implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

rapid assembly and rejuvenation of a large silicic magmatic system insights from mineral diffusive profiles in the kidnappers and rocky hill deposits new zealand abstract the timescales over which magmas in large silicic systems are reactivated assembled and stored remains a fundamental question in volcanology to address this question we study timescales from fe mg interdiffusion in orthopyroxenes and ti diffusion in quartz from the caldera forming 1200 km3 kidnappers and 200 km3 rocky hill eruptions from the mangakino volcanic centre taupo volcanic zone new zealand the two eruptions came from the same source area have indistinguishable 40ar 39ar ages 1 0 ma and zircon u pb age spectra but their respective deposits are separated by a short period of erosion compositions of pumice glass and mineral species in the collective eruption deposits define multiple melt dominant bodies but indicate that these shared a common magmatic mush zone diffusion timescales from both eruptions are used to build on chemical and textural crystal signatures and interpret both the crystal growth histories and the timing of magma accumulation fe mg interdiffusion profiles in orthopyroxenes imply that the three melt dominant bodies established through extraction of melt and crystals from the common source were generated within 600 years and with peak accumulation rates within 100 years of each eruption in addition a less evolved melt interacted with the kidnappers magma beginning 30 years prior to and peaking within 3 years of the eruption this interaction did not directly trigger the eruption but may have primed the magmatic system orthopyroxene crystals with the same zoning patterns from the kidnappers and rocky hill pumices yield consistently different diffusion timescales suggesting a time break between the eruptions of 20 years from core rim zones to 10 years outer rim zones diffusion of ti in quartz reveals similarly short timescales and magmatic residence times of 200 km 3 eruption our data support concepts of large silicic systems being stored as long lived crystal mushes with eruptible melts generated over extraordinarily short timescales prior to eruption

eruptive history of the late quaternary ciomadul csomad volcano east carpathians part i timing of lava dome activity located at the southern tip of the intra carpathian volcanic range in romania and composed of a dozen dacitic lava domes the ciomadul csomad volcanic complex is the youngest eruptive centre of the carpatho pannonian region whereas in the last decade the explosive history of ciomadul since 50 ka has been well constrained by numerous studies the chronology of the dome sequence still lacks robust chronological constraints and an extended analysis of all available data here we apply a detailed k ar dating approach to refine the chronology of the lava dome eruptions using the unspiked k ar cassignol gillot technique our dating focused on the most voluminous central part of the lava dome complex new eruption ages were determined following a strict separation of 10 g of groundmass from about 3 kg of unaltered sample rocks thereby isolating material whose cooling was contemporaneous with the eruption the newly applied methodology mainly consisting of a double full preparation first at larger grain size 0 4 mm and then at 100 m provides an appropriate procedure to separate suitable material to obtain the k ar age of the eruption i e the sample s groundmass in which there is no risk of the presence of older inherited crystals our new geochronological data set gives an improved insight into the temporal construction of the ciomadul volcanic complex where due to the method applied here all ages are younger than those from previous studies that used whole rock k ar ages our new results show that ciomadul s volcanic activity began with the construction of the south eastern peripheral domes from ca 850 ka to 440 ka after a ca 250 ky long repose period the activity resumed in the northern part at around 200 ka with subsequent domes emplaced between 200 and 130 ka aligned roughly north south in the western central part of the complex following a 30 ky long quiescence period the eastern central domes formed between 100 and 60 ka in addition to the chronological history of lava dome volcanism we also investigated the sequence of crystallization of mineral phases present in the lavas with respect to the modification of eruption ages ages obtained on pure minerals plagioclase amphibole and biotite are systematically older than those obtained on groundmass showing that most of them formed up to 1 85 myr before eruption in a long lived pre ciomadul magmatic system crystal size distribution csd data support the age contrasts between juvenile groundmass and older inherited minerals after injection of new magma and convective mixing with crystal clots ascent of the resulting mixture led to eruptions of material representing contrasting ages

