volcanic plutonic parity and the differentiation of the continental crust the continental crust is central to the biological and geological history of earth however crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks volcanic and plutonic rocks and the processes by which they differentiate to felsic compositions our analysis of a comprehensive global data set of volcanic and plutonic whole rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics in both tectonic settings our results indicate that fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas emphasizing the role of mafic cumulates as a residue of crustal differentiation a global geochemical data set of volcanic and plutonic rocks indicates that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts brenhin keller and co authors present a global geochemical dataset of the two fundamental building blocks of the continental crust volcanic externally erupted and plutonic internally solidified rocks their results indicate that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics this work indicates that in both tectonic settings fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas

zircons reveal magma fluxes in the earth s crust magma fluxes regulate the planetary thermal budget the growth of continents and the frequency and magnitude of volcanic eruptions and play a part in the genesis and size of magmatic ore deposits however because a large fraction of the magma produced on the earth does not erupt at the surface determinations of magma fluxes are rare and this compromises our ability to establish a link between global heat transfer and large scale geological processes here we show that age distributions of zircons a mineral often present in crustal magmatic rocks in combination with thermal modelling provide an accurate means of retrieving magma fluxes the characteristics of zircon age populations vary significantly and systematically as a function of the flux and total volume of magma accumulated in the earth s crust our approach produces results that are consistent with independent determinations of magma fluxes and volumes of magmatic systems analysis of existing age population data sets using our method suggests that porphyry type deposits plutons and large eruptions each require magma input over different timescales at different characteristic average fluxes we anticipate that more extensive and complete magma flux data sets will serve to clarify the control that the global heat flux exerts on the frequency of geological events such as volcanic eruptions and to determine the main factors controlling the distribution of resources on our planet

dike transport of granitoid magmas thermal and fluid dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc alkalic granitoids dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust we present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 pa s and a density contrast between magma and crust of 200 kg m 3 can be transported 30 km through the crust in 1 month corresponding to a mean ascent velocity of 1 cm s using analysis modified from numerical studies of the flow of basaltic magmas in dikes we also present an expression that allows the calculation of the critical minimum dike or fault width required for granitic magma to ascend without freezing for all reasonable estimates of cordilleran granitoid viscosity and density contrast the critical dike width is determined to be between 2 and 7 m calculated peak batholith filling rates are orders of magnitude greater than mean cavity opening rates based on estimated fault slippage which is consistent with chemical evidence for intermittent supply of magma pulses

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

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

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

correlating the end triassic mass extinction and flood basalt volcanism at the 100 ka level new high precision u pb geochronology from volcanic ashes shows that the triassic juras sic boundary and end triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial fl ood volcanism in the central atlantic mag matic province to 150 ka this narrows the correlation between volcanism and mass extinc tion by an order of magnitude for any such catastrophe in earth history we also show that a concomitant drop and rise in sea level and negative 13 c spike in the very latest triassic occurred locally in 290 ka such rapid sea level fl uctuations on a global scale require that global cooling and glaciation were closely associated with the end triassic extinction and potentially driven by central atlantic magmatic province volcanism

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

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

metrology and traceability of u pb isotope dilution geochronology earthtime tracer calibration part i abstract mixed 235u 233u 205pb 202pb tracers for u pb isotope dilution isotope ratio mass spectrometry have been prepared under the auspices of the earthtime initiative the methods and results for the preparation and calibration of the u pb ratio and isotopic abundances are given and the various sources of uncertainty are discussed and quantified the accuracy of the earthtime u pb tracer isotopic composition can be traced back to si units via a series of assay and isotopic composition reference materials combined with the experiments described herein the parameters used in calculating u pb ratios and inferentially u pb dates have correlated uncertainties that result in a total uncertainty contribution to 206pb 238u dates of

quantum magmatism magmatic compositional gaps generated by melt crystal dynamics compositional gaps are common in volcanic series worldwide the pervasive generation of compositional gaps influences the mechanical and thermal properties of the crust and holds clues on how our planet differentiates we have explored potential mechanisms to generate these gaps using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs we show that gaps are inherent to crystal fractionation for all compositions as crystal liquid separation takes place most efficiently within a crystallinity window of 50 70 vol crystals the probability of melt extraction from a crystal residue in a cooling magma chamber is highest in this crystallinity window due to 1 enhanced melt segregation in the absence of chamber wide convection 2 buffering by latent heat of crystallization and 3 diminished chamber wall thermal gradients this mechanical control of igneous distillation is likely to have played a dominant role in the formation of the compositionally layered earth9s crust by allowing multiple and overlapping intrusive episodes of relatively discrete or quantized composition that become more silicic upward

