sulfur dioxide from nevado del ruiz volcano colombia total flux and isotopic constraints on its origin abstract nevado del ruiz volcano has been releasing extraordinarily large volumes of sulfur dioxide gas from its arenas crater since september 1985 and probably since the renewal of volcanic activity began in late 1984 the combined release from eruptive and passive flux has been approximately 3 4 1 0 10 6 metric tons in the four years after november 1984 this value combined with data of others on pre eruption sulfur content of the magma allows the calculation of a minimum required volume of magma of 0 92 km 3 consideration of the continued high flux and typical ratios of erupted to degassed magma indicate that the true minimum volume of magma involved is probably at least 4 6 9 2 km 3 a systematic study of the various species of sulfur in the volcanic plume and the hydrothermal system has provided a characterization of the geochemistry and sulfur isotopic variation consideration has been given to five potential sources of the sulfur dioxide dissolution of subvolcanic evaporites remobilization and oxidation of native sulfur within summit crater fumaroles large scale release from the hydrothermal system by reduction of sulfate or water rock interaction assimilation and oxidation of a pre existing sulfide deposit and magmatic volatiles the data are most realistically explained by the passage of magmatic gas through the hydrothermal system with some disproportionation of sulfur dioxide to produce the high sulfate content and low ph of the waters this model is also most consistent with the distribution of acid sulfate chloride hot springs high chloride and fluoride content of the hydrothermal waters sulfide deposition within the volcanic pile and the transient increase in sulfate content of the acid sulfate chloride hot springs that reached a maximum approximately one year after the november 1985 eruption the magmatic model is in apparent conflict with the absence of general deformation from the time that data began to be collected about one week before the eruption until the time this manuscript was finalized september 1989 although this conflict cannot be resolved with certainty it is noted that the absence of deformation is only an indication of the lack of magma movement not of the absence of magma an alternative magmatic model considers the available geological data and proposes that the system is related to a large scale caldera in this model the degassing is related to a long term process of exsolution of magmatic volatiles from a large volume of crystallizing magma left from the construction of modern ruiz this model can better explain the combined geological and geophysical data as well as the geochemical and isotopic data the nature of the degassing data at ruiz is interesting because it is an example of an explosive eruption cycle in which the gas flux is actually dominated by passive release the nature of the degassing allows us to hypothesize that some major change in the system occurred in april may 1986 and to predict that the present high flux can be expected to last for another decade

phase equilibrium constraints on the viscosity of silicic magmas 1 volcanic plutonic comparison by using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at 104 5 pa s irrespective of their temperature and level of emplacement magmas crystallizing as granitic plutons are not water poor and thus not more viscous than their extrusive equivalents instead comparison between pre eruption magma viscosities of extrusive silicic intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous given the typical strain rates in silicic magma chambers magma rheological behavior is expected to be dominantly newtonian bubbles having a minor rheological influence at depth although exceptions can exist thus whether a silicic intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors but not on the rheology of the magma itself however preemptive viscosities of extrusive magmas rarely exceed 106 pa s which suggests that crystal melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes the narrow range of viscosities displayed by silicic intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas considerations of the relationships between magma crystallinities bulk sio2 and preemptive melt h2o contents show that the higher the melt h2o content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted an empirical correlation is proposed that enables us to estimate preemptive melt h2o contents of erupted magmas by knowing their crystallinity and bulk sio2

cerro toledo rhyolite jemez volcanic field new mexico 40ar 39ar geochronology of eruptions between two caldera forming events the cerro toledo rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera forming ignimbrites the tshirege member and otowi member of the bandelier tuff in the jemez volcanic field new mexico to provide a chronologic framework for geochemical and isotopic studies on these rhyolites which record the evolution of the bandelier magma system during this interval a 40 ar 39 ar geochronology study was undertaken pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing bandelier tuff analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages in most cases dominant groups of juvenile sanidine crystals define 40 ar 39 ar ages which agree with stratigraphic constraints plagioclase analyses are distinctly more scattered and do not typically define reasonable ages the 40 ar 39 ar ages for the two members of the bandelier tuff yield an interval of 380 20 k y between these caldera forming eruptions during this interval nine major pyroclastic pumice units were deposited in the sections studied for which six yield isochron ages one a weighted mean age one a maximum age and one no reliable age due to lack of sanidine 40 ar 39 ar dates on pumice fall units within the cerro toledo rhyolite tephra indicate that eruptive activity occurred at 1 59 1 54 1 48 1 37 and 1 22 ma 40 ar 39 ar dating of cerro toledo rhyolite domes indicates these were erupted within the caldera at 1 54 1 45 1 38 1 34 and 1 27 ma the dates obtained indicate that eruptive activity occurred throughout the 380 k y interval between the two members of the bandelier tuff but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca 1 54 1 48 and 1 38 1 34 ma the interval from 1 34 to 1 38 ma was particularly active 7 of 18 units dated are these ages