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

age of the rockland tephra western usa abstract the age of the rockland tephra which includes an ash flow tuff south and west of lassen peak in northern california and a widespread ash fall deposit that produced a distinct stratigraphic marker in western north america is constrained to 565 000 to 610 000 yr by 40ar 39ar and u pb dating 40ar 39ar ages on plagioclase from pumice in the rockland have a weighted mean age of 609 000 7000 yr isotopic ages of spots on individual zircon crystals analyzed by the shrimp rg ion microprobe range from 500 000 to 800 000 yr a subpopulation representing crystal rims yielded a weighted mean age of 573 000 19 000 yr overall stratigraphic constraints on the age are provided by two volcanic units including the underlying tephra of the lava creek tuff erupted within yellowstone national park that has an age of 639 000 2000 yr the basaltic andesite of hootman ranch stratigraphically overlies the rockland in the lassen peak area and has 40ar 39ar ages of 565 000 29 000 and 565 000 12 000 yr for plagioclase and groundmass respectively identification of rockland tephra in odp core 1018 offshore of central california is an important stratigraphic age that also constrains the eruption age to between 580 000 and 600 000 yr

mid pleistocene silicic tephra beds in the auckland region new zealand their correlation and origins based on the trace element analyses of single glass shards abstract the trace element composition of individual glass shards from twelve distinct mid pleistocene silicic tephra beds from the auckland region new zealand has been determined by laser ablation la icp ms analysis this is the first application of such data to tephra correlation studies in new zealand these analyses support earlier major element based correlations between three surface sections oruarangi and okariha at port waikato and st kentigern s at east auckland and a long core patiki road west auckland the correlations provide a higher resolution record of taupo volcanic zone tvz activity than is preserved in more proximal deposits where burial and erosion obscure much of the volcanic record a combination of major and trace element analysis enables the correlation of three units exposed in the auckland region with their proximal equivalents ignimbrites from the tvz namely deposits associated with 1 the ongatiti ignimbrite 1 21 0 04 ma 2 an un named post ahuroa pre unit e ignimbrite sample ut1274 from the rocky hill section king country and 3 the unit e ignimbrite 1 ma combined major and trace element data show a marked evolution in magma chemistry with the large ongatiti and unit e ignimbrite eruptions occurring after a change in composition caused by replenishment reactivation of the magma chamber changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase dominated assemblage trace element data and the wider dispersal of the silicic tephras in the auckland region suggests that they are all sourced from the mangakino volcanic centre mvc in the tvz this study illustrates for the first time in a new zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry

zircon and titanite recording 1 5 million years of magma accretion crystallization and initial cooling in a composite pluton southern adamello batholith northern italy abstract the southern part of the adamello batholith the so called re di castello unit is an example of a composite pluton ranging from gabbro to granodiorite in composition u pb dating of single zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies showing apparent durations of growth between 90 and 700 ka the youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite the formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization in contrast to antecrystic zircons present in the same sample which had formed earlier in the magmatic column or were derived from re mobilized earlier magma the autocryst derived emplacement ages range from 42 43 0 09 ma to 40 90 0 05 ma recording 1 5 ma of intrusion and crystallization history we anticipate that extended periods of zircon crystallization may be common in silicic rocks whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0 1 0 2 in 206 pb 238 u age decreasing th u ratios of dated zircons within one melt batch document the depletion of the residual melt portion in th due to the contemporaneous crystallization of titanite preliminary hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern re di castello unit represent hybrid melts with an important crustal component e hf between 2 8 and 3 0 subsequently emplaced melts are more juvenile at e hf values at 6 4 to 8 9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system

