volatile characteristics of peralkaline rhyolites from kenya an ion microprobe infrared spectroscopic and hydrogen isotope study peralkaline rhyolites of the greater olkaria volcanic complex kenya rift valley were derived from separated though closely related magma chambers ion microprobe analyses of glass inclusions in quartz phenocrysts show pre eruptive water contents of up to 3 4 wt contrasting with previous estimates that the magmas were anhydrous the values approach predicted solubility levels corresponding to water saturation at low crustal pressures 1 kbar the glass matrices of the rhyolites have low water contents ranging from 0 07 to 0 46 wt suggesting significant degassing during or prior to eruption infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses the amount of molecular water determined semi quantitatively apparently increases with increased fluorine content and peralkalinity this suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure the mechanism of degassing has been investigated using hydrogen isotopes the range of d values in most rocks can be produced by varied degrees of open system degassing of rhyolite melt initially in equilibrium with water of a fixed or limited d value there is evidence to suggest that closed system degassing may also have been a significant component in some rhyolites the exact mechanisms of degassing remain uncertain particular problems include the relative contribution of open and closed system degassing during eruption and the initial vapour compositions and solubility relationships

experimental and thermodynamic constraints on mineral equilibrium inpantelleritic magmas abstract crystallization experiments on two pantellerites from pantelleria italy provide new evidence for the relationships between mineral phases in pantelleritic rocks as well as the influence of temperature and redox conditions on mineral assemblages experiments were performed at 1 kbar with temperature ranging between 750 900 c and fluid saturation conditions with xh2o h2o h2o co2 between 0 and 1 redox conditions were fixed at or slightly below the fmq buffer results show that at temperature of 900 c pantelleritic magmas are well above the liquidus regardless their water content we also observed a decrease in liquidus temperature 800 c with increasingly reducing conditions mineral assemblages of the natural rocks have been successfully reproduced particularly the relationship between fayalite and aenigmatite which appear to be strongly controlled by melt peralkalinity temperature and redox conditions this is the first time that fayalitic olivine have been synthetized in experimental studies on pantellerites which was found to be stable only for temperatures 750 c while amphibole can be stable at temperatures as high as 800 c at high ff2 experimental results have been compared with the composition of mineral phases as well as with the results obtained from mineral equilibrium geothermometry and oxygen barometry studies on pantelleritic lava and tuffs from pantelleria italy eburru kenya and menengai kenya petrological characteristics appear similar at different locations worldwide with typical assemblages of anorthoclase and sodian clinopyroxene with variable fayalite aenigmatite fe ti oxides and amphibole

phase equilibria of pantelleria trachytes italy constraints on pre eruptive conditions and on the metaluminous to peralkaline transition in silicic magmas the relationships between trachytes and peralkaline rhyolites i e pantellerites and comendites which occur in many continental rift systems oceanic islands and continental intraplate settings is unclear to fill this gap we have performed phase equilibrium experiments on two representative metaluminous trachytes from pantelleria to determine both their pre eruptive equilibration conditions pressure temperature h2o content and redox state and liquid lines of descent experiments were performed in the temperature range 750 950 c pressure 0 5 1 5 kbar and fluid saturation conditions with xh2o h2o h2o co2 ranging between zero and unity redox conditions were fixed below the nickel nickel oxide buffer nno the results show that at 950 c and melt water contents h2omelt close to saturation trachytes are at liquidus conditions at all pressures clinopyroxene is the liquidus phase being followed by iron rich olivine and alkali feldspar comparison of experimental and natural phases abundances and compositions yields the following pre eruptive conditions p 160 5 kbar t 925 25 c h2omelt 261 wt and fo2 between nno 0 5 and nno 2 a decrease in temperature from 950 c to 750 c as well as of h2omelt promotes a massive crystallization of alkali feldspar to over 80 wt iron bearing minerals show gradual iron enrichment when t and fo2 decrease trending towards the compositions of the phenocrysts of natural pantellerites despite the metaluminous character of the bulk rock compositions residual glasses obtained after 80 wt crystallization evolve toward comenditic compositions owing to profuse alkali feldspar crystallization which decreases the al2o3 of the melt leading to a consequent increase in the peralkalinity index pi molar na2o k2 o al2 o3 this is the first experimental demonstration that peralkaline felsic derivatives can be produced by low pressure fractional crystallization of metaluminous mafic magmas our results show that the pantelleritic magmas of basalt trachyte rhyolite igneous suites require at least 95 wt of parental basalt crystallization consistent with trace element evidence redox conditions through their effect on fe ti oxide stabilities control the final iron content of the evolving melt

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

a permian kuroko type hydrothermal system afterthought ingot area shasta county california lateral and vertical sections and geochemical evolution the afterthought and smaller kuroko deposits in the east shasta mining district formed at the culmination of a late permian mafic to felsic volcanic cycle they were succeeded by triassic elastic sediments and reworked tuff in a presumed rifting environment with local evidence of renewed mafic volcanism and large scale slumping the afterthought host horizon is repeated in outcrop by folding fold closures are tight east of ingot where preore volcanic rocks are more altered they are open basins and domes west of ingot where the preore volcanic rocks are less altered igneous pyroxene and titanomagnetite and secondary pumpellyite occur only in preore andesite and basalt more than 2 5 km from known kuroko deposits in outcrops closer to the deposits disseminated pyrite type e is locally abundant in preore dacite the kuroko deposits comprise three ore types a banded fine grained sulfides and gangue b recrystallized coarser grained sulfides and gangue with a consistent paragenetic sequence and c coarse grained pyrite and barite b and c appear to replace type a a further type of mineralization type d consists of fine grained pyrite occurring locally within silicified dacite on the afterthought host horizon the paragenetic sequence of type b implies a sequential introduction of metals in a fluid evolving toward higher h 2 s activity cu was introduced early as chalcopyrite and bornite later fluids altered preexisting cu minerals rather than introducing significant new cu type b fluids probably cooled from 300 degrees c chalcopyrite stage to about 270 degrees c sphalerite barite stage the last type b fluids dissolved much base metal sulfide and deposited calcite quartz and sericite types a b c and e sulfides have a delta 34 s value range of 5 to 12 per mil implying a common origin barites from type a and b ore have delta 34 s values near 30 per mil partially reduced contemporaneous marine sulfate 10 12ppm is a possible sulfur source but a concealed rock sulfur source of high delta 34 s value is more probable type d pyrite with delta 34 s values between 28 and 0 per mil contains sulfur of a different probably biogenic origin hydrothermal carbonates formed from fluids with delta 13 c values from near 0 to 12 per mil consistent with oxidation of organic c in the hydrothermal system hydrothermal and postore carbonates have delta 18 o values of 17 to 19 per mil and probably equilibrated with a single fluid after the deposition of the hosselkus limestone