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

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

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

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

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

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

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

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

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

low pressure origin of high silica rhyolites and granites abstracthigh silica rhyolites and granites 75 wt sio2 anhydrous basis are common features of the crust as part of both the volcanic and the plutonic records while low crystallization pressure 500 mpa we use glass compositions derived from the earthchem portal selected natural examples from the literature and rhyolite melts calculations to show that the phase relations in the quartz albite orthoclase ternary dictate the silica content of silicic melts in particular we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz sanidine cotectic toward the qz apex with decreasing pressure it follows from our analysis that 1 the crust is expected to be stratified in terms of the silica content of residing melts 2 high silica glasses form at low pressure requiring shallow level cryst

is stoping a volumetrically significant pluton emplacement process magmatic stoping is widely called upon to explain discordant pluton contacts but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons first xenoliths rarely make up more than a few percent of most plutons this sparseness even on well exposed pluton floors argues that few xenoliths make it into plutons second piles of rock fragments have sufficiently high porosities 50 that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much if not all of the available magma producing pluton scale magmatic breccias that are not observed third although settling calculations show that large meter scale or larger xenoliths could sink rapidly in magma natural fragmentation processes are likely to produce a fractal distribution of particle sizes with small particles far more abundant than large ones the absence of small xenoliths at pluton contacts where magma cools most rapidly thus argues against stoping fourth although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks finally there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally stoping cannot be volumetrically important in incrementally emplaced plutons because blocks can only fall through the volume that is molten at any given time where xenoliths are abundant in plutons they may commonly represent isolation of wall rock bodies between successive intrusive increments rather than wall rock fragments that were engulfed by magma however even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber although stoping undoubtedly occurs during the emplacement of some plutons it is unlikely to be a volumetrically significant process

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

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

nd sr o isotopic and chemical evidence for a two stage contamination history of mantle magma in the central alpine bergell intrusion two different contamination processes have been identified as having been operative in the genesis of a plutonic suite initial contamination of a mantle source and subsequent crustal contamination of uprising partial melts from the mantle these processes are indicated by a detailed analyses of nd sr and oxygen isotopes together with major and trace elements of the 32 30 ma calc alkaline bergell intrusion this intrusion is located at the suture of the alpine continental collision zone and contains rock types capable of discriminating between mantle and intracrustal processes a range from basaltic andesitic dykes in the surrounding country rocks cumulitic hornblendites gabbros tonalite granodiorite and lamprophyres to pegmatites and aplites is exposed in this single intrusion the results of ree modelling and isotopic compositions of the basic members suggest that the cumulates were fractionated from a picrobasaltic liquid originating by partial melting of enriched subcontinental mantle end 4 increases in 87sr 86sr 0 7055 and 18o 6 7 in these samples relative to the mantle array and compositions of other periadriatic intrusions are most likely the result of an initial contamination of the mantle source by dehydration or partial melting of altered subducted oceanic crust slight differentiation of such a picrobasaltic liquid produced the basaltic andesitic dykes simultaneous fractional crystallization and contamination of the uprising magma by continental crust produced crustal isotopic signatures which increase with acidity to values of end 7 6 87sr 86sr 0 716 and 18o 10 the crustal imprint and lree enrichment in the dominating tonalite increase with decreasing crystallization depth which indicates that the tonalites were emplaced in several distinct batches with different degrees of contamination shoshonitic lamprophyres which intruded into the partly solidified granodiorite were generated in a deep strongly contaminated mantle source the posttectonic 26 ma novate leucogranite is not cogenetic with the main bergell body but rather formed from a predominantly crustal source if the described features are indeed due to mantle source contamination processes which are well known for volcanic arcs it must be concluded that these may also play a significant role in the genesis of calcalkaline plutonic suites

syn accretionary multistage assembly of an early jurassic alaskan type intrusion in the canadian cordillera u pb and 40ar 39ar geochronology of the turnagain ultramafic mafic intrusive complex yukon tanana terrane the magmatic evolution of the early jurassic turnagain alaskan type intrusion is inextricably linked to terrane accretionary events at the north american continental margin we have calibrated the

zircon petrochronology and 40ar 39ar thermochronology of the adamello intrusive suite n italy monitoring the growth and decay of an incrementally assembled magmatic system the adamello intrusive suite is a composite batholith in northern italy with an estimated 2000 km3 volume assembled incrementally over a time span of 10 to 12 million years the history of crysta