sub micron scale trace element distributions in natural zircons of known provenance implications for ti in zircon thermometry nanosims measurements of ti p y ce and hf in zircon separates from the youngest toba tuff the bishop tuff the quottoon igneous complex the sierra nevada batholith and an adirondack migmatite show that micron scale oscillatory zoning of ti is common the zircons we have studied typically display banded concentration gradients having between 1 and 7 peaks in ti abundance over length scales of 10 20 m the beam diameter commonly used for sims trace element zircon analyses with amplitudes of up to 4 5 ppm ti baseline values are 1 4 ppm and widths measured at half height of between 0 2 and 4 m spatial correlations between concentrations of ti and other trace elements p y and ce are also common but variable in character ranging from oscillatory co variation of ti p y and ce to cases where only a subset of peaks for a given element is spatially correlated with peaks in ti there are also longer length scale generally positive correlations among concentrations of ti p y and ce i e gradients on which narrower peaks are superimposed in contrast hf concentrations are either uncorrelated or inversely correlated with these longer length scale variations in ti concentrations the wide range in ti concentrations over distances of less than 1 m and the various correlations between ti concentrations and those of the other analyzed elements suggest that on the micron scale and at temperatures between 700 and 800 c zircon liquid ti partitioning is not controlled by bulk or lattice equilibrium treating our nanosims ti ion maps as though they were conventional sims analyses i e generating an average ti concentration for each map we evaluate the hypothesis that while micron scale variations in ti concentrations might be kinetically controlled when averaged over 100 400 m2 such variations capture the thermal state of the growing zircon using these average ti concentrations independent petrologic pressure and temperature constraints estimates of silica and titania activities based on phase assemblages as well as calculations using rhyolite melts we show that crystallization temperatures predicted by ti in zircon geothermometry generally do not agree with the independently constrained temperatures for the samples

a relationship between alluvial backswamps and avulsion cycles an example from the willwood formation of the bighorn basin wyoming abstract stratigraphically limited intervals from the lower eocene willwood formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion we present data from one such interval that documents the avulsion sequence vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine grained deposits on which weakly developed hydromorphic paleosols formed the paleosols enclose and are locally incised by ribbon sandstones some of which cut down to and partly through the carbonaceous shale the ribbons have width thickness ratios between 3 and 13 some ribbons cluster at a particular stratigraphic level which together with paleocurrent trends suggests that they formed channel networks sections are capped by yellow brown paleosols showing moderate pedogenic development we suggest that the carbonaceous shales developed in low lying topogeneous swamps in distal portions of the floodplain far from the trunk channel such a location set limits on the sediment that they received the mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse splay complexes resulting from avulsion of the trunk river the ribbon sandstones represent ancient feeder channels of the avulsion complex the rapid influx of avulsion deposits appears to have been crucial to preserving the organic material and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic rich deposits in distal alluvial swamps similar deposits are found in other stratigraphic units in the rocky mountain region and the development of these and other organic rich deposits should be reassessed in terms of channel avulsion

reappraisal of los humeros volcanic complex by new u th zircon and 40ar 39ar dating implications for greater geothermal potential

u pb and 40ar 39ar dating of the miocene fossil track site at ipolytarnoc hungary and its implications abstract abundant early miocene mammal and bird tracks and a rich plant assemblage is preserved by the emplacement of an ignimbrite sheet of the gyulakeszi rhyolite tuff formation grtf near ipolytarnoc in northern hungary the tuff that overlies the track bearing sandstone yielded a single crystal zircon u pb total isochron age of 17 42 0 04 ma and a single crystal laser fusion plagioclase 40 ar 39 ar age of 17 02 0 14 ma uncertainties are quoted at the 2 level an additional 40 ar 39 ar age of 16 99 0 16 ma was obtained from the equivalent rhyolite tuff near nemti where the underlying terrestrial clay yielded early proboscidean remains assigned to the mn4 mammal zone the new high precision dates allow revision of the numeric age and correlation of the ipolytarnoc fossil site and the grtf previously based on an average k ar age of 19 6 1 4 ma the difference of 0 40 0 15 ma between the u pb and 40 ar 39 ar ages support the growing evidence for a systematic bias between the two isotopic systems due to the inaccurately known 40 k decay constant but likely also includes an undetermined pre eruptive residence time of zircon published biostratigraphic data from under and overlying marine strata establish correlation with the nn3 nannoplankton zone and together with the new radioisotopic ages suggest assignment of the fossils and the tuff to the ottnangian regional stage of the central paratethys a global correlation lends support to the recently suggested astronomical calibration of the early miocene time scale that revised the previous scales towards younger ages the 40 ar 39 ar age from nemti provides a reliable correlation of the mn4 mammal zone in central europe with the numeric time scale and places a minimum constraint on the age of the regional proboscidean datum the migration event of proboscideans from africa to europe through the emerging gomphotherium landbridge contrary to suggestions for a significantly earlier european proboscidean datum it appears that the originally suggested age of c 17 5 ma is realistic but it is significantly younger than the south asian proboscidean datum

