synchronizing terrestrial and marine records of environmental change across the eocene oligocene transition records of terrestrial environmental change indicate that continental cooling and or aridification may have predated the greenhouse icehouse climate shift at the eocene oligocene transition eot by ca 600 kyr in north america marine terrestrial environmental change asynchronicity is inferred from a direct comparison between the astronomically tuned marine eot record and published 40ar 39ar geochronology of volcanic tuffs from the white river group wrg sampled at flagstaff rim wyoming and toadstool geologic park nebraska which are type sections for the chadronian and orellan north american land mammal ages we present a new age model for the wrg underpinned by high precision 206pb 238u zircon dates from 15 volcanic tuffs including six tuffs previously dated using the 40ar 39ar technique weighted mean zircon 206pb 238u dates from this study are up to 1 0 myr younger than published anorthoclase and biotite 40ar 39ar data calibrated relative to fish canyon sanidine at 28 201 ma giving consideration to the complexities strengths and limitations associated with both the 40ar 39ar and 206pb 238u datasets our interpretation is that the recalculated 40ar 39ar dates are anomalously old and the 206pb 238u zircon dates more accurately constrain deposition 206pb 238u calibrated age depth models were developed in order to facilitate a robust intercomparison between marine and terrestrial archives of environmental change and indicate that i early orellan terrestrial cooling recorded at toadstool geologic park was synchronous with the onset of early oligocene antarctic glaciation and ii the last appearance datums of key chadronian mammal taxa are diachronous by ca 0 7 myr between central wyoming and nw nebraska

zircon petrochronology reveals the temporal link between porphyry systems and the magmatic evolution of their hidden plutonic roots the eocene coroccohuayco deposit peru abstract we present zircon geochronologic la icpms and id tims trace element and hf isotopic evidence for a complex evolution of the plutonic roots of the eocene coroccohuayco porphyry system southern peru la icpms u pb dating has initially been carried out to optimize grain selection for subsequent high precision id tims dating and to characterize crustal assimilation xenocrystic cores this combined in situ and whole grain u pb dating of the same grains has been further exploited to derive a robust temporal interpretation of the complex magmatic system associated with the coroccohuayco porphyry skarn deposit our data reveal that a heterogeneous gabbrodioritic complex was emplaced at ca 40 4 ma and was followed by a nearly 5 ma long magmatic lull until the emplacement of dacitic porphyry stocks and dykes associated with the mineralizing event at ca 35 6 ma however at the sample scale zircons from the porphyries provide insight into a 2 ma long lived hidden magmatism probably at 4 9 km paleodepth prior to porphyry intrusion and mineralization for which no other evidence can be found on the surface today these dates together with zircon trace element analysis and hf isotopes argue for the development of a long lived magmatic system dominated by amphibole fractionation with an increasing amount of crustal assimilation and the development of a large and sustained thermal anomaly the system was probably rejuvenated at an increasing rate from 37 5 to 35 6 ma with injection of fresh and oxidized magma from the lower crust which caused cannibalism and remelting of proto plutons the porphyry intrusions at coroccohuayco were emplaced at the peak thermal conditions of this upper crustal magma chamber which subsequently cooled and expelled ore fluids zircon xenocrysts and hf isotopes in the porphyritic rocks suggest that this large upper crustal system evolved at stratigraphic levels corresponding to triassic sediments similar to the mitu group that may be present below the district using the zircon ce anomaly as a proxy for oxidation state of the magma through time we show that the high oxidation state of the porphyries is not the result of upper crustal processes but is rather controlled by magmatic processes occurring at deeper levels a comparison of our data with available high precision geochronologic data at other porphyry systems suggests that such deposits may form when injection rate volume and heat of their long lived upper crustal magmatic system reach their peaks these features might be diagnostic of a productive deposit