observations from the floor of a granitoid pluton inferences on the driving force of final emplacement an east west profile across the tilted bergell pluton exposes a 10 km thick interval in terms of crustal depth consequently the floor as well as the root and side of the main intrusive body of the pluton crop out at the surface and a tentative three dimensional geometry is constructed at the highest crustal level the geometry and deformation features at the margin of the pluton indicate ballooning whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation these contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level final emplacement of the pluton into higher crustal levels was therefore not driven primarily by buoyancy but rather by regional deformation within deeper levels of the crust

emplacement related deformation of granitoid magmas southern adamello massif italy the lago della vacca suite in the tertiary adamello massif italian alps is a small 10 km 2 crudely zoned granitoid pluton emplaced at a pressure of 3 5 kbar into folded mesozoic sediments and precursor intrusive rocks field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early passive stoping by the mafic to intermediate composition marginal units to later forceful emplacement of the more silicic core marginal units of the pluton have highly discordant stoped outer contacts against folded triassic carbonates and precursor mafic ultramafic rocks in contrast the core units have steeply inward dipping contacts against these marginal units magmatic foliations defined by the planar alignment of plagioclase amphibole and biotite and by mafic enclaves are well developed in all units they describe steeply inward dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin lineations are rarely present the most intensely foliated rocks occur in the marginal units as anastomosing sinuous bands that wrap around lenses of less intensely foliated granitoid three dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain strain ellipsoids are weakly prolate within the core of the pluton becoming increasingly oblate as strain increases toward the pluton margin the short axis z of the strain ellipsoid is uniformly subnormal to the foliation suggestive of radial flattening textures of the marginal rocks which record high strain coupled with amphibole plagioclase thermometry indicate that deformation occurred in the magmatic or submagmatic state 650 780 c and was accompanied by significant migration of interstitial melt the documented transition from passive to forceful emplacement and final geometry of the suite resulted from temporal changes in the rheology of both the magma and adjacent country rocks pulses of low density hydrous magma were initially emplaced by stoping denser country rocks on cooling and solidification these magmas contracted by 15 20 creating an unpressured volume which allowed subsequent magma pulses to be emplaced into the pluton core these pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas emplacement of the lago della vacca suite was therefore the product of several internal or magma related processes an interaction between stoping radial expansion and volume contraction on solidification of the hydrous magmas the heterogeneous distribution of strain in the marginal units reflects local variations in temperature and most importantly in the distribution of interstitial melt during emplacement

protracted timescales of lower crustal growth at the fast spreading east pacific rise the formation of oceanic crust at mid ocean ridges is a fundamental component of plate tectonics a large fraction of the new crust is created when magmas in the lower crust cool to form gabbroic rocks the duration of magmatism during formation of the new gabbroic crust is expected to vary with plate spreading rate and has been constrained by dating gabbroic rocks at the slow spreading southwest indian ridge and mid atlantic ridge1 2 3 4 here we present high precision u pb dating of zircon minerals from gabbroic rocks formed at the fast spreading east pacific rise and exposed at hess deep we find that the zircons formed between 1 420 and 1 271 0 006 0 081 million years ago within individual samples the zircon minerals exhibit a range in formation dates of up to 0 124 myr consistent with either protracted crystallization from a magma or assimilation into the magma of older zircons from adjacent rocks the variability of zircon dates is comparable to that measured at the slow spreading mid atlantic ridge3 we conclude that the timescales of magmatic processes in the lower crust may be similar at slow and fast spreading ridges implying that the duration of crust formation at mid ocean ridges is not only controlled by spreading rate

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

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

high precision u pb zircon age calibration of the global carboniferous time scale and milankovitch band cyclicity in the donets basin eastern ukraine 1 high precision id tims u pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic cyclostratigraphic and biostratigraphic framework of the carboniferous donets basin of eastern europe chemical abrasion of zircons use of the internationally calibrated earthtime mixed u pb isotope dilution tracer and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of 0 05 or 100 ka for these carboniferous volcanics this precision allows the resolution of time in the milankovitch band and confirms the long standing hypothesis that individual high frequency pennsylvanian cyclothems and bundles of cyclothems into fourth order sequences are the eustatic response to orbital eccentricity 100 and 400 ka forcing tuning of the fourth order sequences in the donets basin to the long period eccentricity cycle results in a continuous age model for the middle to late pennsylvanian moscovian kasimovian ghzelian strata of the basin and their record of biological and climatic changes through the latter portion of the late paleozoic ice age detailed fusulinid and conodont zonations allow the export of this age model to sections throughout euramerica additional ages for mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations

