uplift and submarine formation of some melanesian porphyry copper deposits stable isotope evidence hydrogen and oxygen isotope analyses of sericites and kaolinites from four young porphyry copper deposits ok tedi 1 2 ma and yandera 6 5 ma papua new guinea koloula 1 5 ma solomon islands and waisoi 5 ma fiji indicate that the fluids from which these minerals precipitated were of mixed magmatic and non magmatic sources the non magmatic component of the fluid from the island arc deposits koloula waisoi was ocean water for ok tedi the non magmatic component was a meteoric water with an isotopic composition different from that of the present meteoric water in the region the isotopic signature of the former meteoric water is consistent with a surface elevation of 200 m a s l or less at the time of mineralization the deposit was later exposed and supergene kaolinitization commenced at approximately 1200 m a s l uplift and erosion has continued to the present at which time the elevation of the exposed deposit is 1800 m a s l this rate of uplift is consistent with that known from other geological evidence if the rate of uplift were approximately constant during the last 1 2 ma the age of supergene enrichment can be dated at approximately 0 4 ma b p similarly influx of meteoric water at yandera occurred when the ground surface above the deposit was at an elevation of approximately 600 m a s l the deposit s present elevation is 1600 m a s l in this case a total uplift of approximately 2 2 km is indicated with removal of 1 2 km of overburden by erosion

geology of the don manuel igneous complex central chile implications for igneous processes in porphyry copper systems the don manuel igneous complex and associated porphyry copper mineralization in the andean cordillera of central chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics the don manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes which intrude the older units mineralization is associated with the biotite tonalite and intermediate porphyries which also contain the greatest abundance of mafic enclaves zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing new zircon u pb ages and whole rock ar ar ages indicate that the don manuel igneous complex was emplaced between ca 4 and 3 6 ma the time scale for the episodic intrusion of the don manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes observations from the don manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement the observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation

rapid thermal rejuvenation of high crystallinity magma linked to porphyry copper deposit formation evidence from the koloula porphyry prospect solomon islands magmas containing the components needed to form porphyry copper deposits are relatively common within arcs yet mineralising events are uncommon within the long lived magmatic systems that host them understanding what causes the transition from barren to productive intrusions is critical to the development of conceptual deposit models we have constrained the tempo of pre and syn mineralisation magmatic events in relationship to the thermal evolution of the plutonic body that underlies one of the world s youngest exposed plutonic porphyry systems the inamumu zoned pluton koloula porphyry prospect solomon islands high precision id tims u pb dates of texturally and chemically characterised zircons indicate pluton emplacement over 50 crystals state past the point of rheological lock up we estimate that thermal rejuvenation of the deeper high crystallinity magma and generation of a mobile melt fraction may have occurred 10 kyr before its transport and emplacement within the porphyry environment the underlying pluton likely cooled and returned to high crystallinity states prior to subsequent remobilisation emplacement events titanium in zircon geothermometry and whole rock geochemistry suggest pre mineralisation intrusions were remobilised by mixing of a silicic magma with a high temperature less evolved melt in contrast syn mineralisation melts were most likely remobilised by the percolation of hot volatiles exsolved from contemporaneous less evolved intrusions cooling beneath the crystalline silicic magma the evidence for the rapid thermal rejuvenation and long term storage of highly crystalline silicic magmas is consistent with previous studies that indicate two components of exsolved volatiles contribute to ore forming fluids we conclude that the liberation of crystal rich porphyry copper deposit forming magmas and the addition of the chemical components required for ore formation are intrinsically linked to the volatiles released during the recharge of less evolved melt into a highly crystalline silicic magma