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

a new method integrating high precision u pb geochronology with zircon trace element analysis u pb tims tea increased precision in isotope dilution thermal ionization mass spectrometry id tims u pb geochronology has revealed age complexities in zircon populations that require new tools for understanding how the growth of zircon is related to geologic processes u and pb are routinely separated from other elements in dated minerals by ion exchange separation prior to tims isotope measurement we develop a method in which trace elements in the exact same volume of zircon are redissolved and analyzed using solution nebulization inductively coupled plasma sector field mass spectrometry with matrix matched external liquid calibration using 0 5 ml solution resulting concentrations are between 1 ppt for elements such as ti nb and ta and tens of ppb for zr by analyzing a series of standard solutions zircons and procedural blanks we show that accurate measurements are performed on zr hf y sc and the hree while low concentration elements can be measured accurately to 5 ppt we performed combined u pb id tims geochronology with trace element analysis here called u pb tims tea on zircons from eight volcanic rocks comprising several volcanic systems and one metamorphic sample similar to previous in situ trace element analyses zircon geochemistry is distinct between different samples and records petrogenetic processes such as fractional crystallization assimilation and or magma mixing unique from in situ analysis u pb tims tea can trace geochemical evolution in accessory minerals with adequate age precision to resolve magmatic processes in rocks at least 200 million years old this provides a means to identify auto ante and xenocrystic zircon and lead to more robust age interpretations in id tims u pb geochronology one suite of cretaceous andesitic zircons shows correlations in geochemistry and absolute time that record evolution of a magmatic system over 250 ka prior to eruption future work will combine u pb tims tea with solution isotopic analysis of nd sr and hf and will be applied to a host of datable minerals such as monazite sphene apatite rutile xenotime and baddeleyite these combined tools will provide access to an improved understanding of a wide range of igneous and metamorphic processes as a function of time

a rapid mechanism to remobilize and homogenize highly crystalline magma bodies the largest products of magmatic activity on earth the great bodies of granite and their corresponding large eruptions have a dual nature homogeneity at the large scale and spatial and temporal heterogeneity at the small scale this duality calls for a mechanism that selectively removes the large scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal rich viscous conditions seemingly resistant to mixing here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization unzipping of highly viscous crystal rich mushes we demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning what distinguishes our model is the recognition that the process has two stages initially a stiff mushy magma is reheated from below producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer when the thickening mobile layer becomes sufficiently buoyant it penetrates the overlying viscous mushy magma this second stage rapidly exports homogenized material from the lower mobile layer to the top of the system and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption the agreement between calculated and observed unzipping rates for historical eruptions at pinatubo and at montserrat demonstrates the general applicability of the model this mechanism furthers our understanding of both the formation of periodically homogenized plutons crust building and of ignimbrites by large eruptions

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

voluminous plutonism during volcanic quiescence revealed by thermochemical modeling of zircon five late pleistocene lava domes with a combined eruptive volume of 40 km 3 distributed over an area of 2000 km 2 represent the waning stages of the 10 1 ma ignimbrite flare up in the altiplano puna volcanic complex apvc of the central andes zircon crystal face on unsectioned rims and interior on sectioned crystals ages u th and u pb respectively for a total of 252 crystals indicate remarkably consistent zircon crystallization histories the youngest zircon surface ages ca 104 83 ka are near 40 ar 39 ar eruption ages from sanidine and biotite ca 120 87 ka but a significant population of surface ages predates eruption ranging to secular equilibrium with u pb interior ages to 3 5 ma the essentially continuous zircon crystallization history implies protracted magma presence which agrees with temporally invariant ti in zircon model temperatures backed by the homogeneity of indirectly temperature dependent compositional parameters zircon age spectra modeled using a finite difference thermal and mass balance model for open system magma evolution indicate protracted zircon production in the magma reservoirs that require time integrated recharge rates of 1 10 3 km 3 yr corresponding to high intrusive to extrusive ratios of 75 1 this rate is below the 5 10 3 km 3 yr threshold proposed in the literature for incubating the supereruptions defining the flare up when accounting for the shorter durations of high versus low recharge episodes over the 10 m y lifetime of the apvc flare up the contributions to composite batholith formation in the shallow crust of the apvc remained broadly constant during peaks and lulls in eruptive activity this connotes that eruptive fluxes are a poor measure for intrusive fluxes a corollary of this interpretation is that commonly applied intrusive to extrusive ratios will severely underestimate pluton formation rates during periods of low eruptive flux

