proximity of the lithosphere upper mantle interface in order to study the effects of mantle relaxation coseismic and post seismic deformation can result in several tens of centimetres of displacement with a deformation pattern strongly dependent on the distance from the source and the azimuth from the strike of the fault strike slip faulting is found to be more sensitive to mantle rheology than dip slip faulting deviations between earth models with a homogeneous crust and lithosphere and those based on prem are of the same order as the coseismic and post seismic signals ranging from a few centimetres to several tens of centimetres for large earthquakes with seismic moments of 1022 n m and epicentral depths of 75 100 km at the base of the lithosphere this finding lends further support to the conclusion of a preliminary study based on dip slip point sources that observations based on vlbi gps and sar techniques which can easily resolve crustal deformation of the order of the deviations among the various lithospheric models can be badly misinterpreted if the results of surveying in seismogenic regions are compared with earth models that have a limited number of layers in general and a homogeneous crust in particular

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

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

observations of plagioclase zoning using interference imaging abstract interference imaging techniques laser interference microscopy in transmitted light and nomarski differential interference contrast in reflected light have been used to study different types of zoning in magmatic plagioclase crystals samples studied come from a wide range of rock types and geological environments analysis of zoning profiles for individual unit zones shows that a saw tooth pattern in which an initially high an decreases as growth proceeds is the most common type of zoning configuration we recognize 2 major types of zoning type 1 small zones usually less than 10 microns thick with compositional amplitudes up to 10 an and type 2 zones up to50 microns thick with large compositional amplitudes of up to 35 an detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave it is discontinuous unit zones are characteristically bounded by discontinuities this common pattern is consistent with alternating periods of growth and periods of dissolution reaction and lack of growth or possibly very slow growth our observations firmly establish that dissolution and magmatic reaction involving both growth and dissolution are common during growth of the magmatic plagioclase which we have studied type 1 zonation appears consistent with perturbations of local conditions in a diffusion controlled boundary layer around the growing crystal type 2 zoning is probably due to large scale conditions faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular convolute zonation can occur the repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self organization

a review and analysis of silicate mineral dissolution experiments in natural silicate melts abstract we present a database and a graphical analysis of published experimental results for dissolution rates of olivine quartz plagioclase clinopyroxene orthopyroxene spinel and garnet in basaltic and andesitic melts covering a range of experimental temperatures 1100 1500 c and pressures 10 5 pa 3 0 gpa the published datasets of donaldson 1985 1990 and brearly and scarfe 1986 are the most complete experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals dissolution rates range from 5 10 10 oxygen equivalent moles o e m cm 2 s 1 for olivine in a basaltic melt to 1 3 10 5 o e m cm 2 s 1 for garnet in a basaltic melt values of ln are arthenian for the experiments examined and activation energies range from 118 to 1800 kj o e m for quartz and clinopyroxene respectively the relationship between calculated a rt for the dissolution reactions where a is the thermodynamic potential affinity and values of is linear for olivine plagioclase and quartz we interpret this as strong evidence in support of using calculated a as a predictor of for at least superliquidus melt conditions

homogenization of potential vorticity in planetary gyres the mean circulation of planetary fluids tends to develop uniform potential vorticity q in regions where closed time mean streamlines or closed isolines of mean potential vorticity exist this state is established in statistically steady flows by geostrophic turbulence or by wave induced potential vorticity flux at the outer edge of the closed contours the expelled gradients of q are concentrated beyond this transition lies motionless fluid or a different flow regime in which the planetary gradient of q may be dominant the homogenized regions occur where direct forcing by external stress or heating within the closed isoline is negligible upon the potential density surface under consideration in the stably stratified ocean such regions are found at depths greater than those of direct wind induced stress or penetrative cooling in channel models of the atmosphere we again find constant q when mesoscale eddies cause the dominant potential vorticity flux in the real atmosphere the results here can apply only where internal heating is negligible the derivations given here build upon the prandtl batchelor theorem which applies to non rotating steady two dimensional flow supporting evidence is found in numerical circulation models and oceanic observations

inter and intracrystalline isotopic disequilibria techniques and applications in the last 20 years we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences from constraining pre eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes these techniques have had significant impacts in a wide variety of scientific fields nowhere has the impact been greater than in identifying the sources processes and timing of processes involved in igneous magmatic systems both technique refinements and the development of new technologies have aided in advancing microsampling applications thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems in this chapter we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems including melts and minerals from the use of petrographic microscopes and the later introduction of the electron microprobe the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks even today these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context for trace elements and isotopes early studies e g cortini and van calsteren 1985 confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost later studies such as geist et al 1988 focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic andesitic and rhyolitic magmas and davidson et al 1990 used

the magma mixing origin of mantled feldspars the key to mantled feldspar genesis is epitaxial nucleation of plagioclase on k feldspar or k feldspar on plagioclase once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures the most common mantling is plagioclase on k feldspar which occurs in both volcanic and plutonic environments in the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non cellular shell of plagioclase in the plutonic environment early stages of plagioclase overgrowth also tend to be dendritic although with coarser grained characteristics dendritic morphology is thus a common denominator in rapakivi genesis since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma such magma mixing yields a heterogeneous system at first one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on k feldspar mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals

enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber inclusions and pumices from the rattlesnake tuff oregon a variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7 ma rattlesnake tuff of eastern oregon the tuff represents 280 km3 of high silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels the mafic inclusions provide a window into the processes acting beneath a large silicic chamber quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high alumina olivine tholeiite haot lavas but continuous chemical and mineralogical trends indicate a genetic relationship between them basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles 10 of mafic recharge which enriched incompatible elements while maintaining a mafic bulk composition the crystal fractionation history is partially preserved in the mineralogy of crystal rich inclusions olivine plagioclase clinopyroxene and the recharge history is supported by the presence of mafic inclusions containing olivines of fo80 small amounts of assimilation 2 of high silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed the composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least evolved high silica rhyolite in support of the mixing model most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage equilibrium dacite phenocrysts are rare dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two the rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both low density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input the basaltic root zone in turn thermally maintains the rhyolitic magma chamber and promotes compositional zonation

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

triggering explosive eruptions the case for silicic magma recharge at huaynaputina southern peru recharge by mafic magma is established as a trigger of major explosive volcanic eruptions however recharge by silicic magma can also induce perched resident magma to erupt the evidence of such recharge may be more cryptic than for mafic recharge and requires detailed petrologic and geochemical study to identify we present a study of plagioclase petrology supported by other data that implicates silicic silicic magma interaction during the eruption and evolution of the a d 1600 eruption of huaynaputina one of the largest historical eruptions recharge by silicic magma is probably more common than recognized and should be considered an important eruption trigger

evidence for step function zoning of magma and eruptive dynamics toquima caldera complex nevada analyses of bulk tuff fiamme groundmasses and biotite of eruptive units from the toquima caldera complex provide insights on the nature of compositional zoning of magma and on the eruptive dynamics of a zoned chamber samples of bulk tuff consist of one to several petrographically and compositionally different types of fiamme the diversity of fiamme is generally least in the first erupted parts of units but increases with stratigraphic height as less silicic fiamme types join the assemblage as a result analyses of bulk tuff although yielding systematic zoning trends represent only a minimum range of magma compositions tapped because of mixing of fiamme groups microprobe analyses of fiamme groundmasses cluster into tight compositional groups corresponding to petrographically distinctive types of fiamme fiamme groups are separated by small but significant compositional gaps 4 sio2 which are not evident in analyses of bulk tuff biotite compositional data mimic that of fiamme and cluster into discrete groups that also correlate with petrographically identifiable types of fiamme the discontinuous compositional zoning of fiamme and biotite suggests that magma was zoned in a stepwise fashion analogous to zoning patterns developed in experiments modelling boundary layer and convective fractionation in high level magma chambers in such experiments a series of compositionally distinct internally convecting layers are produced whose boundaries are characterized by abrupt transitions in composition and density reversals in the upward mafic trend of units correspond to discontinuous horizons of tuffaceous sedimentary rocks these compositional overlaps are interpreted to reflect eruption of residual silicic layers of magma that restratified below the vent during pauses in the eruptive cycle similarly horizons of tuff containing several types of fiamme and multiple compositional populations of biotite are considered to form during periods of accelerated rates of eruption perhaps occasioned by cauldron subsidence and vent widening in which several compositional layers of magma are tapped concurrently careful analysis of juvenile constituents in ash flow tuffs in conjunction with well constrained field relations can provide meaningful information on compositional zoning of magma and dynamics of eruption

mineral textures and zoning as evidence for open system processes investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades e g milch 1905 kuno 1936 eichelberger 1975 sato 1975 anderson 1976 in fact evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by bunsen 1851 in recent years mineral studies are invigorated by the development of high precision high resolution analytical instruments and techniques through which textural information and compositional data are combined e g jerram and davidson 2007 as minerals respond texturally and compositionally to changing magmatic environments they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions cf ginibre et al 2007 on the other hand magmatic liquids melts are snapshots of current magmatic states and provide less direct evidence of the processes responsible for their evolution in addition liquids may crystallize thereby destroying the direct evidence they provide with an appropriate set of observations and measurements we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone this contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks many of which apply equally to plutonic rocks and the interpretations ascribed to these features in terms of open system magmatic processes first i will address the analytical tools that are used to acquire textural and compositional data followed by a brief review of open system processes and a summary of the relevant textures from that point on i will address some mineral specific aspects followed by a discussion