magma evolution and the formation of porphyry cu au ore fluids evidence from silicate and sulfide melt inclusions silicate and sulfide melt inclusions from the andesitic farallon negro volcanic complex in nw argentina were analyzed by laser ablation icpms to track the behavior of cu and au during magma evolution and to identify the processes in the source of fluids responsible for porphyry cu au mineralization at the 600 mt bajo de la alumbrera deposit the combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle derived mafic magma that contained approximately 6 wt h2o and 200 ppm cu this magma and a rhyodacitic magma mixed in an upper crustal magma chamber feeding the volcanic systems and associated subvolcanic intrusions over 2 6 million years generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps 1 generation of a sulfide melt upon magma mixing in some parts of the magma chamber 2 partitioning of cu and au into the sulfide melt enrichment factor of 10 000 for cu leading to cu and au concentrations of several wt or ppm respectively 3 a change in the tectonic regime from local extension to compression at the end of protracted volcanism 4 intrusion of a dacitic magma stock from the upper part of the layered magma chamber 5 volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber generating a fluid with a cu au ratio equal to that of the precursor sulfide 6 focused fluid transport and precipitation of the two metals in the porphyry yielding an ore body containing au and cu in the proportions dictated by the magmatic fluid source the cu s ratio in the sulfide melt inclusions requires that approximately 4 000 ppm sulfur is extracted from the andesitic magma upon mixing this exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for s the extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma it appears thus that unusually sulfur rich not cu rich magmas are the key to the formation of porphyry type ore deposits our observations imply that dacitic intrusions hosting the porphyry cu au mineralization are not representative of the magma from which the ore fluid exsolved the source of the ore fluid is the underlying more mafic magma and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated at alumbrera these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole sulfide inclusions in similar dykes of other less explored magmatic complexes may be used to predict the au cu ratio of potential ore forming fluids and the expected metal ratio in any undiscovered porphyry deposit

the behavior of cu zn and pb during magmatic hydrothermal activity at merapi volcano indonesia abstract understanding the behavior of cu zn and pb in magmatic hydrothermal systems is essential in developing models for the genesis of hydrothermal ore deposits in subduction environments a commonly held view is that the metals originate in magmas of intermediate to felsic composition from which they partition directly into exsolving aqueous fluids in this paper we build on the results of an earlier study in which we showed that fe cu co and ni at merapi volcano indonesia were transferred from a mafic melt to an immiscible sulfide melt and then to a magmatic volatile phase which carried them to the surface here we examine the pathways taken by the volatile phase and the behavior of cu zn and pb in the upper part of the magmatic system beneath merapi volcano based on the composition of a suite of silicate melt inclusions and thermodynamic modeling we show that the mafic melt did not hybridize with the resident felsic melt but instead transferred metals to the latter by exsolving a magmatic volatile phase which dissolved the cu rich sulfide melt and then percolated upwards through the felsic melt enriching it in cu and pb it is this fluid which transported the metals to the surface and potentially also to subsurface environments of ore formation

magmatic hydrothermal evolution and devolatilization beneath merapi volcano indonesia abstract at merapi volcano indonesia explosive eruptions lasting several months result from injections of reduced mafic magmas and are interspersed by periods of lava dome growth and collapse quiescent degassing assimilation of wall rock and fractional crystallization lasting a few years silicate melt inclusions and volcanic gases were analyzed to study processes of magmatic volatile exsolution and volcanic degassing during periods of quiescent degassing at merapi volcanic gases were sampled at merapi in 1994 during a phase of quiescent degassing and in 2006 at the end of a dome growth and collapse explosive eruption silicate melt inclusions were collected and analyzed from lavas and scorias for their major element and volatile components solubility relationships between h 2 o and co 2 in melt inclusions demonstrate that melt was being trapped beneath merapi to depths reaching 19 km host amphibole geobarometry indicates crystallization pressures reaching up to 26 km hence there is evidence of deep pooling and crystallization of magma beneath merapi at appreciable depths using a mass balance model of magmatic volatile exsolution we demonstrate that the magma degassing from 19 km to the surface approaches closed system behavior compared to the bulk magmatic volatiles analyzed in melt inclusions volcanic gases had similar amounts of co 2 were enriched in h 2 o and s and depleted in cl and f we thus propose that the magmatic volatile phase was initially exsolved from the magma as a supercritical fluid and that it subsequently exsolved into a h 2 o cl f rich brine and co 2 s rich vapor according to the h 2 o nacl model brine vapor exsolution occurred at 5 km in depth for a 5 wt nacl 900 c to 1000 c fluid although the presence of h 2 s so 2 and co 2 may cause the supercritical fluid to unmix at greater depths magma at shallower depths exsolved a brine and a vapor directly from the melt the volcanic gases at merapi thus represent the vapor fraction of a magmatic volatile phase as brine ponded below the surface we further suggest that carbon dioxide liberated by the magmatic assimilation of limestone as proposed by other studies fluxed the melt thereby promoting co 2 enrichment of the melt and h 2 o degassing finally we propose that merapi volcanic gases are enriched in sulfur compared to the modeled volatile phase composition because deep reduced mafic magma supplies sulfur to the shallow magmatic plumbing system