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

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

re evaluation of the ages of 40ar 39ar sanidine standards and supereruptions in the western u s using a noblesse multi collector mass spectrometer abstract the rewards from resolving the tempo of magmatic paleobiologic paleoclimatic and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision accuracy and calibration of several chronometers including the 40 ar 39 ar variant of the k ar clock a new generation of multi collector mass spectrometers for 40 ar 39 ar geochronology provides the means to address these issues as well as to evaluate the ages of 40 ar 39 ar standards and key marker horizons deposited by large volume caldera forming supereruptions in the western u s at unprecedented temporal resolution we highlight the utility of a 5 collector noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 ar h 35 cl from a 36 ar h 35 cl 12 c 3 sum peak at m e 36 maintains a remarkably low and stable background and thus yields 40 ar 39 ar dates of exceptionally high precision we have calibrated a reference gas and developed a simple analytical routine involving one peak hop both of which are used to evaluate detector bias correction factors because the 40 ar 39 ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age precise and accurate determination of the ages of these standards and verification of isotopic composition and homogeneity is essential we present results from both total fusion and incremental heating experiments on single crystals of the widely used alder creek acs and fish canyon fcs sanidine standards several recent independent measurements of acs using multi collector mass spectrometers including those presented herein favor an age of 1 1864 0 0003 0 0012 ma 95 confidence analytical full external uncertainty that is 1 younger than the widely used value obtained by nomade et al 2005 using a single collector mass spectrometer incremental heating experiments on single crystals of fcs most commonly do not yield plateau ages instead many crystals appear to be affected by ar loss or excess ar our findings obtained using ion counters independently confirm those recently obtained using a multiple faraday detectors or a combination of faraday and ion counting detectors thus careful consideration must be given to using fcs in future high precision 40 ar 39 ar calibrations of the geologic timescale notwithstanding 40 ar 39 ar dating of sanidine using the 5 collector noblesse can now provide leverage critical to interpreting the large range of u pb zircon dates commonly found in rhyolitic lavas and tuffs

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

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

a review of molybdenite and fluorite mineralisation in caledonian granite basement western ireland incorporating new field and fluid inclusion studies and re os and u pb geochronology abstract the recent discovery of late magmatic quartz vein hosted molybdenite and exceptional gem quality vein fluorite in the caledonian galway granite complex ggc has prompted a review of these contrasting styles of mineralisation in the late caledonian granite basement connemara western ireland existing published u pb and re os chronometry and fluid inclusion microthermometry are combined with new a geological field observations b u pb zircon and re os molybdenite geochronometry and c fluid inclusion microthermometry to generate a new pressure temperature time model p t t of mineralisation for the ggc re os chronometry molybdenite indicates that granite related molybdenite mineralisation extended from 423 to 380ma overlapping with the ggc emplacement history determined by u pb zircon chronometry the p t t model reflects initial granite emplacement and mo mineralisation at 423ma followed by lower p and t granite emplacement and related quartz vein hosted mo mineralisation at 410ma carna pluton 400ma kilkieran pluton and at 380ma costelloe murvey granite the gem quality fluorite veins in the ggc represent late triassic hydrothermal mineralisation these veins form part of a regional n atlantic european triassic jurassic hydrothermal mineralisation province vein emplacement was triggered by the rifting of the n atlantic facilitating crustal thinning and subsidence of continental crust and initiating hydrothermal activity at the margins of mesozoic basins

protracted late magmatic stage of the caleu pluton central chile as a consequence of heat redistribution by diking insights from zircon data and thermal modeling abstract zircon u pb geochronology and geochemistry are combined with whole rock composition and thermal modeling to decipher the late magmatic stage of the composite cretaceous caleu pluton which consists of four lithological zones gabbro diorite zone gdz quartz monzodiorite zone qmdz granodiorite zone gz and monzogranite zone mgz the four lithological zones include felsic dikes and veins of variable thickness and distribution zircons of four representative samples each from the mentioned zones were dated and chemically analyzed the u pb ages exhibit sample scale scatter derived from protracted zircon crystallization at pluton scale the ages are substantially overlapped with a subtle decrease of ages from mafic to felsic sample the latter has a normal age span distribution with a mean age of 94 68 0 71 2 confidence and a mswd of 0 95 zircon grains from the uppermost zone of the pluton where the qmdz is emplaced have the highest ree and hfse contents zircon crystallization temperatures oscillate between 680 and 850 c regardless of the zircon age and sample composition differences in temperature and age of zircon crystallization of up to 185 c and 2 6 myr were identified at sample scale respectively numerical modeling indicates that the melts from which zircon crystallized are highly crystalline mostly higher than 60 crystal and resemble mgz in compositions time dependent thermal models were performed to account for preservation of the system above solidus temperature for long time intervals consistent with those of zircon crystallization two non exclusive scenarios for the late stage development of caleu pluton were considered i pluton construction by magma pulses assembled incrementally and ii upward transport of residual melts by diking through a mush system to yield heat redistribution to the levels where the samples collected the first scenario does not preserve residual melts for intervals as long as 2 6 myr unless an extremely thick magma reservoir is considered the second scenario is more favorable because it could provide enough heat that allows preserving residual melts above the solidus temperature depending on i dike width ii melt transport velocity and iii dike intensity vol dike for a dike width of 0 2 m and dike intensity of 10 consistent with field observations a transport velocity of 300 m yr is required to maintain the upper mush zone above 700 c the melt transport would have occurred as successive events to allow developing the protracted late magmatic stage of the caleu pluton