textural constraints on effusive silicic volcanism beyond the permeable foam model transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit one way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows in this paper we present textural observations and isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions after magma has reached the earth s surface this view is in contrast to the widely promoted permeable foam model which states that all volatiles escape during ascent of the magma and that all dense glassy material in lava flows forms from the collapse of pumiceous lava i e that silicic lavas emerge as highly inflated foam flows such interpretations which claim that silicic lavas are completely degassed upon extrusion and that all degassing must take place on the time scale of the eruption neglect several important pieces of evidence including the presence of obsidian in extremely small domes and of vesicular zones in the interiors of silicic flows the copious loss of volatiles through eruption plumes between eruptive phases and direct observations of surface vesiculation during growth of the mount st helens lava dome the permeable foam model also implies the unlikely requirement that explosive to effusive transitions be associated with an increase in eruption rate we present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent as well as late stage vesiculation we then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days weeks or months after the lava emerges from the vent

experimental investigation of h2o and cl solubilities in f enriched silicate liquids implications for volatile saturation of topaz rhyolite magmas to interpret the degassing of f bearing felsic magmas the solubilities of h2o nacl and kcl in topaz rhyolite liquids have been investigated experimentally at 2000 500 and 1 bar and 700 to 975 c chloride solubility in these liquids increases with decreasing h2o activity increasing pressure increasing f content of the liquid from 0 2 to 1 2 wt f and increasing the molar ratio of al na ca mg si small quantities of cl exert a strong influence on the exsolution of magmatic volatile phases mvps from f bearing topaz rhyolite melts at shallow crustal pressures water and chloride bearing volatile phases such as vapor brine or fluid exsolve from f enriched silicate liquids containing as little as 1 wt h2o and 0 2 to 0 6 wt cl at 2000 bar compared with 5 to 6 wt h2o required for volatile phase exsolution in chloride free liquids the maximum solubility of cl in h2o poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of h2o in chloride poor liquids by simple linear and negative relationships there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the mvp s plots of h2o versus cl in rhyolite liquids for experiments conducted at 500 bar and 910 930 c show a distinct 90 break in slope pattern that is indicative of coexisting vapor and brine under closed system conditions the presence of two mvps buffers the h2o and cl concentrations of the silicate liquids comparison of these experimentally determined volatile solubilities with the pre eruptive h2o and cl concentrations of five north american topaz and tin rhyolite melts determined from melt inclusion compositions provides evidence for the exsolution of mvps from felsic magmas one of these the cerro el lobo magma appears to have exsolved alkali chloride bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption

silicate melt inclusion evidence for extreme pre eruptive enrichment and post eruptive depletion of lithium in silicic volcanic rocks of the western united states implications for the origin of lithium rich brines to evaluate whether anatectic and or highly fractionated lithophile element enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption lithification and weathering to generate significant li brine resources pre eruptive melt compositions preserved in inclusions and the magnitude of post eruptive li depletions evident in host rhyolites were documented at six sites in the western united states each rhyolite is a member of the bimodal basalt rhyolite assemblage associated with extensional tectonics that produced the basin and range province and rio grande rift an evolving pattern of closed drainage basins and geothermal energy or mineral resources results from the 0 8 ma bishop tuff geothermal in california 1 3 to 1 6 ma cerro toledo and upper bandelier tephra geothermal and 27 9 ma taylor creek rhyolite sn in new mexico 21 7 ma spor mountain tuff be u f and 24 6 ma pine grove tuff mo in utah and 27 6 ma hideaway park tuff mo in colorado support the following conclusions melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of sn mo and be are extremely enriched in li 1 000s of ppm those from spor mountain have the highest li abundance yet recorded max 5 200 ppm median 3 750 ppm forty five to 98 of the li present in pre eruptive magma was lost to the environment from these rhyolite tuffs the amount of li lost from the small volumes 1 10 km3 of li enriched rhyolite deposited in closed basins is sufficient to produce world class li brine resources after each eruption meteoric water leaches li from tuff which drains into playas where it is concentrated by evaporation the localized occurrence of li enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of li considering that hydrothermal deposits of sn mo be u and f may indicate potential for li brines in nearby basins we surmise that the world s largest li brine resource in the salar de uyuni 10 mt received li from nearby rhyolite tuffs in the bolivian tin belt