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

structure and evolution of the wairakei tauhara geothermal system taupo volcanic zone new zealand revisited with a new zircon geochronology abstract the central part of the taupo volcanic zone tvz new zealand is notable for the number 23 and size of its high temperature geothermal systems of these the wairakei tauhara geothermal system collectively represents the largest commercially developed example natural thermal output prior to utilisation i e before 1950 was 530 mw since 1950 drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework within this framework the strata are dominantly rhyolitic pyroclastic rocks and lava bodies with interbedded fluvial and lacustrine deposits until now no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25 4 ka oruanui ignimbrite or to buried ignimbrite that is correlated with the 349 ka whakamaru group this paper presents 33 suites of u pb age data and a limited number of reconnaissance u th disequilibrium age data from magmatic zircon crystals zircons to date the rocks of the wairakei tauhara system the oldest units dated are intermediate lavas from ca 1 7 ma while the youngest locally vented eruptives of tauhara dacite are tentatively dated to ca 60 ka placing the age data within the established drillhole record allows for the recognition of major volcano tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the wairakei tauhara area in contrast to the rotokawa and ngatamariki geothermal systems 10 20 km to the northeast the wairakei tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka leading to major faulting and abrupt variations in thicknesses of units the faulting and deposit thickness variations established in these two events remain major controls on present day permeability of the geothermal system the new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity however from zircon age data from the rhyolite andesite mixed tauhara dacite we propose that the current geothermal system was initiated at ca 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since

ion microprobe u pb dating of quaternary zircon implication for magma cooling and residence time abstract an ion microprobe u pb dating method of quaternary zircon is developed using the sensitive high resolution ion microprobe shrimp and applied to igneous rocks from japan total pb u isochron age of 1 36 0 23 ma 95 confidence limit cl mswd 1 03 was obtained for zircons extracted from takidani granodiorite in central japan this may provide a constraint on magma cooling when coupled with fission track ages of the zircon as well as rb sr and k ar ages on co existing minerals the closure temperature determined by each method has lead to exponentially declining cooling history the history is consistent with a simple conductive cooling model of a small magma body 10 2 10 3 km 3 a pb u isochron age of 271 58 ka 95 cl mswd 0 85 has been determined for volcanic zircons from torihama dacite sw japan the age is significantly older than the 238 u 230 th ionium age of 145 33 ka and fission track age of 163 8 ka found in the literature suggesting either the magma residence time of 100 kyr or a rapid cooling rate of about 6 c kyr in contrast a tephrochronological estimate of 240 10 ka agrees well with the observed pb u age implying that the ionium and fission track ages are incorrect the pb u isochron age of 90 29 ka 95 cl mswd 1 04 is obtained for zircons in pyroclastic rocks from the unzen eruption in 1991 sw japan the age is again older than the eruptive age which suggests a magma residence time of 90 kyr in a shallow reservoir