magma emplacement differentiation and cooling in the middle crust integrated zircon geochronological geochemical constraints from the bergell intrusion central alps abstract u th pb zircon geochronology is an essential tool for quantifying the emplacement differentiation and thermal evolution of crustal magmatic systems however the power of u pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry as this permits more detailed interpretations of geochronological datasets than conventionally achieved our approach to better relating zircon dates and geological processes consists of a multi method analytical workflow including cathodoluminescence imaging cl in situ la icpms epma zircon geochemistry u pb zircon id tims geochronology and solution icpms zircon trace element analysis u pb tims tea these methods are here applied to zircon from the bergell intrusion a composite alpine pluton preserving a 10 km mid crustal transect hand samples of tonalite granodiorite and hybridized granitoid each record 250 700 kyr of autocrystic zircon growth bergell zircons are ubiquitously zoned with ca 10 4 10 6 yr growth histories as evidenced by id tims analysis of microsampled fragments from single crystals u pb tims tea data exhibit compositional trends on multiple spatiotemporal scales including the handsample scale representing in situ differentiation at the emplacement level e g th u lithology scale defining trajectories corresponding to the production of tonalitic versus granodioritic magmas lu hf and pluton scale indicating increasingly evolved melts over 1 6 myr of pluton assembly zr hf these absolute tims tea temporal trends are corroborated by relative la icpms epma core to rim geochemistry we compare records of trace element evolution from tims tea bergell whole rock geochemistry and a global compilation of whole rock geochemical data these findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

bursting the bubble of ballooning plutons a return to nested diapirs emplaced by multiple processes a popular model for the emplacement of roughly spherical plutons is that of ballooning or in situ inflation of a magma chamber in a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent while the hotter tail of magma continues to rise and expand the already crystallized outer margin this expansion forms a concentric gneissic to mylonitic foliation and flattening type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures our reexamination of three supposedly ballooned plutons ardara ireland cannibal creek australia papoose flat california and evaluation of published descriptions of many others indicate that this model is largely incorrect deflections of country rock structures strains and porphyroblast matrix relationships indicate that only minor to moderate expansion usually 30 or less occurred during emplacement that other emplacement mechanisms must have occurred and that regional deformation continued during and after emplacement internal structures indicate that when magma chamber expansion did occur it did so by flow of magma that magmatic foliations and lineations formed late in the magma chamber evolution that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion and that only minor internal subsolidus deformation results from emplacement this study indicates that many plutons previously interpreted as post tectonic ballooning plutons are better viewed as syntectonic nested diapirs emplaced by a variety of country rock material transfer processes this nested diapir model implies that magma ascent may occur by rise of large magma batches instead of transport in dikes followed by ballooning that magma chamber dynamics differ from that in the ballooning model and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt

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

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

in situ u th pb geochronology of urano thorite a laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms study of seven thorite and uranothorite th u sio 4 samples ranging in age from 13 to 500 ma provides new insights into the u th pb isotope systematics of these geologically significant high th mineral species despite extreme actinide enrichment and complex intra crystal zonation in actinides and rare earth elements this study demonstrates that radiogenic lead loss and or metamictization is minimal and restricted to domains that have undergone significant hydration dating of four igneous uranothorites yields ages that are concordant in u pb and th pb space consistent with other high temperature chronometers and are inferred to accurately reflect the timing of crystallization of each rock similarly th pb ages of three thorite and or huttonite bearing samples yield geologically plausible dates consistent with other mineral chronometers no evidence of isotopic inheritance was observed in any of the samples data presented here demonstrate for the first time the feasibility of extracting accurate and precise u pb and th pb ages from phanerozoic thorite uranothorite and huttonite using la mc icpms at the 5 m spatial resolution these phases have the potential to be robust chronometers in igneous and metamorphic rocks as well as to provide important provenance information complementary to more widely used minerals such as detrital zircon

the metamorphic evolution of garnet cordierite sillimanite gneisses of the gruf complex eastern pennine alps metapelitic gneisses occuring as lenses and bands within the migmatites of the gruf complex in the eastern pennine alps contain various combinations of the minerals quartz biotite cordierite garnet sillimanite plagioclase k feldspar spinel orthopyroxene anthophyllite and muscovite the most common rock type is represented by a darkschistosebiotiterichcordierite garnet sillimanite gneiss a consistent pressure temperature range of 3 4 kb and 600 650 c has been calculated for the last metamorphic equilibration from six geological thermobarometers