preeruptive magma viscosity an important measure of magma eruptibility 1 using a compilation of melt compositions meltwater contents temperatures and phenocryst contents the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas the basaltic to rhyolitic magmas have preeruptive viscosities over the range 101 to 108 pa s although bulk sio2 content has often been used as a qualitative measure of preeruptive magma viscosity the bulk sio2 content shows a weak correlation with magma viscosity correlation coefficient r 0 5 because of a wide range of phenocryst contents from 0 to 50 vol andesitic magmas have viscosities ranging from 102 to 107 pa s which are lower or higher than those of phenocryst poor rhyolitic magmas with 105 to 106 pa s focusing on andesitic to rhyolitic magmas the r between bulk sio2 contents and magma viscosities changes to 0 1 in contrast the melt only sio2 content from a basaltic to rhyolitic melt shows a good linear correlation with melt only viscosity r 0 9 although most of the calculated viscosities of erupted magmas fall below 106 pa s as consistent with the previous compilation study this paper describes 20 examples of highly viscous magmas with 106 pa s in most cases composed of mixtures of high silica rhyolitic melt 75 79 wt sio2 and abundant phenocrysts 30 55 vol in these highly viscous magmas 9 examples have erupted following the precursory eruption of less viscous magma suggesting that precursory dike propagation and conduit formation by the less viscous magma with 106 pa s induced the following eruption of less eruptible highly viscous magmas

light lithophile element boron lithium abundances in the valles caldera new mexico vc 1 core hole boron and lithium abundance distributions are very different in the sedimentary and volcanic sections of the vc 1 core hole valles caldera new mexico in the sedimentary section the wide variations in boron and lithium abundances probably reflect both diagenesis and introduction of these elements in early hydrothermal fluids late higher temperature fluids appear to have leached boron and lithium from some rocks adjacent to fault zones and aquifers the correlation of boron with lithium suggests that these elements behave in similar ways during diagenesis and hydrothermal alteration the small variation of boron and lithium abundances in most of the vitreous volcanic rocks from the upper part of the core implies that their source was relatively homogeneous following primary devitrification boron appears to have been stable but lithium was mobile boron may have been stabilized by the development of boron bearing submicroscopic phenocrysts in the lavas or boron may be less soluble than lithium in relatively cool groundwater

volatile composition of the phonolitic laacher see magma 12 900 yr bp implications for syn eruptive degassing of s f cl and h 2 o a volatile rich and chemically zoned phonolitic magma reservoir was tapped successively during the eruption of laacher see volcano ca 12 900 years bp and produced a tephra sequence consisting of phenocryst poor highly evolved phonolite at the base and phenocryst rich more mafic phonolite at the top the stratospheric volatile loading was estimated by comparing pre and post eruptive s f cl and h2o contents of undegassed glass inclusions and partially degassed matrix glasses glass inclusions 150 1490 ppm s and host matrix glasses 150 820 ppm s both document a strong s decrease during progressive magmatic differentiation which is interpreted to be partially caused by crystallization of hauyne the s6 stotal ratio of the pre eruptive melt increased with differentiation from 8 to 71 as indicated by s k wavelength shift measurements in glass inclusions sulfate rich highly evolved phonolitic magma was erupted during plinian and sulfide rich more mafic phonolitic magma during late phreatomagmatic phases f and cl became enriched during late stages of differentiation glass inclusions 690 4060 ppm f 1770 4400 ppm cl matrix glasses 680 3660 ppm f 2130 4330 ppm cl the most differentiated melts maximum 13 wt na2o occur only as matrix glass and are extremely f enriched 5080 8780 ppm but cl depleted 460 2820 ppm suggesting that f was retained in the melt whereas some cl was lost during pre and or syn eruptive degassing the h2o contents of glass inclusions increase irregularly with differentiation 2 5 5 7 wt matrix glasses are h2o depleted 0 2 2 8 wt compared to most glass inclusions showing that most h2o was released to the atmosphere by explosive degassing a mass balance calculation yields a syn eruptive volatile release of 1 9 tg stotal 6 6 tg cl and 403 tg h2o from the melt this is a minimum estimate since s and cl could have accumulated in a separate fluid phase as indicated by fluid inclusions in hauyne and pre eruption h2o contents close to saturation level at the likely pressure temperature conditions in the laacher see magma reservoir we estimate that at least ca 20 tg so2 were injected into the stratosphere causing a significant negative climate forcing as reflected by several paleoclimate proxies and as shown by recent modeling