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

u pb ages constraining batholith emplacement contact metamorphism and the formation of gold and w mo skarns in the southern cross area yilgarn craton western australia gold mines in the archean southern cross greenstone belt central yilgarn craton have produced more than 220 metric tons t of gold the deposits are characterized by a pyroxene rich gangue and have been interpreted as synmetamorphic amphibolite facies replacement deposits and as postmetamorphic intrusion related skarns we present ion microprobe u pb zircon data constraining the age of amphibolite facies contact metamorphism in the aureole of the ghooli dome the largest granitoid batholith intruding the southern cross belt drill cores from the copperhead gold mine provided zircons from an altered quartz porphyry sill located 300 m southwest of the ghooli dome and zircons from the monzogranite gneiss at the border of the batholith the porphyry sill contains embayed igneous zircons dated by a concordant u pb age of 2912 5 ma and euhedral zircons of metamorphic crystal habitus dated by a second concordant age of 2772 5 ma the igneous zircons are slightly younger than those recovered from another sill of quartz porphyry sampled at the southern star mine which define a single concordant u pb age of 2934 7 ma the metamorphic zircons in the porphyry at copperhead record the same age as the igneous zircons 2775 10 ma in the monzogranite gneiss of the ghooli dome this pluton extends more than 40 km to the southeast of copperhead and also forms the batholith border at the corinthian and fraser s mines these gold deposits too are hosted by amphibolite facies greenstones contact metamorphosed at 2772 5 ma the corinthian skarn 2620 6 ma formed 150 m y after high grade metamorphism and consequently cannot be classified as synmetamorphic or metamorphogenic a review of field relationships and of published geochronometric data suggests that the gold skarns in the southern cross area are related in space and time to a suite of magnetite series granite pegmatite complexes these granites dated regionally at 2 66 to 2 60 ga are also associated with gold bearing w mo skarn greisen or skarn systems of up to 6 km strike length the numerous skarns exposed in the deeply eroded 14 km paleodepth central yilgarn craton are members of a recently recognized group of intrusion related gold deposits which are part of continental margin tungsten tin provinces

rejuvenation of crustal magma mush a tale of multiply nested processes and timescales some relatively crystal rich silicic volcanic deposits including large volume ignimbrites preserve evidence of a history where a rheologically locked high crystallinity magma was rejuvenated unlocked through enthalpy mass exchange with newly injected recharge magma of higher specific enthalpy this is one event in a possible complex history that is the volcanic product of an eruption reflects an array of sequential and nested processes including melt formation and segregation ascent cooling crystallization crustal assimilation magma recharge unlocking shallow ascent fluid exsolution and eruption deconvolution of these these nested processes and concomitant timescales is complicated and relies on a multidisciplinary approach studies that do not clearly associate process and correlated timescale have the potential to provide misleading timescale information we report the results of thermodynamic and heat transfer calculations that document mass energy and phase equilibria constraints for the unlocking of near solidus rhyolite mush via magma mingling heat exchange only with basaltic recharge magma of higher specific enthalpy to achieve unlocking defined as the transition from near solidus to 50 percent melt of the host silicic magma phase equilibria computations provide 1 the enthalpy required to unlock mush 2 the mass ratio of recharge magma to mush mr mm when the two magmas achieve thermal equilibrium and 3 the changes in melt mineral and fluid phase masses compositions and temperatures during the approach to unlocking the behavior of trace elements is computed with knowledge of mineral fluid and melt proportions and solid fluid and solid melt partition coefficients evaluation of unlocking for relatively dry 0 5 wt h2o and wet 3 9 wt h2o rhyolitic mushy locked magma by basaltic recharge at upper crustal pressures indicates minimum values of mr mm can be significantly less than 1 assuming the mingling process is isenthalpic with no waste heat for active volcanic systems estimates of mr may be tested using geodetic data wet mush has lower energy requirements for unlocking and thus requires lower mr mm than dry mush wet rejuvenated magmas therefore may be more abundant in the volcanic rock record and unlocked dry mushes may be restricted to extensional tectonic settings with high recharge flux temperature changes in dry mush as it unlocks are pronounced greater than 150 c compared to those in wet mush which are smaller than the resolution of classical geothermometry 15 c phase equilibria calculations show that as required the net volume of crystals decreases during unlocking interestingly calculations also indicate reactive crystal growth by chemical re equilibration at the crystal size scale during unlocking may also take place in either dissolution by unidirectional resorption or reactive dissolution new growth the chemical signatures of unlocking potentially preserved in crystals or parts of crystals for example rims are predictable and hence testable independent of unlocking thermodynamics the phase equilibria and elemental consequences of isentropic magma ascent a transport event that follows unlocking can also be predicted detailed examination of several canonical cases reveals a marked contrast with isenthalpic unlocking thereby providing a means of process deconvolution unlocking timescales are estimated by two methods one that calculates the time to reach thermal equilibrium for recharge magma dispersed in mush as clumps of fixed size and the second where the required volume of recharge magma is initially a single clump and evolves to smaller size through clump stretching and folding for a range of magma volumes from 0 1 to 5000 km3 unlocking times range from 10 2 to 106 years the shorter timescales for any magma volume requires a large number of relatively small clumps n 106 which implies that large volumes of mush purported to unlock over short timescales 102 103 years and less should preserve and exhibit evidence of intimate magma mingling the key result of our analysis is that multiple timescales are operative during the potentially long and complex history of silicic mush formation rejuvenation and ascent to correctly ascribe timescales to unlocking requires a holistic understanding of the myriad processes that affect the magma before during and after enthalpy exchange in the absence of this context unlocking timescales may be incorrectly constrained which in turn may hinder eruption forecasting and associated hazard mitigation