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

multiscale magmatic cyclicity duration of pluton construction and the paradoxical relationship between tectonism and plutonism in continental arcs abstract the close relationship between crustal magmatism an expression of heat dissipation and tectonics an expression of stress dissipation leads to the question of their mutual relationships indeed the low viscosity of magmas and the large viscosity contrast between magmas and surrounding rocks favor strain localization in magmas and then possible magmatic initiation of structures at a wide range of scales however new data about 3 d pluton shape and duration of pluton construction perturb this simple geological image and indicate some independence between magmatism and tectonics in some cases we observe a direct genetic link and strong arguments for physical interactions between magmas and tectonics in other cases we observe an absence of these interactions and it is unclear how magma transfer and emplacement are related to lithospheric plate dynamics a simple explanation of this complexity follows directly from the pulsed incremental assembly of plutons and its spatial and temporal characteristics the size of each pluton is related to a magmatic pulsation at a particular time scale and each of these coupled time space scales is related to a specific process in small plutons we can observe the incremental process the building block of plutons in larger plutons the incremental process is lost and the pulsation which consists of a cycle of injections at different timescales must be related to the composition and thermal regime of the source region itself driving magmatic processes melting segregation and transfer that interact with tectonic boundary conditions the dynamics of pulsed magmatism observed in plutonic systems is then a proxy for deep lithospheric and magmatic processes from our data and a review of published work we find a positive corelation between volume and duration of pluton construction the larger a pluton the longer its construction time large fast or small slow plutons have not been identified to date one consequence of this observation is that plutonic magmatic fluxes seem to be comparable from one geodynamic setting to another and also over various geologic time spans a second consequence of this correlation is that small plutons which are constructed in a geologically short length of time commonly record little about tectonic conditions and result only from the interference between magma dynamics and the local geologic setting the fast rate of magma transfer in the crust on the order of cm s relative to tectonic rates on the order of cm yr explain why the incremental process of pluton construction is independent of but not insensitive to the tectonic setting however in large plutonic bodies which correspond to longer duration magmatic events regional deformation has time to interact with the growing pluton and can be recorded within the pluton wall rock structure magma transfer operates at a very short timescale comparable to volcanic timescales which can be sustained over variable periods depending on the fertility of the magma source region and its ability to feed the system the fast operation of magmatic processes relative to crustal tectonic processes ensures that the former control the system from below

incremental growth of the patagonian torres del paine laccolith over 90 k y the miocene paine granite in the torres del paine intrusive complex southern chile is an extraordinary example of an upper crustal mafic and granitic intrusion the granite intruded as a series of three sheets each one underplating the previous sheet along the top of the basal paine mafic complex high precision u pb geochronology on single zircons using isotope dilution thermal ionization mass spectrometry yields distinct ages of 12 59 0 02 ma and 12 50 0 02 ma respectively for the first and last sheet of the laccolith this age relationship is consistent with field observations the zircon ages define a time frame of 90 40 k y for the emplacement of a 2000 m thick granite laccolith these precise u pb zircon ages permit identification of the pulses in a 20 k y range the data obtained for the paine granite fill the gap between 100 k y and 100 1000 yr pulses described in the literature for crustal magma chambers

symmagmatic folding of the base of the bergell pluton central alps abstract evidence for magmatic submagmatic and solid state deformation in tonalite granodiorite and country rocks found at the deep seated floor 22 26 km of the bergell pluton demonstrates that final emplacement and crystallization occurred during regional deformation of the pluton and the underlying country rocks after northward emplacement over the country rocks but before complete crystallization the floor of the pluton was folded during simultaneous n s shortening and e w stretching this is evidenced by synmagmatic folds with e w striking nearly vertical axial planes and by regional east plunging stretching lineations in the country rocks which are parallel to the regional scale fold axes and the magmatic mineral lineations in the pluton opposite senses of shear from the well foliated occasionally mylonitic contact suggest that deformation was mostly accomplished by pure shear synmagmatic deformation is related to late stage n s shortening of the alpine orogen and shows that the still partially molten pluton responded to low differential stress very much like the country rocks deformed in the solid state at high temperatures post emplacement tilting associated with backthrusting along the insubric mylonites led to the exposure of the pluton s floor at its present day western margin