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

the 1891 submarine eruption offshore pantelleria island sicily channel italy identification of the vent and characterization of products and eruptive style high resolution bathymetry and seafloor sampling have been used to characterize the 1891 submarine eruption of the pantelleria volcanic complex this submarine eruption has been documented mainly by historical reports describing basaltic scoria bombs floating on the sea surface i e lava balloons in this study the 1891 eruptive vent has been identified as a small cone volume of 700 000 m3 rising 90 m from 350 m w d and located within a newly discovered submarine volcanic field covering a wide area offshore from the nw coast of pantelleria recently kelly et al 2012 confirmed this location by a multibeam and rov survey pyroclasts from the 1891 eruption crop out directly on the seafloor and are fresh scoria clasts i e small bombs bomb fragments and lapilli and glass ash sized grains both have been characterized in their morphology textures and geochemistry the distinctive vesicularity and crystallization characteristics displayed by the scoriaceous pyroclasts reflect modes of degassing in both syn and posteruptive regimes these characteristics along with the distribution of deposits suggest for the strongest eruptive phase of the 1891 eruption a style analogous to hawaiian fountaining glass grains from a buoyant plume were dispersed northward from the vent up to distances of 1 5 km redirected by the levantine intermediate water the identification of the 1891 submarine eruptive vent offshore pantelleria as well as the features of erupted pyroclasts improve our knowledge of submarine explosive eruptions that occur at shallow intermediate depths and among these of the rare eruptions producing lava balloons

archaeomagnetic results from southern italy and their bearing on geomagnetic secular variation archaeodirectional results from kilns and other baked structures in southern italy are presented they are generally compatible with the much larger data sets from france and bulgaria in particular a summary of all the results associated with the well known eruption of vesuvius that destroyed pompeii n 9 d 355 i 5 8 95 1 5 provides a reliable archaeomagnetic anchor point supporting the french and bulgarian master curves it is extremely well constrained in time and it comprises independent studies carried out in four different countries furthermore it is derived from a diverse set of features agreement amongst which argues strongly against significant perturbations due to magnetic refraction structural disturbance or depositional shallowing in terms of geomagnetic secular variation we interpret the western european archaeomagnetic data summarized here in terms of an open loop caused by westward drift followed by an inclination low spanning the first few centuries ce representing the signal of a static flux pulse that reaches a maximum magnetic moment of a few percent of the earth s main central dipole 2005 elsevier b v all rights reserved

compositional and thermodynamic variability in a stratified magma chamber evidence from the green tuff ignimbrite pantelleria italy the green tuff ignimbrite pantelleria is compositionally zoned from pantellerite at the base to comenditic trachyte at the top the variation apparently representing an inverted vertical zonation in the pre eruptive reservoir the main phenocryst assemblages are alkali feldspar olivine clinopyroxene ilmenite apatite in the trachytes and alkali feldspar aenigmatite clinopyroxene quartz in the rhyolites thermodynamic modelling indicates that the temperature range was 900 700 c fo2 fmq 1 5 to fmq 0 5 where fmq is fayalite magnetite quartz buffer and asio2 relative to quartz saturation 0 74 1 00 melt water contents ranged from 1 wt in the trachytes to 4 wt in the pantellerites matrix glass analyses in the more evolved rocks are highly variable showing that compositional layers in the upper parts of the reservoir formed by fractional crystallization were mixed during eruption the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir some areas of glass have low al2o3 contents 5 16 5 46 wt and high feo contents 9 66 10 02 wt making them the most evolved melts yet reported from pantelleria the new glass data reveal how whole rock analyses do not truly reflect the complete range of melt compositions in the pre eruptive reservoir the trachytes contain 40 modal phenocrysts which with relatively high ba contents and positive eu anomalies are considered to have originated in a feldspar accumulitic layer phenocrysts in the trachytes are commonly heavily resorbed inferred to be a result of heating by influxes of intermediate composition magma which however were not erupted it is argued that magmas of intermediate composition were present in the green tuff reservoir but were efficiently trapped in a crystal rich layer below the green tuff magmas which was eventually erupted during a resurgent phase as the montagna grande trachyte

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

the pre eruptive volatile contents of recent basaltic and pantelleritic magmas at pantelleria italy abstract pantelleria island located in the sicily channel rift zone italy is the type locality for the peralkaline rhyolitic rocks called pantellerites in the last 50 ka after the large green tuff caldera forming eruption volcanic activity at pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows lava domes and poorly dispersed pantelleritic pumice fall deposits basaltic cinder cones and lava flows are only present outside the caldera in the nw sector of the island the most recent basaltic cuddie rosse 20 ka and pantelleritic cuddia randazzo and cuddia del gallo 6 ka pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions the melt inclusions in pyroxene and olivine phenocrysts of cuddie rosse scoriae have an alkali basalt composition the dissolved volatiles comprise 0 9 1 6 wt h2o several hundred ppm of co2 1600 2000 ppm of sulphur and 500 900 ppm of chlorine the water carbon dioxide couple gives a confining pressure 2 kbar prior to the eruption this result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption the melt inclusions in feldspar fayalite and aenigmatite phenocrysts of cuddia del gallo and cuddia randazzo pumice have a pantelleritic composition agpaitic indices 1 3 2 1 up to 4 4 wt h2o 8700 ppm cl 6000 ppm f and co2 below the detection limit sulphur averaging 420 ppm has been measured in cuddia randazzo melt inclusions these data indicate relatively high volatile contents for these low energy strombolian type eruptions melt inclusions in cuddia del gallo pumice show the most evolved composition agpaitic indices 2 2 1 and the highest volatile content in agreement with fluid saturation conditions in the magma chamber prior to the eruption this implies a confining pressure of 1 kbar for the top of the pantelleritic reservoir the composition of melt inclusions and mineralogical assemblage of cuddia randazzo pumice indicate that it has a lower evolutionary degree agpaitic indices 1 3 1 8 and lower pre eruptive cl and h2o contents than cuddia del gallo pumice an increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the cuddia del gallo explosive eruption evidence of widespread pre eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the cuddia randazzo event

carbon dioxide emission and heat release estimation for pantelleria island sicily italy abstract detailed surveys of diffuse co2 flux soil temperature thermal gradients and sampling of high t fumaroles were carried out in the favare area and lake specchio di venere on pantelleria island spatial patterns of diffuse co2 emissions in the favare area reflect structural discontinuities faults fractures or cracks in the soil associated with the volcano tectonic structures of the young monastero caldera nne ssw to ne sw trending the estimated diffuse co2 output from two adjacent sites in the favare area 93 000 m2 is 7 8 t d 1 equivalent to 2 62 kt a 1 whereas that from the west shore of the lake 450 m2 is 0 041 t d 1 or 0 015 kt a 1 the extrapolation of diffuse co2 fluxes across the entire altered area of favare suggests that co2 emissions are 19 3 t d 1 the diffuse co2 flux correlates with shallow soil temperatures indicating a similar source for both the heat and volatiles from the underlying geothermal reservoir gas equilibria applied to fumarolic effluents define p t conditions for this reservoir at 2 6 bar and 120 160 c in good agreement with measurements from exploratory wells in these areas e g 135 c at a depth of 290 m using the co2 flux as a tracer for steam output and consequently for heat flow the calculated thermal energy for the shallow reservoir is 10 12 mw this represents the minimum geothermal potential of the reservoir on pantelleria island

volatiles in pantellerite magmas a case study of the green tuff plinian eruption island of pantelleria italy abstract the green tuff gt plinian eruption the largest in magnitude at pantelleria erupted 3 to 7 km3 dre of pantellerite magma and a small volume of trachyte fifty nine anorthoclase hosted melt inclusions from the two basal pumice members were analyzed by ft ir spectroscopy in order to assess the pre eruptive h2o content in the pantellerite melt microanalytical methods were used to determine major element cl f and s contents melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition peralkaline index 1 9 2 2 and variable water contents ranging from 1 4 to as high as 4 2 wt i e much higher than the 1 4 wt of earlier published studies the chlorine content is constant at about 1 wt combined cl and h2o data were used to estimate a confining pressure of about 50 mpa depth around 2 3 km for the gt magma chamber the chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma soon after the gt eruption intra caldera volcanism was dominated by the eruption of voluminous trachyte lava flows while pantellerite melt production resumed after about 20 ka with numerous low volume mildly explosive strombolian to effusive eruptions comparison with data from the literature reveals that despite the different explosivity the post caldera strombolian eruptions and the gt plinian eruption were fed by pantelleritic magmas with similar water contents chlorine and co2 contents suggest that the young magma reservoirs feeding the strombolian to effusive activity were deeper h 4 5 km than the much larger based on erupted volumes magma chamber which fed the gt eruption