magma evolution leading to porphyry au cu mineralization at the ok tedi deposit papua new guinea trace element geochemistry and high precision geochronology of igneous zircon

tracking the evolution of large volume silicic magma reservoirs from assembly to supereruption the most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs the thermal evolution of these subcaldera magma reservoirs controls the volume of eruptible magma and eruptive style here we combine high precision zircon u pb geochronology trace element analyses of the same mineral grains and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the oligocene fish canyon tuff magma reservoir colorado united states as a function of absolute time systematic compositional variations in u pb dated zircons record 440 k y of magma evolution an early phase of volumetric growth was followed by a period of cooling and crystallization during which the fish canyon magma approached complete solidifi cation subsequent remelting due to underplated andesitic recharge magmas began 219 45 k y prior to eruption and led to the generation of 5000 km3 of eruptible crystal rich 45 vol dacite age equivalent but compositionally different zircons in an andesite enclave from late erupted fish canyon tuff tie the growth and thermal evolution of the upper crustal reservoir to a lower crustal magma processing zone our results demonstrate that the combination of high precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fl uid dynamic modeling

a strategy for cross calibrating u pb chronology and astrochronology of sedimentary sequences an example from the green river formation wyoming usa abstract astronomical calibration of the geological timescale has been limited until recently by the precision and accuracy of radioisotopic dates especially for pre neogene records uncertainties for radioisotopic dates of older strata were typically much larger than a single precessional cycle and dates were often sparse leading to the practice of orbital tuning of cyclic strata in order to astronomically calibrate the desired interval ideally in order to test the assumptions of astronomical calibration with geochronology it is necessary that the precision of radioisotopic dates be comparable to the period of the cycle being tested the new u pb ca tims chemical abrasion thermal ionization mass spectrometry zircon dates reported here conform to this precision requirement with 2 analytical uncertainties from 11 000 to 52 000 years for seven volcanic ashes from the wilkins peak member of the green river formation the zircon dates have simple distributions with few outliers and allow accurate estimations of the eruption ages with potential inaccuracies of less than precessional cycle the eocene green river formation wyoming usa has long been recognized as a record of cyclicly deposited lacustrine sediments and the abundant intercalated volcanic ashes make it a suitable place to test new approaches to astronomical calibration of cyclic strata the abundance of different types of marker beds including tuffs that are intercalated with the sedimentary cycles guarantee an unambiguous correlation between sampling locations of dated tuffs on the margins of the basin and the basin center where the cyclicity is best developed thus reducing any stratigraphic uncertainties to a fraction of hypothesized precession cycle tuning based orbital age models accepted by the previous geochronology significantly deviate from the new geochronology whereas a previously rejected model that assumes a short eccentricity period of 125 ky is now allowed in order to test possible explanations for the apparent 125 ky period such as changes in orbital periods or gaps in the sedimentary record we present an iterative strategy to select future ashes for dating such that the astronomical calibration testing is optimized we iteratively contrast two ad hoc age models that bracket the linear interpolation between the dated ashes the optimal intervals for further dating are located where the deviations between the models exceed our reported uncertainties we propose that the iterative approach described here should become the standard for establishing a rigorous orbital calibration of the stratigraphic record where sufficient ashes exist