time resolved construction of a bimodal laccolith torres del paine patagonia abstract granitic and mafic magma pulses were sequentially accreted in the spectacularly exposed shallow crustal torres del paine laccolith in southern patagonia this 12 5 ma pluton forms a composite intrusion with a subvertical feeding system in the west and a laccolith in the east a key unknown in the formation of sill complexes is how individual magma pulses are assembled over time and the geometry and localization of their feeding system high resolution zircon ca id tims u pb dating shows that the laccolith grew first by under accretion of granitic sills over 90 30 ka linked to a sheet like feeding system followed by underplating of mafic sills after 20 ka of quiescence in the mafic sills complex individual sills were injected by over accretion during 41 11 ka our data show that successive granitic and mafic magmas emplacement generated a volume of 88 km3 in 162 11 ka

diversity of primary cl textures in quartz from porphyry environments implication for origin of quartz eyes porphyry style mineralization is related to the intrusion and crystallization of small stocks which can be of different compositions from intermediate to felsic and can intrude into different host rocks from magmatic to sedimentary we used cathodoluminescence and electron probe microanalysis to study the internal textures of more than 300 quartz eyes from six porphyry deposits panguna papua new guinea far southeast porphyry philippines batu hijau indonesia antapaccay peru rio blanco chile and climax usa significant diversity of the internal textures in quartz eyes was revealed sometimes even within a single sample quartz grains with ti rich cores surrounded by ti poor mantles were found next to the grains showing the opposite ti distribution or only slight ti fluctuations we propose that diversity of the internal patterns in quartz eyes can actually reflect in situ crystallization history and that prolonged crystallization after magma emplacement under conditions of continuous cooling can account for the observed features of internal textures formation of quartz eyes begins at high temperatures with crystallization of high titanium quartz 1 which as the melt becomes more and more evolved and cooler is overgrown by low ti quartz 2 subsequent fluid exsolution brings about dramatic change in the melt composition oh alkalis and other cl complexed elements partition into the fluid phase whereas ti stays in the melt contributing to a rapid increase in ti activity separation of the fluid and its further cooling causes disequilibrium in the system and the quartz 2 becomes partially resorbed exsolution of the fluid gradually builds up the pressure until it exceeds the yield strength of the host rocks and they then fracture this pressure release most likely triggers crystallization of quartz 3 which is higher in ti than quartz 2 because ti activity in the melt is higher and pressure of crystallization is lower as a result of the reaction between the exsolved fluid and quartz a new phase a so called heavy fluid forms from this phase quartz 4 crystallizes this phase has extremely high metal carrying capacity and may give a rise to mineralizing fluids finally on the brink of the subsolidus stage groundmass quartz crystallizes prolonged crystallization under conditions of continuous cooling accounts better for the diversity of cl textures than crystallization in different parts of a deep magma chamber it is also in a better agreement with the existing model for formation of porphyry style deposits

zircon crystallization and the lifetimes of ore forming magmatic hydrothermal systems magmatic hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion vein opening and hydrothermal ore deposition it is driven by larger subjacent magma reservoirs acting as the source of fluid and ore forming components high precision u pb ages of individual zircon crystals from porphyries immediately predating and postdating cu au mineralization at bingham canyon utah united states and bajo de la alumbrera northwestern argentina show a significant spread of reliably concordant ages this demonstrates zircon crystal formation over a protracted period of 1 m y which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted the youngest zircons in all pre ore and post ore intrusions overlap within a much shorter time interval of 0 32 m y at bingham canyon and 0 090 m y at alumbrera these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods consistent with thermal constraints this study illustrates that age brackets based on individual magmatic zircon grains are geologically more informative than the calculation of means and standard deviations based on apparently normal age distributions in zircon populations