pre eruptive volatile content and degassing history of an evolving peralkaline volcano abstract the volatile contents of melt inclusions trapped within phenocrysts of quartz and feldspar in peralkaline rhyolites from mayor island new zealand have been studied using fourier transform infrared spectroscopy and ion microprobe analyses the glass inclusions analyzed span the 130 ka subaerial eruptive history of the island and come from volcanic deposits representing a wide range of eruptive styles explosive vs effusive magmatic vs phreatomagmatic low vs high discharge rates the water content of all inclusions analysed is uniformly high at 4 4 wt h2o whereas co2 contents are below the limits of detection 50 ppm chlorine in melt inclusions ranges from 2070 to 5200 ppm while coexisting matrix glasses have generally lower cl concentrations of 1700 4200 ppm the apparent bulk distribution coefficient describing cl partitioning between melt and vapor phase cl fluid cl melt during degassing is 5 15 fluorine appears to be less affected by eruptive degassing than is cl and melt inclusion and matrix glass f concentrations show significant overlap 1400 2520 ppm f in inclusions 1550 2890 ppm f in matrix glass the observed invariance of melt inclusion water content with sample age contrasts with ion microprobe data on incompatible trace elements e g zr nb which suggest 35 fractionation between the oldest and youngest samples this along with the cl partitioning behaviour suggests that at least the upper erupting portion of the mayor island magma chamber was water saturated throughout the volcano s 130 ka eruptive history furthermore the large range of eruptive styles observed on mayor island are not due to differences in water concentration of the erupting magma but instead must reflect differences in rates of magma ascent and supply which control the efficiency of non explosive degassing

the probable occurrence of interstitial al in hydrous f bearing and f free aluminosilicate melts experiments carried out on the system sio2 naalsi3o8 kalsi3o8 qz ab or at 1 kbar in the presence of h2o and f show that the quartz alkali feldspar field boundary is progressively displaced towards the feldspar join as f contents increase from 0 to 4 wt f increasing p h 2 o in the absence of f has already been shown to have a similar effect tuttle and bowen 1958 luth jahns and tuttle 1964 the increased size of the quartz field in the f bearing system compared to the hydrous system is believed to be caused by progressive removal of al from the tetrahedral network of the melt by complexing with f the residual network in the melt is thus enriched in si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions the common presence of quench cryolite na3alf6 in certain experiments carried out with 4 wt f supports this interpretation and indicates that some al in the melt may be present in six fold coordination with f the effect of h2o in the absence of f may be similar with al being progressively removed from four fold coordination as more h2o is dissolved in the melt although a proportion of al in hydrous melts may occur in six fold coordination dry melts predominantly contain al in four fold coordination this major difference in al complexing may be one of the main causes for differences in the high pressure phase relations of wet and dry albearing silicate systems

evolution of peralkaline silicic complexes lessons from the extrusive rocks abstract young 4 wt and the rhyolitic varieties evolve to temperatures 4 10 6 pa s result in rapid growth of volcanic edifices highly efficient crystal melt separation and the ubiquitous development of compositionally zoned caps to reservoirs no peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc alkaline systems has yet been found

measuring the degree of nanotilization of volcanic glasses understanding syn eruptive processes recorded in melt inclusions iron and water content substantially affect the physical properties of natural silicate melts and may therefore influence magmatic and volcanic processes such as crystallization degassing flow behaviour and eruptive style here we present raman spectroscopic data for a set of synthetic and natural multicomponent glasses with varying iron oxidation state and water content we systematically study the effect of different raman excitation sources on the spectral response of both crystal free and magnetite nanolite bearing glasses spanning basaltic to calc and per alkaline rhyolitic compositions based on these data we document changes in raman spectra resulting from the formation of nano scale crystals we show that the peak located at similar to 970 cm 1 is directly related to the presence of fe2o3 dissolved in the quenched melt structure and that this feature is present regardless of the chemical composition of the sample and the raman excitation source we further show that a peak between 670 and 690 cm 1 which is not related to any other spectral feature of the glass structure reveals the presence of nanolites based on systematic spectral investigations of this feature we present a new index that allows to identify if iron is present in the nanocrystalline state and or bound in the glass structure since the melt structural and physical effects of the formation of nanolites can heavily affect the flow behaviour of melts and the eruptive style of volcanoes the results presented in this study significantly broaden the application of raman spectroscopy for investigations of nano heterogeneity in synthetic and natural glasses we apply this method to study both the degree of nanolitization as well as the h2o content and iron oxidation state of groundmass glasses as well as melt inclusions and glass embayments in explosive products from pantelleria island italy we observe that the process of nanotilization is not purely restricted to magnetite nanolites but that raman spectroscopy may also identify the incipient crystalization of pyroxene and feldspar at sub micron scale the data document that nanolite formation correlates well with the observed intensity of the respective eruptions suggesting that structural changes in the melt caused by incipient crystallization play an important role in defining the eruptive style of relatively low viscosity magmas

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

simple shearing block resurgence in caldera depressions a model from pantelleria and ischia abstract similar resurgent blocks occur in caldera depressions at the islands of pantelleria and ischia the two blocks which are tilted show similar deformation patterns and similar distributions of volcanic vents related to resurgence the remaining parts of the floors of the two calderas are not deformed similarities between the two features are so striking that the resurgence mechanism must have been alike a dynamic model for resurgence is presented and called simple shearing block resurgence the triggering mechanism is an increase of the magmatic pressure in the upper part of a shallow magma chamber which results in a vertical direction of the maximum stress applied on the roof rocks the block is first defined by high angle inland dipping marginal detachments and then deformed through a simple shearing mechanism reverse faults on its mostly uplifted side determine an horizontal shortening of the block which is balanced on the opposite side by normal faults oblique movements take place along the other two sides due to this distribution of the stress around the block magmas can reach the surface only through the extensional faults on the less uplifted side

structural chronology oroclinal deformation and tectonic evolution of the southeastern klamath mountains california devonian to jurassic volcanic sedimentary and minor intrusive rocks of the redding section rs accumulated in an intermittently active island arc setting general stratigraphic concordancy and the downsection continuity of metamorphic facies series indicate that these rocks shared a common postdepositional geologic history burial metamorphism produced secondary mineral assemblages of the zeolite prehnite pumpellyite pumpellyite actinolite and greenschist facies close correspondence between pressures calculated from existing stratigraphic overburden and those predicted from the observed metamorphic facies series indicates that less than 3 km of strata have been eroded from the presently exposed top of the early middle jurassic potem formation shortly after deposition of the potem formation compressional deformation f1 produced open to isoclinal folds foliations and small thrust faults k ar dates from post tectonic intrusions indicate that f1 deformation was completed before 168 ma this minimum age of f1 deformation is corroborated by a postfolding paleomagnetic overprint acquired at about 171 ma a second suite of intrusions dated at 120 130 ma provided a thermal pulse that variably reset the k ar system in f1 fabric defining celadonite and other relatively ar unretentive materials the nevadan orogeny late jurassic did not significantly deform rocks of the rs if at all nor is evidence for permo triassic sonoman deformation apparent the geometry of f1 structures varies systematically with location in the western bollibokka quadrangle bq f1 folds are upright with subvertical axial planes and associated foliations in the eastern bq f1 folds are overturned to the east with axial planes and associated foliations dipping as little as 40 to the west regional relationships suggest that f1 may have been a response to coeval tectonism in the western klamaths wherein crustal thickening during regional westward thrusting induced gravitational detachment of the rs from its partly serpentinized ophiolitic basement f1 structures subsequently were broadly warped about vertical axes producing variable paleomagnetic declination anomalies that range from 0 to greater than 100 clockwise the timing and cause of this f2 deformation event are as yet relatively unconstrained but were almost certainly post jurassic and may have been associated with development of the columbia embayment paleomagnetic data indicate north american paleolatitudes for the rs since at least the permian and apparently weigh against an allochthonous or exotic origin for this terrane paleontologic data indicate strong faunal similarities with north america and to a lesser extent with a tethyan province the significance of this apparent dual provinciality remains uncertain but extreme endemism of tethyan fusulinids corals brachiopods and pelecypods is questionable

ion microprobe analyses of trace elements in anorthoclase hedenbergite aenigmatite quartz apatite and glass in pantellerite evidence for high water contents in pantellerite melt