variations in trace element partition coefficients in sanidine in the cerro toledo rhyolite jemez mountains new mexico effects of composition temperature and volatiles trace element partition coefficients have been measured for one plagioclase and five sanidine mineral separates from the cerro toledo rhyolite new mexico sanidine partition coefficients vary substantially and systematically within the cerro toledo rhyolite partition coefficients for ca sr zn la and eu are lowest in the most evolved rhyolites whereas sm and hree partition coefficients are highest rubidium partition coefficients remain constant while those for ba ce and th are variable variations of the sr zn la and eu partition coefficients are correlated with the ca contents and partition coefficients of the sanidines calcium may have controlled the distribution of these elements in the sanidine by modifying the feldspar structure the low ca partition coefficients in sanidines for the most evolved rhyolites may be the consequence of modification of the melt structure possibly due to increased volatile contents at the top of the magma chamber s during evolution of the cerro toledo rhyolite the zn and la partition coefficients between sanidine and melt also may have been controlled by this change in melt structure modelling using major elements and the constant rb partition coefficient for sanidine indicates 70 crystallization of magma during cerro toledo rhyolite time by removal of 68 sanidine and 32 quartz estimates of the volume 1 of initial parental magma and 2 of the magma that crystallized during this period are 11 670 km3 and 8170 km3 respectively the average intrusion rate of silicic magma during cerro toledo rhyolite activity was 35 10 3 km3 a

halogens in volcanic systems the transport degassing and atmospheric release of halogens from active volcanism on earth have been the focus of increasing interest over the last few decades and have recently been the subject of the 1st workshop on halogens in volcanic systems and their environmental impacts that was held in december of 2007 at yosemite lodge in yosemite national park california as an introduction to this chemical geology special issue collecting contributions from many of the participants at the workshop we review here recent advances in this field including experimental and theoretical investigations of halogen behaviour in volcanic and related magmatic systems we discuss previous research on several aspects of halogen geochemistry including halogen abundances in the mantle and magmas on earth the effects of halogens on phase equilibria and melt viscosities their partitioning between melt and fluid phase s upon decompression cooling and crystallisation of magmas in the earth s crust and their final atmospheric release as volcanic gases the role of halogens in the genesis of hydrothermal systems and in the transport of ore forming metals is also reviewed and we discuss our current understanding of atmospheric processing of volcanic halogens in both the troposphere and stratosphere and their consequent impacts in spite of these recent advancements our current understanding of halogen geochemistry at active volcanoes is still far too fragmentary and the key questions that require answers from future research are summarised in our conclusions

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

gas saturation and evolution of volatile and light lithophile elements in the bandelier magma chamber between two caldera forming eruptions we have studied the distribution of light and volatile elements li be b h2o f cl in matrix glasses and melt inclusions in quartz and sanidine phenocrysts from tephras of the cerro toledo rhyolite which were erupted from the bandelier magma chamber between two caldera forming eruptions at 1 608 ma and 1 225 ma since the small volume tephras of the cerro toledo rhyolite record chemical evolution of the upper parts of the magma chamber during this 383 kyr period we can track volatile evolution in the magma during its differentiation specifically the development of a separate fluid phase from the magma the partitioning of volatile and light elements between this fluid and the magma and the timing of the second caldera eruption at 1 225 ma the melt inclusion data reveal that li be and b were progressively enriched with time at the top of the magma chamber showing a similar three fold enrichment by contrast f cl and f2o show more variable behavior f was enriched 3 6 times in the magma and cl increased 2 5 times while water concentrations remained constant evidence for a separate fluid phase includes 1 comparatively small magmatic enrichment factors for cl and h2o elements which are known to partition into fluid 2 decline of the cl be and cl b ratios in melt inclusions with time indicating preferential loss of cl and 3 maximum cl contents of 2900 3000 ppm in melt inclusions which are concentrations at or close to the saturation limit for cl in a two phase gas saturated magma the lack of temporal h2o enrichment in the magma suggests that it became gas saturated at an early stage the cl fluid magma partition coefficient was initially low due to the presence of a low salinity single phase vapor the fluid is calculated to have increased its salinity from 0 76 wt equivalent nacl to 3 1 wt nacl by crystallization and cl enrichment in the magma consequently the fluid was transformed to two immiscible phases as a result the cl partition coefficient increased and the cl content in the magma was buffered at 2900 3000 ppm overpressures in the magma chamber have been modeled as a function of crystallization total pressure and depth h2o solubility in rhyolite resurgence crystallization contraction and initial fluid mass for the large amounts of crystallization 40 70 required by the geochemical relations unrealistically large overpressures 75 mpa are indicated unless 1 the magma existed as a foam or 2 the chamber was able to degas passively and thus buffer the buildup of pressure to maintain realistic overpressures in the magma chamber 50 mpa an equilibrium was required between gas buildup by crystallization and volatile loss by passive degassing the change from a one phase vapor to a two phase immiscible fluid before the upper bandelier tuff eruption may have promoted partial or complete sealing of the magmatic hydrothermal system permitting significant overpressures to develop the result could have been a cataclysmic caldera forming eruption