rates of magma transfer in the crust insights into magma reservoir recharge and pluton growth plutons have long been viewed as crystallized remnants of large magma reservoirs a concept now challenged by high precision geochronological data coupled with thermal models similarly the classical view of silicic eruptions fed by long lived magma reservoirs that slowly differentiate between mafic recharges is being questioned by petrological and geophysical studies in both cases a key and yet unresolved issue is the rate of magma transfer in the crust here we use thermal analysis of magma transport to calculate the minimum rate of magma transfer through dikes we find that unless the crust is exceptionally hot the recharge of magma reservoirs requires a magma supply rate of at least 0 01 km 3 yr much higher than the long term growth rate of plutons which demonstrates unequivocally that igneous bodies must grow incrementally this analysis argues also that magma reservoirs are short lived and erupt rapidly after being recharged by already differentiated magma these findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret geodetic surface signals related to incipient eruptions

rates of magma differentiation and emplacement in a ballooning pluton recorded by u pb tims tea adamello batholith italy abstract geochemical structural field and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths models for pulsed intrusion of small magma batches over long timescales 1 ma versus transfer of larger magma bodies on shorter timescales predict a different thermal metamorphic and rheological state of the crust highlighting the importance of robust time constraints this study focuses on a well characterized upper crustal intrusion the 15 km2 lago della vacca complex lvc adamello batholith n italy previous studies used structural and petrologic data to argue that the lvc was emplaced through pulsed magma injection and in situ expansion or ballooning of a short lived 105 yr magma chamber we test this model using a dense sampling strategy and high precision id tims u pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral u pb tims tea these data show that the marginal mafic pulses of magma crystallized zircon with primitive hf isotopes and negligible eu anomalies during fractional crystallization and ascent through the crust on 10 30 ka timescales subsequent more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the lvc to

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

two competing effects of volatiles on heat transfer in crystal rich magmas thermal insulation vs defrosting the build up and eruption of crystal rich ignimbrites commonly referred to as monotonous intermediates has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal rich magma bodies mushes in this study we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush intrusion system we solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension our calculations span a range of pressures from 1 to 4 kbar mush composition andesite to rhyolite and initial water contents from 1 5 to 6 wt the mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy and as a consequence by the amount of melting and the injection rate of volatiles from the intrusion to the mush we find that volatiles affect the melting of the mush or defrosting in two opposite ways depending on pressure and the initial water content of the intrusion when the intrusion volatile content is high and pressure relatively low 44 wt h2o at 2 kbar the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush for lower initial volatile contents 54 wt h2o in the intrusion and or higher pressure 3 4 kbar volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion we propose that the dual role played by volatiles during the thermal evolution of crystal rich magmas reheated by an underplating intrusion can explain the presence of crystal rich ignimbrites in arcs and their absence in drier hotspots or extensional regions

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

short eruption window revealed by absolute crystal growth rates in a granitic magma giant volcanic eruptions occur when large volumes of magma accumulate in crustal reservoirs and do not cool and crystallize to form a solid pluton of rock within the crust geochronological dating of a pluton from elba italy shows that the magma solidified in the crust within just 10 40 thousand years of being injected into the crust implying that the time window for an eruption was short

rutile saturation in hydrous siliceous melts and its bearing on ti thermometry of quartz and zircon the tio2 solubility of rutile saturated hydrous siliceous melts has been investigated at p 1 gpa and t 650 1000 c for several representative felsic compositions the dissolution of a rutile crystal into a tio2 undersaturated melt provides information on both tio2 solubility and ti diffusion results of this study confirm that tio2 solubility is strongly dependent on both temperature and melt composition but not on the amount of h2o present for a given t t io2 content decreases as the melts become more felsic the solubility of tio2 is given by log ti ppm 7 95 5305 t 0 124fm

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

mount pinatubo volcano and negative porphyry copper deposits a major concern in the analysis of the volatile systematics of the 1991 eruption of mount pinatubo in the philippines is the explanation for the release of such large amounts of so 2 about 20 million metric tons into the atmosphere although the total amount of sulfur originally in the magmatic system is important to the explanation of this large release another very significant variable may be what proportion of that sulfur was released into the atmosphere this paper is a thought experiment in which such considerations of elemental distribution are approached by means of an ore deposit model although such models typically are used to evaluate the conditions required for the most effective removal and concentration of a certain desired element they also can be applied to an undesired element in this case sulfur in particular cline and bodnar9s porphyry copper ore deposit model reveals several variables that are significant to the evaluation of the mount pinatubo eruption the most important of which is the partitioning behavior of sulfur among the solid and aqueous fluid liquid gas phases of the system for example one reason for the release of such large quantities of sulfur from mount pinatubo may be that in contrast to many other oxidized calc alkalic volcanoes its melt reached saturation with an so 2 bearing aqueous fluid before the solid phase anhydrite began to precipitate