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

the origin of trachyte and pantellerite from pantelleria italy insights from major element trace element and thermodynamic modelling abstract trachyte and peralkaline rhyolite pantellerite and comendite frequently comprise the felsic end member in bimodal suites in continental rift and oceanic island settings in these settings the relationship between the mafic mildly alkaline or transitional basalt and felsic lavas is ambiguous major and trace element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization in this paper we present representative mineral analyses and whole rock analyses from forty samples of a basalt trachyte pantellerite suite collected at pantelleria italy in the strait of sicily rift zone and compare the results of major and trace element modelling with the results of thermodynamic melts modelling from these results we conclude that metaluminous trachyte formed as a result of 70 to 75 low pressure 0 1 gpa fractional crystallization of an assemblage of plagioclase clinopyroxene olivine magnetite and apatite from a hydrous 1 0 1 5 wt h2o transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite magnetite quartz buffer fmq 1 the daly gap a lack of intermediate 49 62 wt sio2 volcanic rocks at pantelleria is concluded to be primarily the result of rapid differentiation through that interval relatively rapid crystallization at low pressure may have effected the partial degassing of water saturated 4 wt h2o metaluminous trachyte magma some metaluminous trachyte lavas have positive eu anomalies high k rb ratios high concentrations of ba and low concentrations of incompatible trace elements these are interpreted to be the result of up to 40 accumulation of alkali feldspar comenditic trachyte pantelleritic trachyte and pantellerite formed after an additional 20 to 80 fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma the most evolved pantellerite lavas and tuffs are the result of a total of 95 fractional crystallization of transitional basalt with phenocrysts that equilibrated at low temperatures 4 wt water contents

carbon dioxide speciation in silicate melts a restart abstract using infrared spectroscopy we investigated the speciation of carbon dioxide in albitic and synthetic iron free dacitic glasses after annealing the glasses below the glass transition temperature in the temperature range 673 973 k at 0 5 gpa samples were rapidly quenched under constant pressure the experiments show that the ratio of the integrated intensities of the 2350 cm 1 band due to molecular co 2 to that of the band doublet at 1700 1375 cm 1 due to co 3 2 increases with higher annealing temperatures for both glass compositions our study demonstrates that in contrast to recent suggestions the equilibrium of the co 2 species reaction co 2 o 2 co 3 2 in silicate glasses melts shifts towards molecular co 2 with increasing temperature the co 2 species concentrations and an ideal solution model were used to determine equilibrium constants for the homogeneous species reaction we derived preliminary values for the enthalpy h 0 and entropy s 0 of this reaction assuming that the species concentrations reflect those at experimental temperatures h 0 12 2 kj mol 1 and s 0 23 3 j mol 1 k 1 for albitic composition h 0 29 2 kj mol 1 and s 0 32 3 j mol 1 k 1 for dacitic composition

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

continuous geomagnetic field models for the past 7 millennia 2 cals7k 1 we present two continuous global geomagnetic field models for recent millennia cals3k 2 covering the past 3000 years and cals7k 2 covering 7000 years from 5000 bc to 1950 ad the models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic b splines in time they are derived from a greatly increased number of paleomagnetic directional data compared to previous efforts and for the first time a significant amount of archeointensity data is used in this kind of global model allowing the determination of evolution of geomagnetic dipole strength while data accuracy and dating uncertainties remain a limitation reliable low resolution global models can be obtained the results agree well with previous results from virtual axial dipole moment vadm studies from archeomagnetic intensity data apart from a systematic offset in strength a comparison of model predictions with the previous 3000 year model cals3k 1 gives general agreement but also some significant differences particularly for the early epochs the new models suggest that the prominent two northern hemisphere flux lobes are more stationary than cals3k 1 implied extending considerably the time span of stationary flux lobes observed in historical models between 5000 bc and 2000 bc there are time intervals of weak dipole moment where dipole power is exceeded by low degree nondipole power at the core mantle boundary model coefficients and evaluation code can be obtained from the earthref digital archive erda together with animations and snapshots plots for every 100 years at http www earthref org detailed urls for the different material are listed in appendix a

nested calderas and trapdoor uplift at pantelleria strait of sicily pantelleria is dominated by a caldera 6 km in diameter that formed 55 000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte trachytic lavas then built a shield that nearly filled the caldera magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m creating montagna grande by trapdoor uplift the lid eventually failed and pantellerite magma erupted as domes flows and pumice cones along the trapdoor faults and their hingeline we have identified a second caldera 93 000 yr old within which the younger caldera is nested a still older structure may encircle them both replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures resulting in multicyclic collapses that reuse these structures and produce nested calderas

the recent explosive volcanism at pantelleria many of the least eighty eruptions of the six silicic eruptive cycles in the last 50 ka at pantelleria started out with explosive phases and ended with lava effusion the pyroclastic rocks are rhyolitic in composition while the later erupted lavas are less evolved up to trachyte probably due to the presence of a shallow and compositionally zoned magma chamber tapped at increasing depth during the eruptions the products of twenty explosive eruptions have been recognized most of them are represented by fallout deposits the only exceptions being the green tuff and the fossa carbonara tuff hydromagmatism is very rare though the magmas interacted with sea water it is supposed that interaction was not efficient because the magmas were not vesiculated and disrupted horizons showing variable degrees of welding up to vitrophyres have often been identified in the air fall tuffs most of the eruptions were characterized by low eruptive columns and were likely to have involved a strombolian type mechanism

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

geomagnetic field variations in western europe from 1500bc to 200ad part i directional secular variation curve abstract to improve the geomagnetic field secular variation curve svc of western europe during protohistoric times archaeomagnetic directions of 39 archaeological kilns or hearths from france were investigated the dating of each archaeological structure was established with archaeological or chronometric methods thirty seven of these structures are dated from the first millennium bc one from the end of the second millennium bc and the last one from the fourth millennium bc thermomagnetic curves unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a ti poor titanomagnetite archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens the anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy the new archaeodirections are very consistent with previously published data a new directional svc was built using bivariate statistics with selected western europe data located within 1000 km of paris selection criteria include the number of samples the dating reliability and the accuracy of the mean archaeodirection resulting secular variation between 1500bc and 0ad mainly shows large changes in declination while inclinations are bracketed between 65 and 75 the declinations show a strong maximum with values 30 35 around 800 750bc followed by a sharp decrease to values around 0 at 500bc and close to 7 around 250bc the main features of the secular variation from 1500bc to 0ad appear to be a dominant westward drift and two major changes around 800 and 250bc compared to the global and regional geomagnetic models the new reference data are better fitted by arch3k cst 1 and scha dif 3k than by arch3k 1 and cals3k 4 models the strong variation of the archaeodirection between 1000 and 500bc makes archaeomagnetism very useful for dating purposes

archaeomagnetic secular variation in germany during the past 2500 years summary the german archaeomagnetic data set was supplemented with 35 new directions from ger man sites mainly dating from the past 3000 yr together with archaeomagnetic data from the neighbouring countries of germany a database of 166 reliable archaeomagnetic directions has been compiled for the reference area which is defined by a 500 km radius circle around gottingen the retrieved directions come from well dated archaeological structures and about 40 per cent of the dating relay on natural science methods such as radiocarbon thermolumines cence dendrochronology dating or historical documents from this data set a secular variation sv reference curve has been calculated using a bivariate algorithm which fits a natural cubic spline based on roughness penalty to declination inclination and time simultaneously the error tube surrounding this curve was obtained from bayesian modelling of the experimental errors which can also take stratigraphic information into account the obtained sv reference curve for the past 2500 yr is similar to that from france but also significant differences are seen comparison of the curves does not show a simple westward drift of the sv pattern the german reference curve allows archaeomagnetic dating in the reference area and extends this dating technique to sites situated in middle europe