beryllium systematics in young volcanic rocks implications for 10 be beryllium is an incompatible trace element that closely parallels neodymium in its geochemical behavior be analyses conducted on well characterized oceanic and arc volcanic rock suites as well as on marine sediments suggest a bulk solid liquid distribution coefficient of 0 03 0 06 for melting of the mantle and crystallization of basalts the be nd ratio for many volcanic rocks from diverse tectonic environments is approximately 05 similar to the ratio in chondrites be data for samples from volcanic arcs show that there are significant variations in 10be 9be among different arcs and that variations in 10be are not due to variations in be concentration alone for at least one volcano bogoslof the 10be 9be ratio is constant for samples that vary by a factor of three in both their be and 10be concentrations suggesting that 10be is an inherited magmatic signature and not simply a result of contamination near the surface in addition the be nd and pb isotope systems for this volcano are all consistent with a model in which small amounts of sediment were incorporated into the bogoslof source region provided the mantle wedge has the isotopic characteristics of depleted morb since 10be exists only in the uppermost tens of meters of oceanic sediments the data suggest an efficient return flux of sediment to the mantle at subduction zones

water solubility and chlorine partitioning in cl rich granitic systems effects of melt composition at 2 kbar and 800 c abstract experiments have been conducted in the system na2o k2o al2o3 sio2 h2o cl2o 1 at 2 kbar and 800 c to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of cl between aqueous fluid and haplogranite melt the melts studied include peraluminous subaluminous and peralkaline haplogranites and quartz albite and quartz orthoclase compositions the solubility of water at 2 kbar and 800 c in water saturated haplogranite melts varies strongly as a function of melt composition water solubility in melt decreases in the order strongly peralkaline haplogranite quartz albite composition moderately peralkaline haplogranite peraluminous haplogranite subaluminous haplogranite quartz orthoclase composition d cl wt cl in fluid wt cl in melt is a strong function of system composition d cl increases as the concentration of cl in the system increases a comparison of the distribution of cl between haplogranite melts of subaluminous peraluminous and peralkaline compositions and aqueous fluids shows that with similar concentrations of cl in the system d cl is largest for subaluminous melts a nk 1 having relatively high k2o concentrations as the a nk of melt increases or decreases from 1 or as the n nk of the melt increases both d cl and the concentration of cl in the fluid decrease the experimental data imply that complexes involving cl and na in hydrous subaluminous granite melts are dominant furthermore the data imply that in hydrous peraluminous granite melts cl complexes with network modifying a13 and in hydrous peralkaline granite melts cl complexes with network modifying na k on a molar basis cl apparently complexes with network modifying a13 in peraluminous melts as strongly as it does with na and k in peralkaline melts experimental studies of cl partitioning in granitic systems suggest that the strongest enrichment of magmatic hydrothermal fluids in cl and ore metals complexed with cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in k2o sio2 and cl and relatively depleted in f co2 and na2o

changes in silicic melt structure between the two bandelier caldera forming eruptions new mexico usa evidence from zirconium and light rare earth elements

discussion on metallogenic model for sandstone hosted uranium deposits in northeastern ordos basin the complexity diversity and imperfect comparability of metallogenic characteristics result in the situation that the same metallogenic conditions may not form completely identical ore deposits and a metallogenic model based on one deposit does not have sufficient representativeness therefore in prognosis and evaluation of diffe rent ore deposits in different areas relevant metallogenic models should be established based on previous researches on the metallogenic model for sandstone hosted uranium deposits in northeastern ordos basin the authors studied in detail the uranium mineralization and ore controlling regularities of that area a four phase uranium metallogenic model has thus been established the formation of tabular ore bodies of phreatic oxidation zone late zhiluo epoch in middle jurassic the interlayer oxidation in the early stage and the formation of major roll front ore bodies late zhiluo epoch in middle jurassic to early yijinhuoluo epoch in early cretaceous oil and gas reduction and ore preservation late yijinhuoluo epoch to early dongsheng epoch in early cretaceous and the transformation and superposition of ore bodies of the interlayer oxidation zone in the late stage miocene to the present studies show that there exist two stages of old interlayer oxidation zone and one stage of new interlayer oxidation zone in the study area and that where two or three interlayer oxidation zones exist in the same place the uranium ore body usually becomes large and rich it is hence held that prospective areas seem to be such places as both sides of the paleo river course of zhiluo formation the upward position where groundwater meets and the sectors of well developed faults

