three dimensional electrical resistivity image of magma beneath an active continental rift taupo volcanic zone new zealand 1 magmatic activity in regions of continental extension may result in huge 400 km 3 explosive eruptions of viscous gas rich silicic magma geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long lived mush zone a mixture of solid crystals and liquid melt that accumulates in liquid dominated lenses at the top of a much thicker region of lower melt fraction mush such lenses will be highly electrically conductive compared with normal mid crustal rocks here we use results of 220 magnetotelluric mt soundings to construct a 3 d electrical resistivity image of the northern silicic part of new zealand s taupo volcanic zone a young continental rift associated with very high heat flow and intense silicic volcanism the electrical resistivity image shows a plume like structure of high conductivity interpreted to be a zone of interconnected melt rising from depths 35 km beneath the axis of extension

stratigraphy and dispersal of the mangaone subgroup pyroclastic deposits okataina volcanic centre new zealand abstract the mangaone subgroup from the okataina volcanic centre consists of silicic plinian pyroclastic units that lie between the oruanui deposit from the taupo volcanic centre corrected 14 c calendar age 26 5 ka and the rotoiti flow rotoehu fall deposits from okataina volcanic centre k ar age 64 ka in this study we present a new stratigraphy for the subgroup that is significantly revised from the earlier work and was developed after extensive field mapping and laboratory study we now recognize and describe a total of 12 rhyolitic units as opposed to the 8 that had been recognized previously with a total bulk eruptive volume of approximately 81 km 3 the vents for all the mangaone subgroup units were within the haroharo caldera and possibly migrated through time first within a roughly north south trending zone in the central part of the caldera and then toward the eastern part of the caldera we present new radiometric ages suggesting that the whole sequence was erupted in a relatively short time span possibly 10 15 ka our findings of more eruptions in a shorter time span than in previous work underscores the utility of detailed field studies particularly when volcanic hazards are being assessed all the fall units have plinian dispersals but show various degrees of magma water interaction the majority of the eruptions were large and intense enough to generate pyroclastic flows yet flows were observed in only 2 units and is volumetrically small

insights from quartz cathodoluminescence zoning into crystallization of the vinalhaven granite coastal maine the vinalhaven intrusive complex provides field and petrographic evidence for multiple replenishments of mafic and silicic magmas mingling and limited mixing and rejuvenation of granite quartz in granitic rocks preserves a record of those processes in the form of cathodoluminescence cl zoning which is related to concentration of titanium and to temperature of crystallization using the new titaniq ti in quartz geothermometer injection of mafic melts into partly crystalline vinalhaven granite resulted in partial quartz resorption followed by higher temperature growth from h2o undersaturated melt this is shown by steep rimward increases in cl intensity and ti content across discordant boundaries that truncate older growth zones quartz zoning in granite affected by mafic magmas displays large rimward jumps in ti content whereas quartz in granitic feeders and in granite far from mafic rocks typically displays broad rims with decreasing ti contents consistent with slow cooling without thermal disruptions due to mafic recharge

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

rapid generation of both high and low δ18o large volume silicic magmas at the timber mountain oasis valley caldera complex nevada we present an oxygen isotope and pet rologic study of four voluminous zoned ash flow sheets of the southwestern nevada volcanic field swnvf topopah spring ts 1200 km 3 12 8 ma tiva canyon tc 1000 km in the basin and range province and it fa vors nearly total evacuation of the magma chamber during a single eruption each si licic magma unit was generated by a shal low influx of new mafic magma that melted 18 o 16 o depleted as in the case of at or 18 o 16 o enriched rm ts rocks the at tuff and associated pre and post caldera lavas are 2 5 lower in d 18 o than in the rm tuff and represent the largest known low d 18 o magma we find that all units of the at cycle contain isotopically zoned zir cons that have up to 2 core to rim zon ing and correspondingly smaller out of equilibrium quartz zircon and melt zircon fractionations air abraded cores of quartz and sphene do not preserve any d 18 o zon ing the higher d 18 o zircon cores in low d