genesis of the shangjinshan w mo polymetallic deposit in the eastern jiangnan tungsten belt evidences from geochemistry geochronology and zircon hf isotope analyses abstract the newly discovered shangjinshan w mo polymetallic deposit located at the southern anhui southeast china is a large scale skarn type tungsten polymetallic deposit according to mineral assemblages ore textures structures and superposition relationships of the mineralization three ore types have been recognized veinlet disseminated skarn the primary type quartz vein type and porphyry type the mineralized event of this deposit could be divided into four stages i the prograde skarn stage ii retrograde skarn stage iii quartz sulfide vein stage and iv quartz calcite vein stage the intrusion in the tungsten rich district of the shangjinshan is mainly composed of the granodiorite with zircon u pb ages of 138 4 1 2 ma to 136 7 1 3 ma meanwhile re os isotopic dating of five molybdenite samples yields an isochron age of 141 9 3 1 ma and a weighted average age of 139 3 1 7 ma respectively the ore forming ages are consistent with the rock forming ages within errors indicating that the w mo mineralization is genetically related to the shangjinshan granodiorite the granodiorite is i type granites characterized by metaluminous to slightly peraluminous and high k cal alkaline compositions they are also enriched in large ion lithophile elements liles and light rare earth elements lrees and depleted in high field strength elements hfses and heavy rare earth element hrees with obviously negative eu anomalies eu eu 0 50 0 84 the zircon ehf t values and two stage hf model ages tdm2 fall into the range 6 18 to 2 04 and 1 5 1 3 ga respectively the relatively low rhenium contents in molybdenite and high radioactive pb isotopes of the pyrites from stage ii indicate that the ore forming material was derived from shangjinshan granitic magmatism together with previously published data we propose that the w related granitoids in the jnb were likely generated by partial melting of the shangxi group with some additional mantle materials the granitic magmatism was caused by the asthenosphere upwelling triggered by the roll back of subduced paleo pacific plate during late jurassic to early cretaceous these granitic stocks predominately contributed to the production of skarn alteration and mineralization resulting in the giant tungsten polymetallic district in the jiangnan tungsten belt