developing a strategy for accurate definition of a geological boundary through radio isotopic and biochronological dating the early middle triassic boundary south china abstract the ages of critical periods in the geological record of the earth have to be determined and attached uncertainties quantified as precisely and accurately as possible this study tries to find an optimal strategy for sampling for both biochronology and high precision u pb analysis of zircon from ash beds and for the selection and statistical treatment of geochronological data this strategy is applied to the case of the early middle triassic boundary emtb in the southern nanpanjiang basin in south china aiming to obtain the most accurate age information for this boundary with an associated uncertainty the emtb has been located in a sedimentary section near monggan guangxi south china which permitted tight correlation between conodont biozones the carbon isotopic record and high precision zircon u pb data from volcanic ashes two residual maximal horizons were defined using characteristic associations of conodont species accurately bracketing the emtb the lower and upper age limits of these horizons as well as the first occurrence fo of the base middle triassic index fossil chiosella timorensis were numerically calibrated by high precision chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims u pb dates volcanic zircon in the analyzed ash beds reveals complex system behavior in particular i unresolved lead loss after chemical abrasion and ii pre eruptive growth and or recycling of zircon crystals leading to excess scatter in 206 pb 238 u dates and to violations of the stratigraphic sequence a sufficiently high number of closely spaced ash samples n and of u pb analyses per sample n promotes recognition of such complications two different age models were constructed from weighted mean 206 pb 238 u dates comparing smoothing cubic spline and bayesian interpolation approaches the statistical treatment of the combined datasets allows quantification of the lower and upper limits and associated uncertainties of the two conodont biozones as well as of the fo of c timorensis the age for the emtb is calculated from combined interpolated data at 247 05 0 16 ma the age model derived from bayesian interpolation is considered to be more adapted to the present case where discontinuous changes in sedimentation rates are expected to be important the smoothing spline represents a more parsimonious model producing a simpler solution in terms of smoothness but the model assumptions are too strong for many geological problems the additional freedom in constructing the bayesian interpolating curve presumably leads to more realistic albeit larger confidence intervals

on the longevity of large upper crustal silicic magma reservoirs understanding the processes involved in the formation and maturation of upper crustal magma reservoirs ultimately sourcing the largest volcanic eruptions on earth is one of the most fundamental aspects of volcanology while such reservoirs are known to assemble incrementally over extended periods of time debate persists regarding the time scales of melt preservation in the cold upper crust if rapid cooling individually freezes incoming replenishing intrusions accumulations of eruptible magma are impossible precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates and subsist only ephemerally making their long term evolution less predictable motivated by geological observations suggesting the contrary we have improved upon these models by incorporating two fundamental features of natural systems not previously considered 1 a nonlinear crystallization temperature relationship adapted for upper crustal silicic magmas and 2 a temperature dependent thermal conductivity we demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces while the crystallization temperature path plays the most significant role in maintaining a large pool of eruptible magma the incorporation of temperature dependent thermal properties significantly extends the lifetime of such reservoirs furthermore while deeper emplacement levels e g 10 km depth can both extend magma survival and increase melt availability we show that a 5 km depth can also provide an adequate magma storage environment these results provide strong support for long lived upper crustal mushes as a staging ground for accumulation of highly eruptible crystal poor silicic magmas and further assert the evolutionary link between volcanic and plutonic systems

u th pb and 230th 238u disequilibrium isotope systematics precise accessory mineral chronology and melt evolution tracing in the alpine bergell intrusion in order to investigate the potential of combined th u pb isotope and 230th 238u disequilibrium systematics for tracing magmatic crystallization and melt evolution conventional high resolution single crystal tims techniques have been applied to zircon titanite and fragments of geochemically characterized growth zones of allanite these minerals were extracted from a single tonalite specimen collected from the feeder zone of the tertiary bergell pluton southern steep belt s switzerland n italy the isotopic results document an extended history of crystallization and melt evolution of at least 5 ma with well resolved zircon ages defining an early interval of 33 0 to 32 0 ma followed by crystallization of zoned allanite from 32 0 to 28 0 ma and formation of magmatic epidote possibly as late as 26 ma trace and major element patterns in zoned allanite closely mirror melt evolution characterized by increase of u concentration and sharp decrease of th and lree reflecting early crystallization of phases low in u and in particular the dominating control by allanite precipitation preservation of substantial quantities of excess 206pb derived from initial excess 230th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic pb during prolonged residence at magmatic conditions and regional metamorphic cooling and that the measured sequence of 208pb 232th dates translates into a real age sequence major loss of radiogenic pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one as demonstrated by numerical modeling of 232th 208pb 238u 230th 206pb isotopic evolution the results therefore suggest closure temperatures 700 c for magmatic allanite quantification of 230th 238u disequilibrium relationships reveals a smooth initially steep decrease of th u in the magma from values of 2 9 at 32 0 ma to 0 1 at 28 0 ma in equilibrium with sequential allanite zoning comparison of calculated th u magma with measured total rock th u 0 79 requires fractional crystallization of allanite at an early stage this process of removal of allanite and thus th from the melt was operative at least until 31 5 ma providing an upper limit for the time of emplacement of the studied magma batch the fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep seated plutons with intrusion ages the persistence of magmatic conditions over a period of 5 ma concentrated in a narrow collision zone between the european and the african plate has been a major controlling factor for the oligocene evolution of the adjacent insubric fault the main tectonic lineament of the alps being active from late cretaceous to miocene times