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

zircon trace element chemistry at sub micrometer resolution for tarawera volcano new zealand and implications for rhyolite magma evolution zoned crystals can be important recorders of magmatic processes in space and time however in most situations the temporal dimension is difficult to quantify here we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non polished crystal faces of zircon to obtain 5 m resolution u th disequilibrium ages one pit that can be correlated with trace element zoning at sub m resolution derived from a second pit data from 17 crystals representing each of the four rhyolite eruptions of tarawera volcano an intra caldera edifice within the okataina volcanic centre reveal diverse zircon growth conditions over time most crystals display rimward depletions in zr hf and ti broadly consistent with cooling and crystallization however a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization oscillatory zonation patterns in y th and u are superimposed on the zr hf and ti trends despite the limited number of crystals analyzed in this way the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir by combining data from all crystals a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains 100 ka prior to caldera formation and is best explained by the post caldera system consisting of small isolated melt pockets that evolved independently an important conclusion is that the zircon cargo in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma

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

extraction of crystal poor rhyolite from a hornblende bearing intermediate mush a case study of the caldera forming matahina eruption okataina volcanic complex the matahina ignimbrite 160 km3 rhyolite magma 330 ka was deposited during a caldera forming eruption from the okataina volcanic centre taupo volcanic zone tvz new zealand juvenile clasts are divided into three groups group 1 the dominant crystal poor rhyolite type group 2 a minor coarse grained mingled mixed intermediate type and group 3 a rare fine grained basalt the ignimbrite consists of the group 1 type and is divided into three members a lower and middle member which is high silica crystal poor 10 vol rhyolite and the upper member which is low silica and slightly more crystal rich up to 21 vol cognate crystal rich up to 50 vol basalt to intermediate pumice occurs on top of lag breccias and within lithic rich pyroclastic density current deposits along the caldera margin groups 2 and 3 several lines of evidence indicate that the intermediate clasts represent the cumulate complement to the melt rich rhyolite 1 continuity in the compositions of plagioclase orthopyroxene hornblende and oxides and normal zoning of individual phenocrysts 2 the silicic glass from the intermediate magma interstitial melt overlaps compositionally with the bulk rock rhyolite and glass 3 high zr and a slight positive eu anomaly in the intermediate magma relative to quenched enclaves from other intermediate tvz eruptions indicates zircon and plagioclase accumulation respectively 4 an increase in the cl contents in glass from the least evolved to most evolved is consistent with the concentration of volatiles during magma evolution most of the compositional variations in the low to high silica rhyolites can be accounted for by continued rayleigh fractionation up to 15 following melt extraction from the underlying mush under varying fo2 fh2o conditions to form a slightly compositionally zoned rhyolitic cap this link to the varying fo2 fh2o conditions is evidenced by the strong correlation between key geochemical parameters e g dy y that qualitatively reflect fh2o conditions presence or absence of hornblende biotite and fo2 estimated from fe ti oxide equilibrium magma mingling mixing between the basalt andesite and the main slightly compositionally zoned rhyolitic magma occurred during caldera collapse modifying the least evolved rhyolite at the lower portion of the reservoir and effectively destroying any pre eruptive gradients

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

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

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

sub millennial eruptive recurrence in the silicic mangaone subgroup tephra sequence new zealand from bayesian modelling of zircon double dating and radiocarbon ages abstract accurate dating of young 38 4 1 4 1 7 ka bp and constrains the beginning of the msg eruption period to 42 7 3 5 3 7 ka bp unit a and the end to 30 6 1 5 0 6 ka bp unit l thus the entire msg sequence was emplaced in 12 100 years representing an eruption frequency of one event per 930 years on average our study demonstrates the efficacy of zdd to yield accurate eruption ages on pyroclastic deposits highlighting its potential for dating young

time scales and temperatures of crystal storage in magma reservoirs implications for magma reservoir dynamics the thermal and therefore physical state of magma bodies within the crust controls the processes and time scales required to mobilize magmas before eruptions which in turn are critical to hazard a