age intercalibration of 40ar 39ar sanidine and chemically distinct u pb zircon populations from the alder creek rhyolite quaternary geochronology standard abstract we report results from a 40 ar 39 ar sanidine and ca tims 238 u 206 pb zircon dating study of eruption and crystal residence timescales of the alder creek rhyolite acr california extruded during the cobb mountain normal polarity subchron c1r 2n a 40 ar 39 ar acr sanidine date of 1 1850 0 0016 ma 2 external uncertainty determined relative to the astronomically dated a1 tephra sanidine is interpreted as the acr eruption age this age is supported by ca tims u pb zircon dating guided by la icpms trace element analyses titanium in zircon tizr thermometry and cathodoluminescence cl imaging using these data two compositionally distinct zircon populations were revealed pre acr group b zircon exhibit oscillatory zoning large positive ce and negative eu anomalies high incompatible trace element contents tizr temperatures of 650 c 750 c and th u disequilibrium corrected 238 u 206 pb dates of 1 38 1 24 ma these crystals are interpreted as antecrysts inherited from earlier intrusives in the geysers cobb mountain magma source region acr related group a zircon present as discrete grains and overgrowths on group b zircon display less intense cl with diffuse zoning and have less pronounced positive ce and negative eu anomalies lower incompatible trace element contents higher tizr temperatures that range up to 840 c and significantly younger dates the youngest group a dates yield a weighted mean of 1 1978 0 0046 ma 2 including systematic uncertainties that is interpreted as the mean age of zircon crystallization prior to eruption the 13 5 ka offset between the 40 ar 39 ar and 238 u 206 pb dates can be attributed to zircon magma residence time recognition of a young population of acr zircon is consistent with the 40 ar 39 ar eruption age which coincides with the astronomical age estimate for the cobb mountain subchron determined by correlating the oxygen isotope record of the giant piston core md972143 to the la93 1 1 orbital solution on the basis of independent radio isotopic and orbital forcing results we propose the refined age of 1 1850 0 0016 ma 2 external uncertainty for the quaternary acr 40 ar 39 ar sanidine standard

evolution of zircon deformation mechanisms in a shear zone lanzo massif western alps abstract magmatic zircons within two sheared gabbroic dykes from the peridotitic massif of lanzo western alps italy revealed evolution of deformation from crystal plasticity to rigid body rotation during shear zone evolution this is the first time that multiple zircon grains have been analysed in a kinematic context in a shear zone zircon grains recorded crystal plastic deformation activating the commonly inferred 001 and 100 glide systems to the newly identified 110 glide system the exact selection of glide system could be dependant of deformation conditions such as pressure temperature and strain rate moreover the activation of one or several glide systems within a single grain could be favoured by the primary orientation of the grains combined with a high strain rate in these sheared gabbros the deformation mechanisms evolve from plastic deformation at low strain rate conditions to increase strain strain softening and localisation of deformation the progressive shear zone development and the softening of the matrix relative to the zircon has lead to a switch from crystal plasticity to rigid body rotation of zircon the zircon grains rigid body rotation involved that their long axes became parallel to the lineation of the shear zone causing reorientation and dispersion of the misorientation axes away from kinematic y

generation of porphyry copper deposits by gas brine reaction in volcanic arcs we acknowledge research funding from bhp billiton a benjamin meaker visiting professorship to j m and a royal society wolfson research merit award and erc advanced grant critmag to j b