formation of the green tuff pantelleria the strongly peralkaline green tuff pantelleria is an example of a thin densely welded air fall tuff which mantles an area of at least 85 km2 offshore the tuff is correlated with the y 6 ash layer in the central mediterranean sea and the total volume of the eruption is estimated at 7 km3 d r e new petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler more fractionated upper zone relative to be bulk of the magma analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian type eruption it is shown that due to the low viscosity of pantelleritic ejecta dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm minute is suggested for the green tuff this yields an estimate for the eruption duration of rather less than one day it is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism

a regional archeomagnetic model for europe for the last 3000 years scha dif 3k applications to archeomagnetic dating 1 the available european database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in europe for the last 3000 years from 1000 b c to 1900 a d connecting with the epoch covered by the igrf models this new model scha dif 3k constitutes an improvement with respect to the previous regional archeomagnetic model scha di 00 f which used relocated values and was only valid for the last 2000 years the new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time an algorithm has been developed to jointly model the three archeomagnetic elements declination inclination and intensity the resulting model provides the direction and intensity of the earth s magnetic field over the european continent northern africa and western asia for the last 3000 years the fit to the european archeomagnetic database is more accurate than that provided by global archeomagnetic models in addition this model represents a step forward in archeomagnetic dating studies since the relocation error is avoided and can also be used to study the rapid changes of the geomagnetic field archeomagnetic jerks that have been recently proposed

geochemical mass transfer patterns as indicators of the architecture of a complete volcanic hosted massive sulfide hydrothermal alteration system panorama district pilbara western australia outstanding exposure across the panorama district in the archean pilbara block of western australia reveals a cross section through a complete volcanic hosted massive sulfide vhms hydrothermal alteration system in an area of low metamorphic grade and very low strain geochemical maps in combination with alteration maps document mass transfer across this alteration system feldspar bearing rocks at the top of the volcanic pile are enriched in k and si but depleted in na in contrast to those at the bottom of the volcanic pile that are enriched in na and depleted in k sericite quartz altered rocks are typically enriched in si and depleted in ca na and fe whereas chlorite quartz altered rocks which dominate transgressive feldspar destructive alteration zones are marked by k ca na depletion and mg fe enrichment copper and zn are strongly depleted in rocks at the base of the volcanic pile and to a lesser extent in transgressive feldspar destructive alteration zones the amount of metal leached from the volcanic pile is far greater than that contained in known deposits and thus a magmatic metal source is not necessary to explain the formation of the panorama vhms deposits mass transfer patterns at panorama have similarities to existing models for regional vhms hydrothermal alteration systems including the distribution of alkali elements in feldspar bearing semiconformable alteration zones and the presence of a base metal depleted zone at the base of the volcanic pile however there are significant differences both in the architecture of the hydrothermal alteration system and in mass transfer patterns these include si enrichment rather than depletion at the top of the volcanic pile widespread mg enrichment outside of the upper semiconformable alteration zones the absence of a silica cap rock above the base metal depleted zone more hydrous alteration assemblages in the zones of base metal depletion and extensive zones of transgressive feldspar destructive alteration

the redistribution of copper during low grade metamorphism of the karmutsen volcanics vancouver island british columbia the karmutsen volcanics exposed on northern vancouver island are a sequence of chemically uniform tholeiitic basalts with a thickness greater than 5 km within the karmutsen a stratigraphic succession from close packed pillow lava to breccias to flows resulted from a transition from deep marine to subaerial eruption of the parent magmas all lithologies have undergone prehnite pumpellyite facies burial metamorphism which substantially redistributed cu within the flows but not within the subaqueously erupted breccias or pillow lavas there is no evidence for substantial metamorphic redistribution of cr ni ti zn and mn which vary only slightly in the three stratigraphic units like other transition elements cu abundance is confined to a restricted range 120 200 ppm in the subaqueously erupted rocks however in the flows cu is very unevenly distributed and is generally less abundant than in the subaqueously erupted rocks posteruptive redistribution of copper is evidenced by the fact that the present distribution of copper in the flows can be related to posteruptive mineral assemblages lowest copper abundances are in red flows rich in hematite which probably resulted from deuteric alteration highest copper abundances and most native copper occurrences are in green pumpellyite rich flows the initial copper content of the flows is not known for certain but is likely to have been similar to that of the subaqueously erupted rocks which are in other respects chemically similar to the flows it is believed that high f o 2 in low grade metamorphic fluids led to the dissolution of chalcopyrite and sporadic precipitation of native copper in association with amygdular prehnite intense pumpellyitization of some flows locally lowered the f o 2 and possibly also raised the ph of the metamorphic solutions thus favoring deposition of copper in these flows copper was not redistributed in the subaqueously erupted units because these lacked the appropriate deuteric fe ti oxide assemblages

archaeomagnetic investigations in greece and their bearing on geomagnetic secular variation archaeomagnetic results are presented from kilns and other heated structures on mainland greece and on the islands of crete delos euboea and thasos they span the interval from the mid third millennium bce to the mid second millennium ce the results obtained are in good agreement with the comprehensive dataset from bulgaria and reveal secular variation with amplitudes of 25 in inclination and 30 in declination the sharp inclination low in the first few centuries ce now recognized at sites ranging from crete to britain is a robust geomagnetic feature reflecting a static flux pulse due to a magnetic source in the outermost core another much more pronounced inclination low early in the second millennium bce is tentatively identified the emerging greek secular variation curve provides archaeomagnetic dates for five kilns of hitherto uncertain age 2006 elsevier b v all rights reserved

new data on magmatic h 2 o contents of pantellerites with implications for petrogenesis and eruptive dynamics at pantelleria infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at pantelleria italy show that the magmas contained moderate pre eruptive h2o contents ranging from 1 4 to 2 1 wt melt h2o concentrations increase linearly with incompatible elements demonstrating that h2o contents were not buffered significantly during fractionation by any crystalline or vapor phase the relatively low h2o contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt preeruptive h2o concentrations do not correlate with the volume or explosivity of pantellerite eruptions decompression history is critical in determining the style of pantellerite and other eruptions

paleomagnetic intensity data a window on the dynamics of earth s fluid core abstract we consider paleomagnetic intensity data obtained from sedimentary cores as a time sequence related to fluctuations of the fluid velocity in earth s core although this is a gross oversimplification of the relationship between the geomagnetic and velocity fields some aspects of their possible interaction might be obtained through inversion of paleomagnetic intensity data in particular a signature for the onset and development of turbulence as seen in laboratory experiments on rotational parametric instabilities in rotating fluids may be contained in a paleointensity time sequence just as in laboratory experiments changes in flow regimes can be traced using these observations the paleomagnetic intensity itself is subject to several factors which contribute to uncertainty including the determination of the intensity as well as errors in timing due to offsets in age determination non linear inversions of several records from the odp 983 data set of paleomagnetic amplitudes are presented growth and decay with time of paleomagnetic amplitudes are modelled as the sum of an exponential plus two harmonics this simple model accounts for most of the paleomagnetic amplitude signal yielding geophysically plausible growth and decay rates for a proposed rotational parametric instability in earth s core this model provides a new interpretation for observed paleomagnetic intensity changes and thus an improved understanding of the dynamical processes producing the geomagnetic field