wa1ms a 2 61 ga muscovite standard for 40ar 39ar dating abstract the 40ar 39ar dating technique requires the use of neutron fluence monitors standards to allow calculation of the age of a sample precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40ar 39ar dates few fully intercalibrated 40ar 39ar standards with a sufficient total fusion grain to grain reproducibility are currently in use in the argon community for precambrian samples only hb3gr hornblende 1 08 ga yields sufficient grain to grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements here we present chemical and intercalibration results for a new 2 61 ga standard wa1ms is a muscovite extracted from an archaean shear zone in the lake johnston greenstone belt western australia in situ trace element analysis by ela icpms revealed consistent k contents subtle zonation and intra grain and grain to grain heterogeneities in rb sr ti and fe but a lack of mineral inclusions wa1ms has been investigated over 3 irradiations ranging from 25 to 40 h in two reactors with several disc positions and three grains sizes and has been calibrated against fcs and ga1550 and hb3gr overall we carried out 48 total fusion and 4 step heating experiments of wa1ms crystals flat age spectra and average f value 40ar 39ark relative standard deviations ranging from of 0 43 to 0 60 p 0 15 0 83 for 47 48 analyses demonstrate the reproducibility of wa1ms and its suitability as a reliable 40ar 39ar standard we calculated r wa1ms fcs 205 59 0 25 r wa1ms ga1550 57 25 0 06 and r wa1ms hb3gr 3 9713 0 014 all with p 0 14 allowing direct comparison between wa1ms and any standards in used in the community provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards the recently revised decay constant values and standard ages proposed by renne et al 2011 yield a weighted mean age of 2614 2 1 5 ma 0 055 p 0 8 1 for wa1ms against the three standards when calibrated solely against fcs in order to minimize correlated errors between fcs vs ga1550 and hb3gr we calculate a preferred age of 2613 0 2 4 ma 0 09 which is recommended when using this standard additionally this corresponds to a 40ar 40k value of 0 3375 0 0057 10 1 for wa1ms which is independent of the value of the decay constant used for calculation and allows recalculation of the age of wa1ms for any preferred set of decay constants and standard ages using the constants proposed by steiger and jager 1977 directly calculated from the three r values yields an age of 2598 2 3 5 ma corresponding to an identical although less precise 40ar 40k value of 0 3375 0 0086 10 1 benefits of this standard include 1 an increased relative precision per crystal analyzed relative to other standards for proterozoic and archaean samples 2 no post irradiation waiting time due to the absence of ca no need to wait for 37arca decay and 3 loading of standard grains that are optically different from the unknown e g pyroxene or hornblende to avoid standard unknown mixing wa1ms is freely available to the 40ar 39ar community

the role of mantle derived magmas in the isotopic evolution of yellowstone s magmatic system injection of mantle derived magmas into the earth s crust provides the heat necessary to develop and maintain large silicic magmatic systems however the role of mantle derived magmas in controlling the compositional evolution of large silicic systems remains poorly understood here we examine the role of mantle derived magmas in the post caldera magmatic system at yellowstone plateau the youngest magmatism associated with the yellowstone hotspot using microbeam techniques we characterize the age and hf isotope composition of single zircon crystals hosted in rhyolites from the most recent eruptive episode at yellowstone plateau which produced the central plateau member rhyolites we place these zircon data into context by comparing them to new solution hf isotope data for the central plateau member glasses yellowstone basalts and potential local crustal sources zircons in the central plateau member rhyolites record a wide range of hf isotope compositions relative to their host melts and extend from values similar to previously erupted yellowstone rhyolites to values similar to yellowstone basalts most zircons 90 are in isotopic equilibrium with their host melt but a significant proportion show ehf values higher than their host melt thus providing the direct evidence that silicic derivatives of mantle derived basalts have recharged yellowstone s magmatic system mixing models confirm that the isotopic characteristics of the youngest yellowstone rhyolites can be explained by recharge of yellowstone s magma reservoir with silicic derivatives of underplating mantle derived basalts 5 to 10 material added by mass this process helps drive the long term isotopic evolution of yellowstone s magmatic system this article is protected by copyright all rights reserved

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

in situ u pb ages of zircons from the bishop tuff no evidence for long crystal residence times ion microprobe u pb isotope analyses of zircons from fallout and intercalated ash flow deposits of the early most chemically evolved bishop tuff east central california yield a weighted mean age of 823 11 ka mean square of weighted deviates mswd 0 8 n 22 a similar mean age of 839 36 ka mswd 0 6 n 15 obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the bishop tuff obtained by recent ar isotope determinations may be largely analytical in origin considerations based only on the effect of 230 th 234 u disequilibria on the u pb ages yield an apparent maximum mean crystal residence time for zircon of 100 k y the zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that once evolved the early bishop tuff rhyolite was not stored for long before erupting

second reply to comment of r s j sparks h e huppert and c j n wilson on evidence for long residence times of rhyolitic magma in the long valley magmatic system the isotopic record in the precaldera lavas of glass mountain