shrimp u pb dating of the preeruption growth history of zircons from the 340 ka whakamaru ignimbrite new zealand evidence for 250 k y magma residence times the whakamaru group ignimbrites are products of one of the largest caldera forming eruptions from the central taupo volcanic zone new zealand ca 340 ka imaging of zircons separated from low silica rhyolite pumice from whakamaru group ignimbrite reveals a complex magmatic crystallization history some grains contain resorbed cores cores and rims of individual zircons were dated using the sensitive high resolution ion microprobe due to the high u contents of the zircons 206 pb count rates were sufficiently high to obtain 206 pb 238 u ages with precisions of 5 10 1 s using longer than usual 206 pb count times and this has allowed zircon core and rim ages to be distinguished zircon rim and outer growth zone ages range from within 1 s error of the eruption age to 449 20 ka whereas zircon cores produced ages to 608 20 ka indicating magmatic residence times exceeding 250 k y for some zircons the data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years involving magma replenishment and mixing with residual phases of older silicic magma chambers in addition to crystal fractionation processes prior to eruption the study has shown that 206 pb 238 u dating using high resolution ion microprobe can be used to determine zircon preeruptive growth histories in quaternary lavas and pyroclastic rocks

provenance of fossiliferous clasts in carboniferous conglomerate isle madame southern cape breton island nova scotia fossiliferous clasts occur in carboniferous conglomerate in the horton group on western isle madame and in the mabou group on eastern isle madame most of the clasts 21 of 23 examined are calcareous siltstone and sandstone that contain silurian lower devonian faunas comparable to those in the arisaig area northern mainland nova scotia although the lithologies are coarser grained and less calcareous than those of the arisaig section these middle paleozoic faunas are well constrained to the silurian uppermost llandovery through pridoli and lowest devonian and are characteristic of those known from shallow siliciclastic dominated platforms of the avalon microcontinent in wales and england the remaining two clasts have abundant inarticulate brachiopod shells that indicate provenance from middle cambrian proximal marine facies on the avalonian marginal platform no clasts were found that are likely to have been derived from the torbrook formation and thus from the meguma terrane in southwestern nova scotia as has been previously reported the association of relatively large reworked fossiliferous clasts in carboniferous conglomerate on isle madame suggests local derivation from lower and middle paleozoic units not presently exposed although probably present as subcrop under the carboniferous units in southwestern cape breton island and adjacent mainland nova scotia

la icp ms u pb zircon timescale constraints of the pleistocene latest magmatic activity in the sancy stratovolcano french massif central abstract it has been demonstrated for several years that the u pb zircon system can provide useful dating information for understanding the time evolution of the volcanic systems in addition in situ techniques were successfully used for determining the formation age of young pliocene and pleistocene volcanic rocks and to record the spatial and temporal distribution and the lifetime evolution of the magma chambers generally in the case of large volcanic domains here we show that it is possible to obtain a precision at the percent level on zircon from a short size and late volcanic event from the sancy stratovolcano french massif central la icp ms dating of pleistocene magmatic zircons allow to distinguish the timescale of these late magmatic events within the complex volcanic activity of this large stratovolcano a syenite xenolith enclosed into a trachyte pumice flow were dated at 277 6 3 2 ka and 260 1 3 6 ka respectively whereas an earlier trachyandesite dome was emplaced at 284 4 4 2 ka these different ages allow to propose a minimum evolution time of about 24 8 kyrs for a differentiated magma chamber which corresponds to a short lifetime associated with low magma volumes the duration of this discrete volcanic event should be compared to the hundreds of thousands of years reported for the large volcanic systems interestingly the zircon crystals from the pumices were embedded into a thin layer of glass of trachytic composition demonstrating that these zircons crystallized into the melt only a short time before eruption