redistribution of cu zn pb and ba in the bollibokka group east shasta california u s a implications for metallogeny and geochemical balances abstract basalt and andesite of the bollibokka group underlie stratigraphic horizons with volcanogenic massive sulfide vms deposits in the east shasta district but with no known deposits to the north least altered rock remote from known deposits commonly retains clinopyroxene and titanomagnetite phenocrysts and contains abundant secondary pumpellyite and prehnite in contrast to altered rock near known deposits least altered rock contains on average 89 ppm cu 100 ppm zn 10 ppm pb and 160 260 ppm ba alteration within a few kilometers of the bully hill rising star mines led to removal of 59 ppm cu and possibly 7 ppm zn 2 ppm pb and significant ba from basalt one km 3 of altered basalt can account for 7 the cu and 1 7 the zn production of the mines basaltic volcaniclastic wacke in the area contains framboidal pyrite and kerogen and was enriched in cu zn and pb intense leaching within 1 km of the afterthought mine led to removal of 74 ppm cu 34 ppm zn and 4 ppm pb relative to unaltered basalt alteration of 1 km 3 of basalt could supply 40 the cu 8 the zn and 14 the pb produced by the afterthought mine zones of maximum leaching of metals underlie known deposits yet it cannot be proven that leached metals contributed to the deposits the oceanic inventories of cu and zn can be established in thousands of years by hydrothermal and river water fluxes these fluxes cannot account for the rate of fixation of zn in shale a dominant detrital flux is required a dominant detrital flux of cu is permitted but not required

carbon dioxide and radon emissions from the soils of pantelleria island southern italy abstract in the period july 2005 october 2006 several soil gas prospections were performed on the island of pantelleria an active volcanic geothermal system at present in a quiescent state measured parameters were co2 concentrations and 220rn and 222rn activities at 50 cm depth and co2 fluxes from the soil the whole island was covered with a measurement density of 3 5 points km2 for co2 flux and concentration and 1 6 points km2 for rn further three main and seven minor areas of previously ascertained or expected exhalative activity were covered with a measurement density up to 100 points km2 the obtained results ranged from 0 038 to 95 vol for co2 concentration from 10 bq m3 to 164 000 bq m3 from 10 bq m3 to 750 000 bq m3 for 220rn and 222rn activities respectively and from

explosive eruptive history of pantelleria italy repeated caldera collapse and ignimbrite emplacement at a peralkaline volcano a new pre green tuff 46 ka volcanic stratigraphy is presented for the peralkaline pantelleria volcano italy new 40ar 39ar and paleomagnetic data are combined with detailed field studies to develop a comprehensive stratigraphic reconstruction of the island we find that the pre 46 ka succession is characterised by eight silica rich peralkaline trachyte to pantellerite ignimbrites many of which blanketed the entire island the ignimbrites are typically welded to rheomorphic and are often associated with lithic breccias and or pumice deposits they record sustained radial pyroclastic density currents fed by low pyroclastic fountains the onset of ignimbrite emplacement is typically preceded more rarely followed by pumice fallout with limited dispersal and some eruptions lack any associated pumice fall deposit suggesting the absence of tall eruption columns particular attention is given to the correlation of well developed lithic breccias in the ignimbrites interpreted as probable tracers of caldera collapses they record as many as five caldera collapse events in contrast to the two events reported to date inter ignimbrite periods are characterised by explosive and effusive eruptions with limited dispersal such as small pumice cones as well as pedogenesis these periods have similar characteristics as the current post green tuff activity on the island and while not imminent it is reasonable to postulate the occurrence of another ignimbrite forming eruption sometime in the future

a method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating summary spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present day field a method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation for an archaeomagnetic region the size of the british isles 900 km radius the maximum error in relocating vectors to a central location is predicted to be of the order of 1 2

insights on the geomagnetic secular variation in the eastern mediterranean first directional data from cyprus abstract the first directional archaeomagnetic results from cyprus are presented based on the study of seven baked clay structures excavated at the copper smelting site of agia varvara almyras situated some 20 km south of nicosia this ancient copper working site represents a unique example of complete primary production of the copper metal in cyprus in total five furnaces one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation all the studied structures are well dated based on pottery finds and radiocarbon dates with ages ranging from 600 bce to 50 bce several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material the direction of the characteristic remanent magnetization chrm for each structure has been determined by demagnetization of 7 15 specimens in alternating magnetic fields up to 100 120 mt mean directions were calculated assuming a fisherian distribution and are well defined for five out of the seven structures studied the new directions are compared with the directional data and the secular variation curves available from the surrounding area and with the predictions of global geomagnetic field models they are an important contribution to improving our knowledge of the geomagnetic field variations in the eastern mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium bce that were not previously identified by the available geomagnetic field models predictions in this area

the eruptive history of pantelleria sicily channel in the last 50 ka six silicic eruptive cycles have been recognized in the last 50 ka at pantelleria the products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte the relationships between the range of chemical variation the erupted volume and the time of eruptions allow us to estimate an average differentiation rate of 5 crystal fractionation per 1000 years and a constant long term rate of magma discharge of 0 1 km3 per 1000 years pressure increase in the magma chamber caused by the addition of new magma accumulation of highly differentiated volatile rich magma in the roof zone and a concomitant build up of a vapour phase is postulated as a possible triggering mechanism for eruptions

changing modes and rates of mafic magma supply at pantelleria sicily channel southern italy new perspectives on the volcano factory drawn upon olivine records

cooling and crystallization recorded in trachytic enclaves hosted in pantelleritic magmas pantelleria italy implications for pantellerite petrogenesis abstract this study focuses on the comenditic trachyte enclaves hosted in pantelleritic lava and pumices emplaced during one of the most recent eruptive events at pantelleria 6 ka enclaves range from mm sized fragments to dm sized blocks with spheroidal to amoeboid shapes and characteristic globular surfaces they are crystal rich with 30 vol large anorthoclase less abundant fe rich olivine clinopyroxene and fe ti oxides vesicles ranging from a few mm to 1 2 cm in size are distributed throughout the enclave and are commonly filled with microlite free vesicular glass the groundmass presents spectacular textures including fine to coarse spherulites and hopper and skeletal microlites mostly anorthoclase with 10 35 vol residual glass and 10 15 vol small vesicles 2 accordingly the microlite composition is close to that of the host pantellerite these textures were acquired during a magma mixing event prior to the eruption i a vapor saturated comenditic trachyte magma intruded the shallower cooler pantellerite magma body triggering a first degassing event first boiling and rapid crystallization with a high degree of undercooling t 100 150 c ii vapor exsolution induced by rapid crystallization second boiling produced the microvesicular groundmass and iii vapor pressure forced the residual pantelleritic liquid to migrate into the large vesicles and or outside the enclaves based on this interpretation these enclaves can be used as a natural laboratory for identifying the chemical and physical processes driving the evolution of silicic magmas at pantelleria in particular they can be used to assess whether pantellerite melts can be obtained from comenditic trachytes after extensive crystallization followed by gas driven filter pressing and consequent pantellerite melt segregation

experimental study of chlorine behavior in hydrous silicic melts chlorine solubility in silicate melts has been investigated at 830 850 plus minus 5c and at pressures ranging from 50 to 200 mpa using both natural pantellerite rhyolite phonolite and synthetic sio sub 2 al sub 2 o sub 3 k sub 2 o na sub 2 o compositions and a stated h sub 2 o nacl kcl fluid phase near 4 molal at 100 mpa cl contents in pantelleritic melts reach a solubility plateau at initial aqueous phase molality near 2 this plateau coincides with a large immiscibility gap between aqueous and chloride rich fluids with the coexisting cl saturated aqueous phase cl ranges from 2 720 plus minus 120 ppm in rhyolite to 8 960 plus minus 85 ppm in pantellerite and reaches 6 270 plus minus 170 ppm in phonolite at 100 mpa between 50 and 200 mpa the cl content in pantelleritic melt decreases from 9 640 plus minus 200 ppm to 5 040 plus minus 150 ppm although cl solubility increases with increasing feo sup in high sio sub 2 melts it is mainly controlled by the al si and na k al molar ratios of the melt with a minimum at na k al 1 in a series of synthetic rhyolitic to pantelleritic melts the experimental results suggest that chlorine occurs as alkali chloride complexes in high sio sub 2 more melts they also indicate that cl is concentrated in the aqueous fluids in equilibrium with sio sub 2 rich melts the exact value of d depending on melt composition and melt chlorine concentration volcanic degassing will create chlorine rich hydrothermal fluids and decrease chlorine melt content less