zircon reveals protracted magma storage and recycling beneath mount st helens current data and models for mount st helens volcano washington united states suggest relatively rapid transport from magma genesis to eruption with no evidence for protracted storage or recycling of magmas however we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust cool and crystallize beneath the volcano and are then rejuvenated and incorporated by hotter young magmas on their way to the surface estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time recording growth from hotter less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface or changes in whole rock geochemistry and petrology and providing a new time correlated record of this evolution independent of the eruption history zircon data thus reveal the history of the hidden long lived intrusive portion of the mount st helens system where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

magmatic perturbations in the okataina volcanic complex new zealand at thousand year timescales recorded in single zircon crystals abstract using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system if micron scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems then new insights into long term maintenance and storage of eruptible magma can be unlocked this study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50 000 yrs of changing physical and chemical conditions within a magma reservoir trace elements hf and y in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized thus different stages of growth can be associated with discrete time periods in the magmatic system zircon from the two most recent rhyolite eruptions kaharoa and whakatane within the okataina volcanic complex ovc new zealand have both isotopic age and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system this discrete change is associated with caldera collapse and reflects a change from cold wet source rhyolite to a relatively hotter and drier source rhyolite and back the kaharoa and whakatane eruptions are separated by 4000 yrs and 15 km distance yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 10 3 to 10 5 yrs that is periodically extracted during eruption

towards accurate numerical calibration of the late triassic high precision u pb geochronology constraints on the duration of the rhaetian numerical calibration of the late triassic stages is arguably the most controversial issue in mesozoic stratigraphy despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end triassic mass extinction here we report new chemical abrasion isotope dilution thermal ionization mass spectrometry zircon u pb dates for volcanic ash beds within the aramachay formation of the pucara group in northern peru that place precise constraints on the age of the norian rhaetian boundary nrb and the duration of the rhaetian the sampled ash bed bearing interval is located just above the last occurrence of the bivalve monotis subcircularis placing this stratigraphic sequence in the uppermost norian perhaps ranging into the earliest rhaetian zircon u pb dates of ash beds constrain the deposition age of this interval to be between 205 70 0 15 ma and 205 30 0 14 ma providing precise constraints on the age of the nrb combined with previously published zircon u pb dates for ash beds bracketing the triassic jurassic boundary we estimate a duration of 4 14 0 39 m y for the rhaetian this ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end triassic mass extinction

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

zircon petrochronology and 40ar 39ar sanidine dates for the mesa falls tuff crystal scale records of magmatic evolution and the short lifespan of a large yellowstone magma chamber

multiple rejuvenation episodes of a silicic magma reservoir at the origin of the large diatreme dome complex and porphyry type mineralization events at cerro de pasco peru abstract the cerro de pasco district in central peru hosts one of the world largest porphyry related epithermal polymetallic deposits the district is centered onto a large diatreme dome complex crosscut by numerous dacite to rhyodacite bodies showing domal structures and quartz monzonite dykes three temporally distinct high temperature porphyry type mineralization events have been recognized pm1 pm2 and pm3 dating of the latter by molybdenite re os and by zircon u pb la icp ms and ca id tims geochronology complements the already available dataset of zircon u pb ages of the subvolcanic and volcanic rocks mostly dacitic in composition occurring in the district and shows the existence of successive short lived episodes 50 at pressures between 0 9 and 3 4 kbar and at temperatures between 680 c and 725 c at such conditions magma is beyond the point of rheological lock up and is not eruptible the emplacement of the mostly dacitic subvolcanic and volcanic rocks has required a series of rejuvenation events of the upper crustal high crystallinity silicic magma reservoir field relationships and geochronology indicate that emplacement of the subvolcanic and volcanic rocks is preceded each time by high temperature 600 c quartz pyrite magnetite chalcopyrite and quartz molybdenite veining forming the three recognized porphyry type mineralization events hydrothermal quartz of these veins host silicate melt inclusions a rare feature in hydrothermal veins their composition determined by la icp ms analysis indicates that the mineralizing fluids potentially sourced from intermediate magma recharges have circulated in a magma reservoir in which the residual interstitial melt was more evolved than the melt trapped as inclusions in the magmatic quartz the combined geochemical and geochronological data obtained show that several episodes of rejuvenation of a highly crystallized upper crustal silicic magma reservoir probably triggered by intermediate magma recharges and by circulation of co2 and s rich fluids exsolved from them are at the origin of the large diatreme dome complex and porphyry type mineralization events at cerro de pasco