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

mushy magma beneath yellowstone a recent prospective on the yellowstone caldera discounts its explosive potential based on inferences from tomographic studies which suggests a high degree of crystallization of the underlying magma body in this study we show that many of the first teleseismic p wave arrivals observed at seismic stations on the edge of the caldera did not travel through the magma body but have taken longer but faster paths around the edge after applying a number of waveform modeling tools we obtain much lower seismic velocities than previous studies 2 3 km sec v p and 1 1 km sec v s we estimate the physical state of the magma body by assuming a fluid saturated porous material consisting of granite and a mixture of rhyolite melt and water and co 2 at a temperature of 800 c and pressure at 5 km 0 1 gpa we found that this relatively shallow magma body has a volume of over 4 300 km 3 and is about 32 melt saturated with about 8 water plus co 2 by volume

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

laacher see revisited high spatial resolution zircon dating indicates rapid formation of a zoned magma chamber high spatial resolution 230 th 238 u disequilibrium dating reveals rapid zircon crystallization prior to the 12 900 yr b p laacher see germany phonolite eruption zircons from lower laacher see tephra llst and syenitic subvolcanic nodules share ree and low u th characteristics and define an isochron age of 17 1 1 3 ka 1 zircons also show higher initial 230 th 232 th 0 and oxygen isotopic disequilibrium compared to their host phonolite thus llst zircons shortly predate the eruption and yet are not strictly part of the phonolite crystallization sequence based on their similarity to zircons in subvolcanic nodules they are instead interpreted to be scavenged from a precursor syenite intrusion which implies short a few k y filling and differentiation timescales for the laacher see magma chamber thorium and oxygen isotopes in zircon as well as evidence for abundant crustal xenocrysts further indicate extensive crustal contamination which allows for considerably faster basanite phonolite differentiation than the 100 k y time span previously estimated from bulk u series data

accessory mineral behavior during differentiation of a granite suite monazite xenotime and zircon in the sweetwater wash pluton southeastern california u s a abstract compositional and textural characteristics of the accessory minerals monazite xenotime and zircon in the sweetwater wash granites and related aplites indicate that these phases not only controlled much of the trace element geochemistry of the suite but that they also record the melt compositional changes that occurred during magmatic differentiation fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare earth element ree depletion this was accompanied by increasing heavy ree concentrations until xenotime joined the crystallizing assemblage subsequently combined monazite xenotime fractionation resulted in lowering of the entire ree budget zircon which contains inherited cores that were apparently resorbed and rounded during initial anatexis was saturated throughout the differentiation history of the sweetwater wash suite accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales the differences are best displayed by monazite on average crystals in more differentiated rocks aplites are relatively depleted in light ree s and contain higher concentrations of the substituting elements u and th zircon displays complimentary increases in hf and y while xenotime shows a slight increase in th and in gd ho ratios both phases also exhibit higher u concentrations in aplites than in granites these differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites but also within individual thin sections due to in situ fractionation on an even smaller scale strong compositional variations are present within single crystals possibly due to diffusion controlled melt compositional gradients in the regions a adjacent to growing major phases and b adjacent to the growing accessory crystal itself our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes but that the scale of observed variations must be carefully considered

millennial timescale resolution of rhyolite magma recharge at tarawera volcano insights from quartz chemistry and melt inclusions most rhyolite eruption episodes of tarawera volcano have emitted several physiochemically distinct magma batches 1 10 km3 these episodes were separated on a millennial timescale the magma batches were relatively homogeneous in temperature and composition at pumice scale 4 cm but experienced isolated crystallisation histories at the sub cm scale matrix glasses have trace element compositions sr ba rb that vary by factors up to 2 5 indicating incomplete mixing of separate melts some quartz hosted melt inclusions are depleted in compatible trace elements sr ti ba compared to enclosing matrix glasses this could reflect re melting of felsic crystals deeper in the crystal pile individual quartz crystals display a variety of cathodoluminescence brightness and ti zoning patterns including rapid changes in melt chemistry and or temperature 50 100 c and point to multi cycle crystallisation histories the tarawera magma system consisted of a crystal rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion

rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals silicic volcanic eruptions pose considerable hazards yet the processes leading to these eruptions remain poorly known a missing link is knowledge of the thermal history of magma feeding such eruptions which largely controls crystallinity and therefore eruptability we have determined the thermal history of individual zircon crystals from an eruption of the taupo volcanic zone new zealand results show that although zircons resided in the magmatic system for 103 to 105 years they experienced temperatures 650 to 750 c for only years to centuries this implies near solidus long term crystal storage punctuated by rapid heating and cooling reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales and our approach can help to quantify heat input to and output from magma reservoirs

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

deformation dewatering and decollement development in the antler foreland basin during the antler orogeny we present evidence from north central nevada that structures in the antler foreland basin formed before lithification of kinderhookian flysch indicating that deformation occurred during emplacement of the roberts mountains allochthon onto the north american shelf during the late paleozoic antler orogeny a decollement coeval with prelithification structures developed within the foreland basin at the transition from fine grained sediments below to coarse grained sediments above and suggests that the roberts mountains allochthon entered its own foreland basin at mid level in the sediment section as do modern accretionary prisms at fore arcs growth of the decollement was facilitated by dewatering of the fine grained sediments and high fluid pressures at the base of the coarse grained section deformation of the footwall suggests either footwall collapse or fluid pressures lower than those along decollements in modern fore arcs

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

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

chronology and evolution of caldera forming and post caldera magma systems at okataina volcano new zealand from zircon u th model age spectra u th disequilibrium model age data are presented for zircons from four young eruptive units from okataina volcano new zealand these data highlight contrasts in the time scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the 61 ka caldera forming rotoiti eruption 80 120 km 3 magma the rotoiti event was followed by 12 explosive eruptions of the mangaone subgroup between 45 and 30 ka a change in eruptive styles between 30 and 25 ka brackets the major oruanui eruption of taupo volcano 80 km away subsequently eight rhyolitic lava dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 ce we present 238 u 230 th zircon model age data determined by secondary ion mass spectrometry sims and thermal ionization mass spectrometry tims from the rotoiti ngamotu unit b 45 ka mangaone unit i 33 ka and rotorua 15 4 ka eruptive units we couple these data with published and new compositional information to trace magma crystallization and storage patterns population density curves of sims model ages from zircons in the two pumice types biotite free and biotite bearing from the rotoiti eruption differ zircons from the former yield a model age peak coincident with eruption age whereas zircons from the latter show a peak at 70 90 ka and little variation in tims model age values from different crystal size fractions concentration weighted means of model ages from the two pumices however are the same within 1 sd error and their sr isotopic values overlap at 2 sd precision suggesting that they are genetically linked model age spectra for the ngamotu mangaone and rotorua pumices are complex indicating recycling of crystals from multiple older populations that largely pre date the 61 ka caldera forming eruption superimposed on the older age spectra are variably developed younger pre eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption a lack of commonality in these younger peaks in the post caldera eruptive rocks along with compositional and isotopic differences between and sometimes within the eruptive units collectively precludes their origin from a single melt bearing mush system okataina contrasts with taupo where comparable age eruption deposits have simpler age spectra consistent with larger scale crystallization cycles and thermal events in the magma chambers there when compared with other caldera related silicic systems for which suitable data are available okataina differs in lacking a simple pre eruptive history prior to its caldera forming event and having a complex non coherent magmatic history of post caldera eruptions