temporal and genetic link between incremental pluton assembly and pulsed porphyry cu mo formation in accretionary orogens economically important porphyry cu mo deposits pcds are generally hosted by upper crustal plutons of variable chemical compositions related to distinct geodynamic settings the absolute timing and duration of pluton assembly and pcd formation are critical to understanding the genetic relationship between these interrelated processes here we present new comprehensive zircon u pb and molybdenite re os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore bearing plutons of the central tethyan metallogenic belt the meghri ordubad pluton southern armenia and nakhitchevan lesser caucasus this composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over 30 m y comprising middle eocene 48 9 43 1 ma calc alkaline subduction related magmatism lasting 5 8 0 8 m y followed by postsubduction late eocene middle oligocene 37 8 28 1 ma shoshonitic magmatism over 9 7 0 9 m y and late oligocene early miocene 26 6 21 2 ma adakitic magmatism consisting of shoshonitic dikes and high k calc alkaline granodioritic magmas emplaced over 5 4 0 4 m y despite the distinct geodynamic settings and magma compositions each intrusive suite culminated in the formation of variably sized pcds including the giant oligocene kadjaran porphyry cu mo deposit associated with high sr y shoshonitic magmas complementary in situ zircon hafnium ehfzircon 8 to 11 3 and oxygen 18ozircon 4 6 to 6 0 isotope data support a mantle dominated magma source with limited crustal contribution and or cannibalization of young and juvenile lower crustal cumulates we conclude that independent of geodynamic setting and magma composition long lived 5 10 m y incremental mantle derived magmatism is a prerequisite to form fertile magmatic hydrothermal systems and especially giant pcds

zircon compositional evidence for sulfur degassing from ore forming arc magmas porphyry cu mo au and epithermal au ag deposits are major sources of mined metals and are commonly formed by magmatic hydrothermal fluids derived from hydrous magmas in phanerozoic convergent margin settings the igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings but while many have produced acidic magmatic fluids very few past or present have produced sufficient metal chlorine and sulfur enrichments necessary to engender an ore deposit the reasons for this remain uncertain we report shrimp rg ion microprobe analyses of hafnium titanium and rare earth element ree abundances in zircon a nearly ubiquitous and robust trace mineral in crustal magmas comparison of the compositions of zircons in ore forming and barren granitic plutons indicate that ore forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies mostly eu n eu n 0 4 we interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to so 2 degassing from magmas with a relatively low fe s ratio oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate s 6 to so 2 s 4 upon degassing this interpretation reinforces the important role of oxidized sulfur rich fluids in porphyry and epithermal mineral deposit formation zircon compositions thus may be used to identify ancient magmas that released significant amounts of so 2 rich gases and regional surveys of zircon composition are potentially a valuable tool for mineral exploration

isoseismal map of the 2015 nepal earthquake and its relationships with ground motion parameters distance and magnitude abstract a devastating earthquake of mw 7 8 struck central nepal on 25th april 2015 6 11 25 ut which resulted in more than 9000 deaths and destroyed millions of houses standing buildings roads and electrical installations worth 25 30 billions of dollars are reduced to rubbles the earthquake was widely felt in the northern parts of india and moderate damage have been observed in the northern part of up and bihar region of india maximum intensity ix according to the usgs report was observed in the meizoseismal zone surrounding the kathmandu region in the present study we have compiled available information from the print electronic media and various reports of damages and other effects caused by the event and interpreted them to obtain modified mercalli intensities mmi at over 175 locations spread over nepal and surrounding indian and tibet region we have also obtained a number of strong motion recordings from india and nepal seismic network and developed an empirical relationship between the mmi and peak ground acceleration pga peak ground velocity pgv we have used least square regression technique to derive the empirical relation between the mmi and ground motion parameters and compared them with the empirical relationships available for other regions of the world further seismic intensity information available for historical earthquakes which have occurred in the nepal himalaya along with the present intensity data has been utilized for developing an attenuation relationship for the studied region using two step regression analyses the derived attenuation relationship is useful for assessing damage of a potential future large earthquake earthquake scenario based planning purposes in the region