high resolution 40ar 39ar chronostratigraphy of the post caldera 20 ka volcanic activity at pantelleria sicily strait abstract the island of pantelleria sicily strait the type locality for pantellerite has been the locus of major caldera forming eruptions that culminated ca 50 ka ago in the formation of the cinque denti caldera produced by the green tuff eruption the post caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor a high resolution 40 ar 39 ar study was conducted on key units spanning the recent post 20 ka intracaldera activity to better characterize the present day status and forecast the short term behavior of the system based on the temporal evolution of the latest eruptions the new 40 ar 39 ar data capture a long term 15 ka decline in eruption frequency with a shift in eruptive pace from 3 5 ka 1 to 0 8 ka 1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12 14 ka this shift in extraction frequency occurs without major changes in eruptive style and is paralleled by a subtle trend of decreasing melt differentiation index we speculate that this decline probably occurred i without short term variations in melt production differentiation rate in a steady state compositionally zoned silicic reservoir progressively tapped deeper through the sequence and ii that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka the intracaldera system is experiencing a protracted stasis since 7 ka coupled with recent geodetic evidence of deflation and subsidence of the caldera floor the system appears today to be on a wane with no temporal evidence for a short term silicic eruption

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

the green tuff of pantelleria rheoignimbrite or rheomorphic fall detailed stratigraphic analysis of the green tuff of pantelleria shows that this formation can be divided into several members designateda throughh from base to top these members have a coherent pattern when traced from outcrop to outcrop throughout the island shedding light on their origin only memberg completely mantles the entire island the distribution of the other members is controlled by prevailing wind direction or by topography membera is entirely of fall origin membersc ande are of fall and or surge type membersb d andh have the characteristics of thin welded ash flow tuffs membersf andg are ash flow tuffs with textural characteristics of compound cooling units most of the ash flow tuffs exhibit characteristics of ignimbrites vertical fluidization pipes local concentrations of lithic lapilli imbrication of clasts and valley ponding memberg is unusual in that it is highly welded exhibits large scale rheomorphic structures contains huge lithic clasts and has near vertical foliation where it adheres to cliffs and caldera walls

problems of recognition of olistostromes an example from the lower pit formation eastern klamath mountains california the recognition of large olistostromes is difficult unless outcrop is nearly complete over broad areas a previously unrecognized olistostrome in the lower pit formation was exposed on the shores of shasta lake california at a time of low water level blocks as much as 1000 2000 m across of diverse volcanic and sedimentary rocks are set in an unsheared tuffaceous matrix the olistostrome formed in response to a middle triassic disturbance possibly involving rifting and it disrupted volcanogenic massive sulfide deposits

mineral stability in peralkaline silicic rocks information from trachytes of the menengai volcano kenya electron microprobe analyses are presented for phenocrysts and matrix glass in peralkaline silica oversaturated trachytes from the menengai volcano kenya rift valley the dominant phenocryst assemblage is alkali feldspar hedenbergite fayalite titanomagnetite apatite aenigmatite amphibole and quartz occur rarely in more peralkaline rocks quilf calculations indicate that the trachytic magmas crystallised at temperatures of 854 870 c at relatively low oxidation states fmq 1 6 to 1 7 and silica activity asio2 qtz of 0 60 0 66 the new analyses are used along with published data to outline the distribution of the main phases in the compositional spectrum of peralkaline quartz trachytes and rhyolites there is uncertainty about the nature or even existence of a low temperature zone in the alkali feldspar primary phase region the equivalent of the thermal valley in the haplogranite system quartz phenocrysts may appear early or late in the crystallisation sequence even in rocks of similar bulk composition its appearance perhaps being a function of the f content of the melts whereas hedenbergite and fayalite show fairly systematic compositional trends with increasing host rock peralkalinity amphibole compositions are variable for reasons not yet understood aenigmatite crystallisation is at least partly controlled by oxygen fugacity and silica activity with rare exceptions ilmenite and titanomagnetite are incompatible phases but the factors controlling their relative stabilities are not clear it appears that peralkaline trachyte rhyolite sequences evolve along many crystallisation paths the paths perhaps being strongly influenced by ph2o pf2 melt f cl ratios and perhaps total pressure

the serra della fastuca tephra at pantelleria physical parameters for an explosive eruption of peralkaline magma abstract the serra della fastuca tephra is the product of the first explosive eruption during the most recent 8 ka silicic cycle of activity at pantelleria it is totally made up of fallout tephra with very limited areal distribution stratimetrical and grain size data have been used to define the physical parameters of the eruption the latter are not easily definable as various critical data for peralkaline magmas are not available in the literature an initial velocity of 62 m s 1 has been tentatively estimated assuming the volatile phase exsolved from the magma is a mixture of hf hcl so 2 co 2 and water in equal molar fraction terminal velocity has been found not to be affected by the distance from the vent the pressure at fragmentation depth has been estimated at around 12 bar these parameters suggest that the eruption column did not reach great height even during the most energetic phases though this eruption cannot be easily and unequivocally classified it can be considered strombolian lack of hydromagmatic activity has been explained in light of the shallow fragmentation depth

experimental and thermodynamic constraints on the sulphur yield of peralkaline and metaluminous silicic flood eruptions many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known we have performed experiments and theoretical calculations to address this issue the melt sulphur content and fluid melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites which are a major component of some flood provinces experiments were performed at 150mpa 800 900 c fo2 in the range nno 2 to nno 3 and under water rich conditions the sulphur content is strongly dependent on the peralkalinity of the melt in addition to fo2 and reaches 1000ppm at nno 1 in the most strongly peralkaline composition at 800 c at all values of fo2 peralkaline melts can carry 5 20 times more sulphur than their metaluminous equivalents mildly peralkaline compositions show little variation in fluid melt sulphur partitioning with changing fo2 ds 270 in the most peralkaline melt ds rises sharply at fo2 nno 1 to values of 500 the partition coefficient increases steadily for sbulk between 1 and 6wt but remains about constant for sbulk between 0 5 and 1wt at bulk sulphur contents lower than 4wt a temperature increase from 800 to 900 c decreases ds by 10 these results along with 1 thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and 2 recent experimental constraints on sulphur solubility in metaluminous rhyolites show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis in particular hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur in contrast peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90 of the original sulphur content of the parental magmas especially when the basalt is co2 rich on this basis we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences the peralkaline ethiopian and deccan rhyolites could have produced 10 17 and 10 18 g of s respectively which are comparable amounts to published estimates for the basaltic activity of each province in contrast despite similar erupted volumes the metaluminous parana etendeka silicic eruptives could have injected only 4 6 10 15 g of s in the atmosphere peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity in agreement with evidence available from mass extinctions and oceanic anoxic events