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

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

intracaldera volcanic activity toledo caldera and embayment jemez mountains new mexico the toledo caldera was formed at 1 47 0 06 ma during the catastrophic eruption of the lower member bandelier tuff the caldera was obscured at 1 12 0 03 ma during eruption of the equally voluminous upper member of the bandelier tuff that led to formation of the valles caldera earlier workers interpreted a 9 km diameter embayment located ne of the valles caldera toledo embayment to be a remnant of the toledo caldera drill hole data and new k ar dates of toledo intracaldera domes redefine the position of toledo caldera nearly coincident with and of the same dimensions as the younger valles caldera the toledo embayment may be of tectonic origin or a small tschicoma volcanic center caldera this interpretation is consistent with distribution of the lower member of the bandelier tuff and with several other field and drilling related observations explosive activity associated with cerro toledo rhyolite domes is recorded in tuff deposits located between the lower and upper members of the bandelier tuff on the northeast flank of the jemez mountains recorded in the tuff deposits are seven cycles of explosive activity most cycles consist of phreatomagmatic tuffs that grade upward into plinian pumice beds a separate deposit of the same age and consisting of pyroclastic surges and flows is associated with rabbit mountain located on the southeast rim of the valles toledo caldera complex these are the surface expression of what may be a thicker more voluminous intracaldera tuff sequence the combined deposits of the lower and upper members of the bandelier tuff toledo and valles intracaldera sediments tuffs and dome lavas form what we interpret to be a wedge shaped caldera fill this sequence is confirmed by deep drill holes and gravity surveys this fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse we interpret the toledo valles caldera complex to be a pair of nearly coincident trapdoor calderas with the hinge on the west side and thick caldera fill in the east

internal geology and evolution of the redondo dome valles caldera new mexico deep geothermal drilling in the resurgent redondo dome of the valles caldera has allowed us to define a consistent intracaldera stratigraphic sequence that differs in a number of respects from the temporally equivalent sequence outside the caldera above the deeply eroded pliocene paliza canyon formation felsic ash flows and sediments in the dome area form a complex sequence of undetermined age that we call the lower tuffs an erosional interval separates these rocks from the overlying otowi member 1 4 m y of the bandelier tuff another period of erosion during which a tuffaceous sandstone was deposited separates the otowi from the overlying tshirege member 1 1 m y of the bandelier both the otowi and tshirege members with maximum thicknesses of 833 and 1155 m respectively are substantially thicker within the caldera than outside indicating simultaneous deposition and cauldron subsidence both are predominantly densely welded with distinctive interior zones of granophyric crystallization resurgent doming was initiated after emplacement of the tshirege member as evidenced by erosion of the upper portions of the cooling unit prior to deposition of overlying units an isopach map of the sandstone deposited during this erosional period shows that streams draining the uplifting dome were localized along the present redondo creek trend subsequent volcanic activity resulted in the formation of at least three additional ash flow tuff cooling units prior to deposition of caldera fill and eruption of the redondo creek member of the valles rhyolite a numerical model applied to formation of the redondo dome suggests that the top of the causative magma body is located at a depth of about 4700 m 1458 m beneath the bottom of the deepest geothermal well in the dome no wells have penetrated intrusives that could be related to this magma we suggest that the locations of faults bounding the apical graben of the dome were influenced by older faults associated with the northeast trending jemez lineament these faults were active early in the uplift history of the dome and account for many of the structural differences between hypothetical dome development and reality

a model of degassing at galeras volcano colombia 1988 1993 galeras volcano reactivated in early 1988 as evidenced by increased seismicity gas emissions and fumarole temperatures and had a series of explosive eruptions in may 1989 during 1989 1990 the volcano was characterized by high so 2 flux reaching 5000 t metric tons day this period of strong degassing was followed by upward movement of magma july september 1991 and the emplacement of a lava dome october november 1991 since november 1991 long period seismicity and so 2 flux have decreased indicating that the conduit has partially sealed itself probably by crystallization and solidification of the magma on july 16 1992 the dome was destroyed by an explosion consistent with pressurization due to sealing and degassing in a nonporous medium continued pressurization beneath the volcano is indicated by eruptions on january 14 march 23 april 4 april 13 and june 7 1993 glass inclusion and matrix glass analyses support a hypothesis of significant magma degassing at depth in the magma chamber and in the conduit this degassing has reduced the h 2 o s and cl contents of the magma but the f content appears to have increased due to crystallization of anhydrous mineral phases and to the low fluid melt partition coefficient for f despite its degassed state galeras is currently a dangerous volcano because the sealing process forces gas saturated magma to release volatile components into confined spaces thereby causing pressurization and explosive eruptions