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

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

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

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

middle late pleistocene tephrostratigraphy of the eastern bay of plenty new zealand abstract up to fifty tephras ranging in age from ca 500 to 50 ka are recorded in a terrestrial sequence of loess and paleosols in the eastern bay of plenty north island new zealand tephra correlations are based on the distinctive physical characteristics of the airfall beds and confirmed by microprobe analysis of glass shards the deposits are divided into six subgroups with their boundaries marked by either major tephras or significant changes in the paleo climate indicator deposits such as loess and paleosols age control on these tephras has been provided by fitting the paleoclimatic data seen in the field to the specmap oxygen isotope curve with correlations to a few well dated tephras and ignimbrites providing key time planes within this record

organization of oscillatory zoning in zircon analysis scaling geochemistry and model of a zircon from kipawa quebec canada abstract the character of oscillatory zoning within a zircon crystal from the syenite kipawa complex quebec varies with scale of observation analysis of an scanning electron microscopy sem back scatter gray scale traverse at a resolution of one pixel 2 43 m revealed 145 zones over 5130 m whereas a detailed high resolution one pixel 0 195 m section near the crystal rim revealed 225 zones over 795 m in order to mathematically characterize the zoning pattern wavelet fourier and nonlinear analysis techniques were used on profiles of the sem gray scale data and a series constructed was from the zone widths results demonstrate that the zircon oscillatory zoning preserves nonlinear and periodic components secondary ion mass spectrometry electron microprobe and sem analyses of trace elements show the sem back scatter bright zones are enriched in u th and rare earth elements ree in comparison to the darker zones ree patterns are sharply heavy ree enriched and have negative eu anomalies and prominent positive ce anomalies we model the oscillatory zoning including a measure of its chemical variation by use of a periodically forced nonlinear system results of this data driven model are quantitatively similar to the natural data we envisage that the small scale oscillatory zoning was the result of a nonlinear feedback process wherein the crystal growth modified the adjacent melt which in turn affected the crystal composition the large scale harmonic zones likely reflect changes in the bulk geochemistry of the system from which the zircon grew

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

magma mixing and the generation of isotopically juvenile silicic magma at yellowstone caldera inferred from coupling 238 u 230 th ages with trace elements and hf and o isotopes in zircon and pb isotopes in sanidine the nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time we examine compositional heterogeneity in the youngest 170 to 70 ka post caldera volcanism at yellowstone caldera the central plateau member cpm rhyolites as a case study we compare 238u 230th age trace element and hf isotopic data from zircons and major element ba and pb isotopic data from sanidines hosted in two cpm rhyolites hayden valley and solfatara plateau flows and one extracaldera rhyolite gibbon river flow all of which erupted near the caldera margin ca 100 ka the hayden valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the cpm reservoir the gibbon river flow hosts one zircon population that is compositionally distinct from hayden valley flow zircons the solfatara plateau flow contains multiple sanidine populations and all three zircon populations found in the hayden valley and gibbon river flows demonstrating that the solfatara plateau flow formed by mixing extracaldera magma with the margin of the cpm reservoir this process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs more generally hf isotopic data from the cpm zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post caldera magmatic system and demonstrate that the sources contributing magma to the cpm reservoir were heterogeneous in 176hf 177hf at ca 100 ka thus the limited compositional variability of cpm glasses reflects homogenization occurring within the cpm reservoir not a homogeneous source

minor and trace element zoning in plagioclase implications for magma chamber processes at parinacota volcano northern chile textural and compositional zoning in plagioclase phenocrysts in a sample from parinacota volcano chile was investigated using backscattered electron images and electron microprobe analysis of major and trace elements large 2 mm oscillatory zoned crystals type i with resorption surfaces of moderate an discontinuities 10 an and decreasing trace element contents sr mg ti towards the rim reflect melt differentiation and turbulent convection in the main magma body early recharge with a low sr mafic magma is seen in the core small scale sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years smaller crystals type ii with low trace element an ratio reflect the influence of an h2o rich melt probably from a differentiated boundary layer closed system in situ crystallisation mafic magma recharge and the role of a water rich differentiated boundary layer can be distinguished from the an trace element relationships crystals apparently move relatively freely between different parts and regimes in the magma chamber evidence for convective crystal dispersion high sr type ii crystals indicate an earlier input of sr rich mafic magma recharge of two distinct mafic magma types is thus identified high sr and low sr which must have been present at increasing recharge rates with time in the plumbing system throughout the volcano s history