time scale controversy accurate orbital calibration of the early paleogene 1 timing is crucial to understanding the causes and consequences of events in earth history the calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating uncertainties in the computations of earth s orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 ma attempts to construct a robust astronomically tuned time scale for the early paleogene by integrating radioisotopic and astronomical dating are only partially consistent here using the new la2010 and la2011 orbital solutions we present the first accurate astronomically calibrated time scale for the early paleogene 47 65 ma uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the fish canyon standard comparison with geological data confirms the stability of the new la2011 solution back to 54 ma subsequent anchoring of floating chronologies to the la2011 solution using the very long eccentricity nodes provides an absolute age of 55 530 0 05 ma for the onset of the paleocene eocene thermal maximum petm 54 850 0 05 ma for the early eocene ash 17 and 65 250 0 06 ma for the k pg boundary the new astrochronology presented here indicates that the intercalibration and synchronization of u pb and 40ar 39ar radioisotopic geochronology is much more challenging than previously thought

seismic wave attenuation and source excitation in la paz los cabos baja california sur mexico we present results from a regional study of seismic wave attenuation and source excitation from small magnitude earthquakes recorded at distances from 6 to 180 km in the la paz los cabos region at the south end of the baja california peninsula data were recorded using 32 strong motion seismic stations from the la paz network lap a least squares regression separating the excitation site and propagation effects was carried out we performed the analyses in the time and frequency domains and we compared these results with results from a coda normalization method the propagation term was parameterized to represent a geometrical spreading function and a frequency dependent q f at a reference distance of 40 km we estimated the regional attenuation by measuring the maximum amplitude of the s or lg waves as a function of frequency defining a continuous piecewise propagation term d r f after separating the excitation and site terms our results show that the attenuation is lower than that of central or northern mexico recorded data were of remarkably good quality despite the fact that the strong motion network recorded only small earthquakes our best model is that of a quality factor of q f 380 f 0 10 with a complex geometrical spreading function

formation of rhyolite at the okataina volcanic complex new zealand new insights from analysis of quartz clusters in plutonic lithics granitoid lithic clasts from the 0 7 ka kaharoa eruption at the tarawera volcano okataina volcanic complex taupo volcanic zone new zealand provide insight into the processes of rhyolite formation the plutonic lithic clasts of the kaharoa eruption consist of 1 quartz phenocrysts which are often grouped into clusters of two to eight quartz grains 2 plagioclase phenocrysts mostly an 40 with up to an 60 cores and 3 interstitial alkali feldspar quartz orientations obtained through electron backscatter diffraction ebsd methods show that 78 of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched variations in cathodoluminescence cl zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events the process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation the quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and or compaction the preservation of oriented quartz clusters from the kaharoa plutonic lithics thus provides evidence for synchronous shallow pluton formation from a cumulate mush during active silicic volcanism this result is consistent with models whereby melt rich high silica rhyolite formation occurs via interstitial melt extraction from a low silica rhyolite mush in the shallow crust

rejuvenation of the fish canyon magma body a window into the evolution of large volume silicic magma systems voluminous unzoned phenocryst rich pyroclastic deposits considered as erupted batholiths provide a unique opportunity to investigate magmatic processes in silicic magmas the fish canyon tuff a well documented example of these monotonous ignimbrites displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near isobaric reheating from 720 to 760 8c these observations along with a high crystallinity 45 and near solidus mineral assemblage suggest that the fish canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption rejuvenation was triggered by intrusion of water rich mafic magmas at the base of the fish canyon mush but the mechanisms of heat transfer remain poorly understood the growth of amphibole during reheating requires addition of mafic components but the absence of any measurable gradients and the paucity of mafic enclaves in the fish canyon magma rule out a reheating event dominated by convective mixing with a mafic magma closed system processes such as heat conduction and convective self mixing could not account for the transport of externally derived mafic components we performed numerical simulations of upward percolation of a hot low density h2o co2 fluid phase gas sparging through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems sparging by 20 40 km 3 of gas extracted from 3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by 40 8c in 150 200 k y moreover the vertical thermal gradient after 150 k y in most of the mush is small 25 8c in the upper 65 gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal poor rhyolites by expelling interstitial melt however our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com