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

timing and conditions of formation of granitoid clasts erupted in recent pyroclastic deposits from tarawera volcano new zealand abstract granitoid clasts in pyroclastic deposits of the 0 7 ka kaharoa eruption from the intra caldera tarawera volcano of the okataina volcanic centre ovc new zealand provide an insight to the plutonic processes beneath one of the most productive quaternary rhyolite centers on earth sims u th data for 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 208 4 ka mswd 1 2 n 72 the remaining crystals are at secular equilibrium and u pb analyses indicate that a few of these outliers date back to 750 ka a period significantly longer than the known volcanic record at ovc probably 550 ka in contrast zircon crystallization in co erupted pumice and lava of the 0 7 ka kaharoa event and that of the three preceding rhyolite eruptions occurred mostly during 0 50 ka reflecting a separate magmatic history brittle deformation features incipient alteration and low 18o whole rock compositions 3 are consistent with a shallow solid intrusion that has interacted with meteoric hydrothermal fluids however 18o sims analyses of zircons 5 4 0 2 n 11 are consistent with magmatic compositions and thus meteoric interaction occurred post emplacement the kaharoa granitoids contrast with those ejected in the 40 60 ka caldera forming rotoiti event that were partly molten and display zircon age spectra indistinguishable from those for co erupted pumices suggesting the latter were derived from contemporaneous crystal mush the 0 7 ka kaharoa case shows that zones of the magmatic system remained solid despite frequent and voluminous magma production that included caldera forming events such long lived sub solidus zones in magma systems would act as barriers to melt connectivity and interaction with country rock but are also a potential source for antecrysts xenocrysts in subsequent eruptions this exemplifies how plutonic and volcanic evolution can diverge even in close proximity

some remarks on u th mineral ages from igneous rocks with prolonged crystallisation histories abstract mineral isochron dating is a frequently used geochronological tool one of its assumptions is that the minerals grow over a time period that is small compared to the half life of the radiogenic isotope system used in recent years increasing analytical precision has promoted the use of the short lived u series isotope system in order to date young crystallisation events three whole rock zircon u th isochrons from the 26 5 ka oruanui eruption in the taupo volcanic zone new zealand yield pre eruptive model ages of 5 5 0 8 ka 9 7 1 7 ka and 12 3 0 8 ka for the sub 63 m 63 125 m and 125 250 m zircon size fractions respectively this suggests that in this case the assumption of instantaneous crystal growth breaks down instead the u th data may be explained by continuous zircon growth over a period of 90 ka however cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector zoned euhedral rim and the u th data can also be modelled by mixing an older 27 ka model age population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the oruanui rhyolite this indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment it is concluded that conventional u series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude in future microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract recent drilling at the ngatamariki geothermal field taupo volcanic zone new zealand has provided new constraints on the stratigraphy and volcanic evolution of the region over 2800 m thickness of volcanic products are present at ngatamariki mainly comprised of rhyolitic ignimbrites linked to large caldera forming events at sources outside the field area but locally sourced andesite and rhyolite lavas and domes are also encountered most of the rocks are allocated to the pre 0 35 ma tahorakuri formation crystallisation age spectra and consequent best estimates of eruption age have been obtained by u pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by secondary ion mass spectrometry techniques using a shrimp rg instrument the oldest rock dated is an ignimbrite which yields an eruption age estimate of 1 85 0 06 ma this ignimbrite plus comparable aged units dated at the adjacent rotokawa and ohaaki geothermal fields are interpreted to represent the oldest silicic volcanic deposits in the area and onlap the basal andesite lava pile that is best developed at rotokawa to the south other pyroclastic units and associated volcaniclastic sediments with another intercalated andesite lava unit return age estimates between 1 85 0 06 and 0 701 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma the area was buried by two ignimbrite packages of the whakamaru group erupted from sources just west and well north respectively of the field ignimbrites of the waiora formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0 282 0 011 and 0 257 0 011 ma coeval with more extensive volcanic activity in the maroa dome complex west of the field sediments of the huka falls formation and deposits of the 25 4 0 2 ka oruanui eruption then cap and seal the system the new u pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the ngatamariki geothermal field the field despite 2 8 km of subsidence does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of quaternary age two chemically and temporally distinct hydrothermal events are located at the ngatamariki field with no evidence of continuity between the two