zircon rock partition coefficients of rees y th u nb and ta in granitic rocks uses for provenance and mineral exploration purposes abstract the trace element composition of magmatic zircon is highly variable even within the same grain and its study is complicated by the abundance of microinclusions metamict portions and strong compositional zoning among other factors to contribute to the understanding of the geochemistry of trace elements in zircon we determined the rare earth y nb ta th and u contents in zircon grains by laser ablation inductively coupled plasma mass spectrometry la icp ms with the use of cathodoluminescence and backscattered mev images zircon rock partition coefficients were determined in nine different types of granitoids and compared with experimental and natural data the obtained data are comparable with those reported by other authors by using the partition coefficient we estimated the composition of the host granitoids based on the zircon grain composition which was interpreted as evidence that the patterns of the trace elements in zircon grains are controlled by the liquid composition at the moment of crystallization a new set of partition coefficients for rees y nb ta th and u is suggested for provenance and exploratory studies la 0 8 ce 1 7 pr 2 1 nd 3 4 sm 9 8 eu 7 9 gd 24 tb 37 dy 69 ho 117 er 167 tm 281 yb 379 lu 437 nb 1 6 ta 2 9 th 27 u 177 and y 109 trace element diagrams eu eu vs yb nb vs th and y 200 lu 10 nb vs nb ce are proposed for discriminating zircon from different types of granitoids particularly for a type i type and shoshonitic rocks the trace element patterns of zircon are useful as a tool for mineral exploration studies because they often reproduce the patterns of mineralized granites

implications of incremental emplacement of magma bodies for magma differentiation thermal aureole dimensions and plutonism volcanism relationships abstract field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets sills emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation country rock metamorphism and assimilation and for the formation of large magma chambers in the upper crust when a magma body begins to grow by slow accretion of sills each successive intrusion solidifies before the injection of the next one when the system is thermally mature sill temperatures equilibrate above the solidus melts accumulate and older sills can re melt the time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body the transition from a mafic crystal poor magma to a partially molten rock that retains a highly differentiated melt is fast whereas the resulting evolved residual melt can reside in the crust for protracted periods as long as temperatures in the system are relatively low highly differentiated melts are generated which may explain the bi modal character and the absence of intermediate compositions in some magmatic provinces the level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth metamorphism partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock magma body boundary if the magma emplacement rate is low the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body understanding emplacement of magma bodies is fundamental for our understanding of the plutonism volcanism relationship magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions the time averaged emplacement rates of plutons are lower than this critical emplacement rate eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust this suggests that the emplacement of magma bodies is a multi timescale process with the development of large magma chambers corresponding to the highest magma fluxes because they control magmatic processes and the impact of magma intrusion on the country rock future studies should focus on magma emplacement rates and on magma emplacement geometries these studies should integrate field observation on plutons and geophysical data on active magmatic systems coupled with laboratory experiments and numerical simulations