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

constraints on magma degassing beneath the far southeast porphyry cu au deposit philippines k silicate alteration in the far southeast fse porphyry cu au introduction deposit formed at 1 4 ma concentric to dikes of quartz diorite magmas become saturated with volatile components as porphyry at 2 km paleodepth the hydrothermal system consisted they ascend in the crust and either erupt or crystallize of magmatic hypersaline liquid and vapor at 550 c and 50 at depth saturated magmas that remain at depth exsolve mpa lithostatic pressure advanced argillic alteration formed at an aqueous fluid that forms a hydrothermal system on the same time over the deposit at 1 km paleodepth from acidic separation from the magma chamber burnham 1979 condensates of the vapor at 1 3 ma k silicate alteration was although part of the exsolved fluid a low density vapor overprinted by 350 c magmatic liquid 5 wt nacl equiv may discharge to the surface from passively degassing at hydrostatic pressure sericite alteration and much of the cu au volcanoes hedenquist 1995 part will remain at depth mineralization formed at this later stage evolution of the magmatic and this dense residual magmatic liquid may be asfluid composition was simulated with a magma chamber cryssociated with hydrothermal mineralization in the portallization model homogeneous crystallization during early stage phyry ore environment hedenquist lowenstern convection is assumed whereas at 50 vol crystals the chamber 1994 becomes stagnant and crystallizes from rim to core over a narrow in this paper we use evidence from a porphyry ore crystallization interval the model calculation based on a magma deposit in the philippines to determine the composition chamber with 2 km thickness at 6 km depth 150 mpa and of the fluid that exsolved from the deeper parent magma 800 c saturated melt composition at 30 vol crystals 5 the evolving composition of the magmatic fluid at the wt h2o 0 2 wt cl and dd of 40 can reproduce the depth of the ore body can then be used to place constraints chemical and isotopic compositions of the early and late magmatic on the nature of the evolved magma chamber far below fluids the most critical factor controlling the compositional evolution the porphyry ore body this approach differs from that of the model hydrothermal system is the transition from convective of other studies where a portion of the parent magma to stagnant magma chamber crystallization there is also a sharp chamber is available to study directly e g at yerington decrease in the rate of fluid exsolution associated with this transition nevada dilles 1987 which can account for the thermal collapse of the fse porphyry the far southeast fse porphyry cu au deposit system from k silicate to sericite alteration and overlying lepanto epithermal cu au ore body are

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles emissions of sulphur 1 2 and transition metals 3 4 from magmas in the shallow crust have global impacts on human society by affecting climate after volcanic eruptions 5 6 and by forming giant copper and gold ore deposits which are massive anomalies of sulphur as well as of the ore metals themselves 3 4 7 8 these emissions vastly exceed the amounts that could have been derived from the associated magmas themselves 2 intrusion of denser hotter vapour and sulphide saturated magma beneath the cooler and more buoyant bodies that are finally erupted is the most probable source of the excess sulphur and metals in eruptive or hydrothermal systems 1 4 9 10 however the mechanism for the selective upward transfer of sulphur and metals remains poorly understood because their main carrier phase sulphide melt is dense and is generally assumed to settle to the bottoms of magma bodies here we show by experimental observation and by gas speciation and mass balance models that such transfers may be achieved by the flotation of compound drops 11 comprising droplets of sulphide melt attached to vapour bubbles by proposing and demonstrating the feasibility of a novel physical mechanism for upward mobility of sulphide liquids our work provides a mechanism for massive atmospheric releases of sulphur and contradicts widely held assumptions that dense sulphide liquids rich in sulphur copper and gold will remain sequestered in the crust

a physical model for metal extraction and transport in shallow magmatic systems 1 the highest concentrations of metals e g cu au ag mo in the earth s crust are found in porphyry type deposits the metals are ultimately sourced from magmas and appear to be concentrated hundred to thousand fold from typical magmatic contents ppm ppb in the exsolved volatile phase to better quantify the purging and transport of metals we develop a physical model of volatile evolution in an incrementally built upper crustal magma reservoirs that considers 1 partitioning of metals from the melt to an exsolved volatile phase and 2 advection of the buoyant volatile phase using a single dimensionless parameter the peclet number pe ratio of advection rate over diffusion rate we propose that metal extraction and segregation from magmas can occur in 3 stages with different pe 1 during exsolution of the magmatic volatile phase in shallow crystal poor magma bodies slow volatile advection pe 1 2 during the growth of volatile channels that develop in the reservoir as crystallinity increases pe 1 and 3 during advection in connected channels rapid volatile advection high pe 1 for each stage a metal enrichment factor can be calculated allowing insight into the optimal conditions to maximize metal mass flux into the overlying hydrothermal system the model predicts that the most efficient purging of metals occurs for magmas with intermediate volatile contents and is enhanced during late stage magmatic activity as the reservoirs reach high crystallinity and are not disturbed by volcanic venting in agreement with natural observations suggesting that ore formation post dates volcanic activity

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

triggers for the formation of porphyry ore deposits in magmatic arcs porphyry ore deposits supply much of the copper molybdenum gold and silver used by humans a review of the main processes that trigger porphyry ore formation suggests that sulphide saturation of the magmas that supply the metals could be the overriding mechanism that helps control the temporal and spatial distribution of the ore deposits

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content