a possible link between gabbros bearing high temperature iddingsite alteration and huge pegmatoid intrusions the society islands french polynesia this paper reports new field observations and new petrological textural and geochemical data on two gabbroic intrusions and one pegmatoid dyke from the french polynesian islands maupiti and bora bora respectively olivine crystals from the faataufi and barque de hiro gabbros maupiti include high temperature iddingsite hti exhibiting three distinct morphological facies chemical and crystallographical data performed by electron microprobe scanning microscope x ray diffractometry and near infrared spectroscopy show that hti results from modifications of olivine structure through intense hydroxylation and fe oxidation the hti free 120 m wide cloche de hiro dyke bora bora exhibits a textural zonation from heterogranular intergranular gabbro peripheral part to typical pegmatoid inner area we propose a textural tetrahedral classificatory diagram which can be used to clearly distinguish the textural characteristics of the pegmatoids from those of the other coarse grained samples through comparison with a lava flow containing vesicle rich segregation sheets we suggest that the cloche de hiro dyke may correspond to a huge segregation structure derived from a maupiti type hti bearing gabbro through a vapor differentiation process both intrusions might be considered as representative of two superposed levels in a vertically arranged magmatic complex in this view the maupiti gabbroic bodies would represent the lower part of the postulated system under high oxygen fugacity olivine crystals would have undergone important modifications of their structure leading to hti the huge bora bora pegmatoid dyke would be the upper part of the complex after expulsion from the hti bearing gabbroic solidification zone by build up of gas pressure vesicle rich residual melt would have gathered into a shallow level and crystallized there as a pegmatoid body

mineral inventory of continuously erupting basaltic andesites at arenal volcano costa rica implications for interpreting monotonous crystal rich mafic arc stratigraphies abstract except for the first 2 years since july 29 1968 arenal volcano has continuously erupted compositionally monotonous and phenocryst rich 35 basaltic andesites composed of plagioclase plag orthopyroxene opx clinopyroxene cpx spinel olivine detailed textural and compositional analyses of phenocrysts mineral inclusions and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments we suggest three distinct crystallization environments crystallized low mg 7 al ti and high 4 wt al2o3 titanomagnetite with considerable al2o3 10 18 wt and possibly olivine but appears to lack plagioclase a fourth crystallization environment is characterized by clinopyroxene e g mg 78 85 cr2o3 0 15 0 7 wt al cr rich spinel olivine fo80 and in some circumstances high an 80 plagioclase this assemblage seems to record mafic inputs into the arenal system and crystallization at high to low pressures single crystals cannot be completely classified as xenocrysts antecrysts cognate crystals or phenocrysts because they often contain different parts each representing a different crystallization environment and thus belong to different categories bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions on the other hand they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources the driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time may also go through compositional modification by fractionation and assimilation of country rocks thus mafic magmas become basaltic andesite through mixing fractionation and assimilation by the time they arrive at subvolcanic depths we infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing subvolcanic magma thus the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite furthermore we propose that this quasi constant supply of small increments of magma is the fundamental cause for small scale decade long continuous volcanic activity that is the current eruption of arenal is flux controlled by inputs of mantle magmas

deciphering magma mixing the application of cluster analysis to the mineral chemistry of crystal populations abstract cluster analysis a classification technique used to group data in many fields is developed here as a tool to study magma mixing and mixed crystal populations in volcanic rocks the method is based on the quantification of the chemical degree of similarity among populations of mineral chemistry data which allows identification of discrete clusters in order to apply the technique for the particular problem of mixed crystal populations the mineral chemistry of a given crystalline phase is represented by a vector with n coordinates in which each coordinate is a real number that represents the amount of a given component in cations per formula unit present in the phase these vectors are in a set which is a subset of rn the real vector space of n dimensions because mineral chemistry data are a particular case of compositional data i e the components sum to a constant value usually 100 or the numbers of cations per formula unit the conventional euclidean distance cannot be used to quantify how similar the data are in order to apply cluster analysis to avoid this predicament aitchison s metric is proposed to measure similarities instead here average linkage a hierarchical clustering technique combined with the aitchison metric and stoichiometrical constraints is applied to mineral chemistry data this approach is evaluated using well characterized lava samples from the vancori period of activity 26 13 8 ky of stromboli volcano italy in which magma mixing has been identified between a basaltic andesite latite hosted in the magma chamber and a less evolved basaltic recharge magma the results are in agreement with previous interpretations of magma mixing which validates the use of the cluster analysis technique in the context of magma mixing relationships and opens the possibility to expand this methodology to other aspects of igneous petrology

sulfur evolution of oxidized arc magmas as recorded in apatite from a porphyry copper batholith uniformly sulfur rich cores abruptly zoned to sulfur poor rims 1 to 0 2 wt so3 in apatite from the yerington batholith nevada indicate that early magma that is crystal poor oxidizing and sulfate rich evolved to sulfate poor magma via crystallization of anhydrite a mineral observed in magmas from pinatubo and el chichon we predict that the characteristic zonation to sulfur poor rims of apatite in the yerington batholith is common in other oxidized hydrous calc alkaline magmas and can be used to track cryptic anhydrite saturation as well as to monitor sulfur evolution sulfate rich arc magmas such as yerington magmas may crystallize to produce hydrothermal fluids rich in chlorine copper and sulfur and porphyry copper ores

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

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