the behavior of light lithophile and halogen elements in felsic magma geochemistry of the post caldera valles rhyolites jemez mountains volcanic field new mexico abstract post collapse rhyolite lava domes lava flows and pyroclastic rocks from valles caldera 1140 ka erupted from 1133 ka to approximately 520 60 ka have been sampled to study variations of light lithophile li be b and halogen f cl elements our principal objectives were 1 to examine the mobility of these elements during post eruptive devitrification and hydration and 2 to study their behavior during magma differentiation compared to fresh glassy samples devitrified rocks from the same dome are depleted in b li f and cl but not in be during devitrification be was immobile while the other elements were progressively more mobile in the order b

micro analytical perspectives on the bishop tuff and its magma chamber new in situ major and trace element analytical data are presented for crystals sanidine plagioclase biotite orthopyroxene clinopyroxene and matrix glasses from juvenile materials representing the full bishop tuff sequence from the earliest fall unit f1 to the latest ignimbrite package ig2nc these data are combined with published information to investigate the nature and zonation of the pre eruptive bishop magma chamber our data confirm that this magma chamber was a single unitary body that was thermally and compositionally zoned the zonation was largely established prior to the growth of crystals and also prior to mixing in the lower parts of the chamber induced by late stage intrusion of a magma of contrasting composition and slightly higher temperature the bright rim magma sparse mixed swirly and dacitic pumices show enrichments in ba sr and ti that identify these pumices as possible representatives of the bright rim magma a model revised from previously published work for the pre eruptive magma chamber comprises three main parts 1 an upper volumetrically dominant 2 3 relatively unzoned region that was the source of the earlier eastern erupted ignimbrite units and their coeval fall units 2 a volumetrically minor transition zone that shows evidence for minor degrees of mixing and was the dominant source for the latest eastern erupted part of ig1eb sherwin subunit and the earlier part of the northern erupted ignimbrite ig 2na 3 a lower volumetrically subordinate 1 3 region that was affected by mixing with the bright rim invasive magma in the lead up to the eruption and fed later northern erupted units ingress of the bright rim magma introduced orthopyroxene and bright rimmed zircon crystals and induced partial resorption then overgrowth of rims enriched in ti sr and ba on sanidine and quartz and development of zoning in clinopyroxene based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence we infer that the roof and floor of the magma chamber were stepped down to the north such that the transition zone magma formed the floor of the southern part of the melt dominant chamber and the roof of the northern part our data reinforce the previous concept of a single compositionally and thermally zoned bishop magma chamber and additionally support a temporally constrained role for pre eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident bishop magma

partitioning of f between h2o and co2 fluids and topaz rhyolite melt implications for mineralizing magmatic hydrothermal fluids in f rich granitic systems fluid melt distribution coefficients for f have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of h2o and h2o co2 at pressures of 0 5 to 5 kbar temperatures of 775 1000 c and concentrations of f in the melt ranging from 0 5 to 6 9 wt the major element f and cl concentrations of the starting material and run product glasses were determined by electron microprobe and the concentration of f in the fluid was calculated by mass balance the h2o concentrations of some run product glasses were determined by ion microprobe sims the solubility of melt in the fluid phase increases with increasing f in the system the solubility of h2o in the melt is independent of the f concentration of the system with up to 6 3 wt f in the melt no evidence of immiscible silica and fluoriderich liquids was detected in the hydrous but water undersaturated starting material glasses 8 5 wt f in melt or in the water saturated run product glasses f concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied however df wt f in fluid wt f in melt increases strongly with increasing f in the system maximum values of df in this study are significantly larger than those previously reported in the literature linear extrapolation of the data suggests that df is greater than one for water saturated peraluminous granitic melts containing 8 wt f at 800 c and 2 kbar df increases as temperature and as h2o h2o co2 of the fluid increase for topaz rhyolite melts containing 1 wt f and with h2o rich fluids df is independent of changes in pressure from 2 to 5 kbar at 800 c for melts containing 1 wt f and in equilibrium with co2 bearing fluids the concentrations of f in fluid increases with increasing pressure f and lithophile element enriched granites may evolve to compositions containing extreme concentrations of f during the final stages of crystallization if f in the melt exceeds 8 wt df is greater than one and the associated magmatic hydrothermal fluid contains 4 molal f such f enriched fluids may be important in the mass transport of ore constituents i e f mo w sn li be rb cs u th nb ta and b from the magma