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

geology and ore petrography of permian kuroko type volcanogenic massive sulfide deposits of the bully hill area east shasta district california kuroko type volcanogenic massive sulfide vms deposits occur in the late permian bully hill rhyolite and the basal part of the overlying pit formation in the bully hill area the bully hill rhyolite at bully hill consists of three tuffaceous units and a lava unit that overlie the dekkas andesite two ore host horizons are present the younger containing the bully hill and probably the copper city deposits and the older the rising star deposit a large olistostrome in the basal pit formation contains disrupted shale mudstone and volcanic rock and includes hydrothermally altered and mineralized material the host strata are folded into tight anticlines at bully hill and copper city distinct zones of hydrothermal alteration ranging from intense silicification to a quartz albite chlorite epidote assemblage are found within the bully hill rhyolite at bully hill the most intensely silicified rock occurs within the lava unit stratigraphically beneath the known vms deposits this rock preserves a stockwork of quartz and barite veinlets some bearing sulfides which represent hydrothermal feeders of the syngenetic mineralization the syngenetic ore has been hydrothermally reworked and is characterized by a paragenetic sequence resembling that of japanese kuroko type deposits eldridge et al 1983 with the added complexity of retrograde barite replacing sulfide minerals

a revision of the structure and stratigraphy of pre green tuff ignimbrites at pantelleria strait of sicily abstract at pantelleria peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes pyroclastic flows plinian to strombolian pumice fallout and lava flows in this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island this age interval 180 85 ka was punctuated by six ignimbrite forming eruptions silicic and variably peralkaline for a cumulative erupted magma volume of approximately 6 km 3 dense rock equivalent based on new 40 ar 39 ar na k feldspar ages and petrographic data we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows i the age of the old la vecchia caldera collapse is now tightly constrained between 140 and 146 ka and the caldera forming eruption can be traced to a lithic rich welded tuff breccia that outcrops in two opposite sectors of the island south west and north east ii four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85 ka in particular the 85 ka eruptive event is comparable in magnitude to the younger caldera forming green tuff plinian eruption iii the recurrence patterns of the 107 and 85 ka eruptions compared to the green tuff allow us to qualitatively assess that the climax in production of low temperature silicic and peralkaline melt was focused in the age interval 85 45 ka

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

archaeomagnetic dating of santorini volcanic eruptions and fired destruction levels of late minoan civilization archaeomagnetic dating on the minoan ash horizons of the santorini volcano and on fired destruction levels at late minoan sites on crete demonstrates that the basal plinian air fall ash of the first minoan pumice is contemporaneous with the destruction levels on central crete while the higher minoan ashes are contemporaneous with the destruction levels in extreme eastern crete these destruction levels were almost certainly caused by seismic activity rather than the ash fall the determination of a time gap between these events lead to a reappraisal of the archaeological evidence and is important volcanologically

intra arc sedimentation and tectonism late paleozoic evolution of the eastern klamath terrane california rocks of the eastern klamath terrane redding section in northern california record deposition magmatism and tectonism in an island arc setting volcanism and sedimentation persisted for more than 220 m y spanning devonian to middle jurassic time and yielding thick successions of water laid volcanic debris and lavas with interstratified carbonate and clastic sequences this study focuses on the upper paleozoic rocks within the section the oldest rocks studied within the upper devonian and mississippian brag don formation contain epiclastic debris that was deposited in a submarine fan setting sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation transitional shallow water facies in the upper part of the bragdon interfinger with the overlying baird formation which contains abundant volcanic debris volcanogenic shelf and strandline deposits characterize the lower part of the formation and volcaniclastic turbidites characterize the upper part the baird formation is locally interfingered with the overlying lower permian mccloud limestone which records two stages of early permian carbonate platform development early wolfcampian and late wolfcampian to early leonardian only the younger of the two platforms is present within the study area and it contains more distal facies than those preserved to the south the strata contain abundant fusulinids and corals which provide age constraints for carbonate platform development and form the basis for regional biogeographic correlation the time transgressive upper permian bollibokka group nosoni and dekkas formations succeeds the mccloud limestone comprising thick sequences of volcanogenic debris intercalated lava and in the upper part lenses of shallow water limestone the upper permian strata reflect volcanism and rapid subsidence within the are all upper paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with upper permian basalts in the upper dekkas formation the upper paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern klamath arc mississippian epiclastic rocks reflect uplift of the volcanic arc basement which contained rocks similar to those now exposed in the adjacent yreka terrane or shoo fly complex pulses in volcanism were restricted to mid carboniferous and late permian time diminished volcanism during the early permian allowed development of carbonate platforms rapid fluctuations of sedimentation and sub sidence rates pronounced lateral facies and age variations accumulation of thick volcanic debris aprons during episodic volcanism and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs the stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates reflecting events and processes within the arc itself evidence for nearby basins or landmasses is sparse the mississippian epiclastic rocks however which were derived from the arc basement reflect an ultimate sialic crustal source comparison of volcanic arc evolution with the late paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo graphic ties between the eastern klamath volcanic arc and rocks of the golconda allochthon or the north american continent

integrated tectonic genetic model for volcanic hosted massive sulfide deposits volcanic hosted massive sulfide deposits exhibit a continuous range in metal composition from copper dominated cyprus type to lead rich zinc copper deposits kuroko type this spectrum is best viewed as a progressive enrichment in large ion lithophile elements lile in the ores e g pb ba and the magmatic systems e g k responsible for their generation thus lead rich volcanic hosted massive sulfide deposits are associated with calc alkalic magmatism in mature are systems e g buchans or in back arc sites where older continental crust can be implicated in magma genesis at the copper rich end of the spectrum ores are related to plagiogranites in supra subduction zone ophiolite complexes and lile enrichment is minimal the ancient back arc assemblages that now constitute greenstone belts host cu zn massive sulfide deposits associated with tholeiitic calc alkalic magmatic systems of intermediate lile enrichment i suggest that these variations in both the metal composition of such ores and the petrochemistry of coeval intrusions relate in a fundamental way to the siting of their emplacement in arc and back arc systems and the duration of the subduction process

heavy metal and sulfur transport during subcritical and supercritical hydrothermal alteration of basalt influence of fluid pressure and basalt composition and crystallinity crystalline basalt diabase and basalt glass have been reacted with a na ca k cl fluid of seawater ionic strength at 350 425 c 375 400 bars pressure and fluid rock mass ratios of 0 5 1 0 to assess the role of temperature basalt diabase chemistry and texture on heavy metal and sulfur mobility during hydrothermal alteration alteration of basalt diabase is characterized by cation fixation and hydrolysis reactions which show increased reaction progress with increasing temperature at constant pressure correspondingly ph in a series of 400 bar experiments ranges from 4 8 to 2 7 at 350 and 425 c respectively and is typically lower for alteration of a sio2 rich crystalline basalt than for other rock types due in part to relatively high sio2 concentrations in solution high sio2 concentrations stabilize hydrous na and ca rich alteration phases causing ph to decrease according to reactions such as 3 0 caal2si2o8 1 0 ca 2 0 h2o 2 0 ca2al3si3o12 oh 2 0 h phases experimentally produced include mixed layer chlorite smectite ca rich amphibole and clinozoisite clinozoisite was identified as a replacement product of plagioclase from diabase solution interaction experiments in direct response to h production dissolved fe mn and h2s concentrations increase dramatically for early stage reaction h2s typically exceeds fe and mn however at 425 c and after long term reaction at 400 c h2s is lost from solution apparently in response to pyrite replacement of oxide and silicate phases pyrrhotite formed at temperatures 375 c whereas magnetite was identified in all run products except from basalt glass alteration cu and zn concentrations in solution are not simple functions of ph these metals achieve greatest solubility in fluids from experiments at 375 400 c except when basalt glass is used as a reactant the relatively low concentrations of these species in solution during basalt glass reaction may be due to adsorption by fine grained alteration phases

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment

geology of the peralkaline volcano at pantelleria strait of sicily situated in a submerged continental rift pantelleria is a volcanic island with a subaerial eruptive history longer than 300 ka its eruptive behavior edifice morphologies and complex multiunit geologic history are representative of strongly peralkaline centers it is dominated by the 6 km wide cinque denti caldera which formed ca 45 ka ago during eruption of the green tuff a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls surges and pyroclastic flows soon after collapse trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it progressive chemical evolution of the chamber between 45 and 18 ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera beginning about 18 ka ago inflation of the chamber opened old ring fractures and new radial fractures along which recently differentiated pantellerite constructed more than 25 pumice cones and shields continued uplift raised the northwest half of the intracaldera trachyte cone 275 m creating the island s present summit montagna grande by trapdoor uplift pantellerite erupted along the trapdoor faults and their hingeline forming numerous pumice cones and agglutinate sheets as well as five lava domes degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18 3 ka episode led to entrainment of subjacent crystal rich pantelleritic trachyte magma as crenulate inclusions progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0 82 km3 of magma erupted during the episode