time scales of pluton construction at differing crustal levels examples from the mount stuart and tenpeak intrusions north cascades washington deciphering the magmatic history of continental magmatic arcs in general and the growth history of individual intrusions in particular is key to understanding the complex history of magma generation segregation and transport that define the dynamics of crustal growth we utilize high precision u pb geochronology to resolve a detailed magmatic history from two composite intrusions the 2 4 kbar mount stuart batholith and the 7 10 kbar tenpeak pluton emplaced in the cretaceous north cascades arc this temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts u pb zircon crystallization ages were obtained from 12 samples of the mount stuart batholith and 8 samples of the tenpeak intrusion representing the range of compositional diversity and geographical extent these dates indicate that the mount stuart batholith was constructed over a 5 5 m y time period that was punctuated by four intervals of high magma flux the durations of the high flux periods are short a few hundred thousand years relative to the duration of the batholith the consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time transgressive magma fabrics in contrast the tenpeak intrusion was constructed over a 2 6 m y time period with magma influx distributed throughout the intrusive history and texturally distinct magma bodies the tenpeak intrusion lacks distinct age domains which suggests that any magma reservoir was smaller in size and potentially more ephemeral although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally neither pluton bears resemblance to a purely end member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little if any interaction in particular ages from the youngest domain of the mount stuart batholith indicate that a melt rich magma reservoir of 520 km 3 existed over a 170 90 k y time span

multistage emplacement of the mcdoogle pluton an early phase of the john muir intrusive suite sierra nevada california by magmatic crack seal growth new mapping of the southern mcdoogle pluton an early intrusion in the john muir suite in the central sierra nevada california has resolved important details of incremental assembly by magmatic crack seal the tabular mcdoogle pluton concordantly intruded the steeply dipping mylonitic sawmill lake shear zone around 95 ma the pluton was previously divided into three phases based on wall rock screen abundance and lithology the earliest central phase consists of steep dikes separated by abundant wall rock screens these screens are largely absent from border phases located on either side of the central phase the inferred relative ages of intrusive phases and the wall rock screen distribution signify a change from antitaxial addition at the margins of the central phase to syntaxial addition at the center of each border phase assembly during pluton growth interaction among the effects of preexisting foliation older nondeformed intrusions and synintrusive static recovery of shear zone rocks likely caused the transition from antitaxial to syntaxial growth the trend from antitaxial to syntaxial growth during emplacement has been observed in other intrusions and intrusive suites and may be a general pattern of incremental intrusion associated with the feedback between preexisting deformation and thermal addition in magmatic arcs

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

integrating 40ar 39ar u pb and astronomical clocks in the cretaceous niobrara formation western interior basin usa this study revises and improves the chronostratigraphic framework for late turonian through early campanian time based on work in the western interior u s and introduces new methods to better quantify uncertainties associated with the development of such time scales building on the unique attributes of the western interior basin which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the niobrara formation new 40ar 39ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples as well as newly collected material these results are complemented by new u pb zircon chemical abrasion isotope dilution thermal ionization mass spectrometry ages from four biozones in the study interval when combined with published radioisotopic data from the cenomanian turonian boundary paired 206pb 238u and 40ar 39ar ages spanning cenomanian to campanian time support an astronomically calibrated fish canyon sanidine standard age of 28 201 ma stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the niobrara formation the ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the niobrara formation to radioisotopic ages with the lowest uncertainty proximal to the boundary and adding or subtracting time by parsing the 405 k y cycles the new stage boundary age determinations are 89 75 0 38 ma for the turonian coniacian 86 49 0 44 ma for the coniacian santonian and 84 19 0 38 ma for the santonian campanian boundary the 2 uncertainties for these estimates include systematic contributions from the radioisotopic measurements astrochronologic methods and geologic uncertainties related to stratigraphic correlation and the presence of hiatuses the latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations each methodological approach employed in this study new radioisotopic analysis stratigraphic correlation astrochronology and ammonite and inoceramid biostratigraphy was critical for achieving the final result

sensitivity of the short to intermediate wavelength geoid to rheologic structure in subduction zones 1 the rheologic structure of subduction zones determines how the negative buoyancy of sinking lithosphere slabs couples to the surface to cause surface deformation and plate motion however the rheologic structures determined using different methods are inconsistent analysis of the geoid and some dynamical models conclude that slabs must be weak whereas other dynamical models and experimentally determined rheology predict that slabs are strong using idealized three dimensional models of a subduction zone we show that lateral variations in viscosity lvvs within a slab caused by yielding render the geoid insensitive to the viscosity structure outside of the bending region of the slab spectral analysis of the geoid up to harmonic degree 360 features at wavelengths down to 110 km shows that lvvs cause significant mode coupling with changes to the long wavelength geoid 1000 km caused by short wavelength 500 km changes to dynamic topography models with either a uniformly weak slab or a strong slab with yielding provide a good match to the observed geoid spectrum and characteristic features of the geoid over subduction zones therefore observations other than the geoid are needed to constrain the strength profile of subducting slabs

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones

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