replenishment and crystallization in epicontinental silicic magma chambers evidence from the bandelier magmatic system abstract the cerro toledo rhyolite jemez volcanic field new mexico records the volcanism that occurred after the eruption of the lower bandelier tuff ignimbrite which formed toledo caldera at 1 51 0 03 ma pumice samples from the top of this ignimbrite and from the base of the cerro toledo rhyolite tephra sequence are stratigraphically and geochemically distinct from younger cerro toledo rhyolite tephras that overlie this basal suite all samples exhibit decreasing la lu with increasing lu concentrations for a given lu content however the lower bandelier tuff and basal member cerro toledo rhyolites have higher la lu compared to overlying cerro toledo rhyolites this difference can be explained either by 1 differentiation of a single batch of magma in which trace mineral solubilities increased with time due to higher volatile contents or 2 replenishment of the magma chamber by an input of silicic magma after caldera collapse at 1 51 ma large scale replenishment of epicontinental silicic magma chambers may be restricted to timescales of 10 5 years

beryllium and boron in subduction zone minerals an ion microprobe study secondary ion mass spectrometry was used to measure the distribution of be b rb sr y nb ba and ce among minerals in rocks from the franciscan complex and catalina schist paleosubduction zone terranes the rocks examined represent a wide range of metamorphic pressure temperature conditions 6 15 kb 300 750 c and include metasomatized and migmatitic samples white mica is the primary host of be 2 5 ppm and b 107 ppm in the rocks examined average concentrations amphibole be 0 7 b 15 ppm lawsonite be 0 9 b 37 ppm and biotite be 0 9 b 25 ppm may also contain significant amounts of both be and b omphacite may host be 4 1 ppm in veins 0 7 ppm groundmass and chlorite is a minor host of b 22 ppm minerals in these subduction zone metamorphic rocks that lack measurable be or b include albite apatite calcite clinozoisite epidote garnet ilmenite sphene stilpnomelane plagioclase quartz rutile and zoisite white mica biotite and stilpnomelane are the major mineral hosts of rb 90 300 and 30 ppm respectively and ba 2100 1000 and 4700 ppm epidote and lawsonite are the major host minerals of sr 600 and 300 ppm and ce 12 and 15 ppm garnet epidote lawsonite and sphene host y 100 30 40 and 60 ppm respectively sphene 77 ppm 1 analysis ilmenite and rutile host nb evidence of veining and metasomatism is strongly correlated with be and or b enrichment of white mica omphacite amphibole and lawsonite at the p t conditions examined both be and b appear to be transported in aqueous fluids at depths of 20 50 km in subduction zones the transfer of be b and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure dependent dehydration reactions or alternatively the be and b in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis

fluid immiscibility in volcanic environments volatile components enter the atmosphere oceans and other surface waters through pre syn and post eruptive volcanic processes that involve a variety of fluids aluminosilicate poor fluids include aqueous or carbonic to sulfide sulfate chloride fluoride carbonate and phosphate rich compositions in volcanic environments but other more complex combinations of these constituents may be involved giggenbach 1977 roedder 1984 1992 lowenstern 1995 other rare gases and dissolved constituents e g h2 n2 he ar h3bo3 hg ch4 and other hydrocarbon compounds metals and metalloids are also present but in general they are not sufficiently concentrated to form their own phases or control bulk fluid composition and volcanic processes as a result of their diverse compositions volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity dielectric behavior viscosity and other chemical and physical characteristics geiger et al 2006a the density of a saline liquid in the system nacl h2o at 50 mpa and 800 c for example is more than 7 times that of the coexisting aqueous vapor e g 0 2 gm cm3 henley and mcnabb 1978 multiple fluids move mix and or unmix in magma chambers and volcanic conduits in the root zone of fumaroles and in the convective hydrothermal systems that are ubiquitous to volcanic environments hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant shallow crustal pressure temperature composition conditions volcanic fluids occur at supersolidus to subsolidus conditions meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present this chapter addresses the evidence for and occurrence of two or more low density aluminosilicate poor fluids

first insights on the metallogenic signature of magmatic fluids exsolved from the active magma chamber of vesuvius ad 79 pompei eruption abstract fluid inclusions hosted in sanidine from xenoliths representing the crystallization front of the ad 79 magma chamber of vesuvius offer a rare opportunity to track the composition of magmatic fluids exsolving at the peripheral part of a growing k alkaline magma chamber disrupted by an explosive eruption this work demonstrates that the exsolving magmatic fluids have a complex composition taking in evidence the great capability of metal transport by means of these solutions inclusions are particularly enriched in zn mo w sb bi tl and pb metals display two different positive correlations 1 pb zn tl bi 2 mo w the different trends may reflect the different complexing agent for these elements probably chloride for pb zn tl bi and probably hydroxy for mo w the occurrence of a silicothermal component in these fluids may furthermore favor the metal transport of these solutions this is probably a major previously underestimated process in the enrichment of some metals in magmatic hydrothermal systems this hypothesis may challenge conventional ideas on fluid phase mass transport of components by means of magmatic hydrothermal fluids and should be taken into account in future works on these topics