ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

a two phase approach to the simulation of the combined isotope salinity signal of marine ice a modified boundary layer box diffusion model is used to simulate the production of the combined oxygen isotope salinity signal of a body of marine ice the final signal is considered to be the result of fractionation and segregation processes occurring first upon the formation of frazil crystals in suspension and second during their accumulation and consolidation due to the advancement of a freezing front the model results are compared to oxygen isotope and salinity values in a 45 m marine ice core from the nansen ice shelf antarctica parameters more suitable to the modeling of marine ice as opposed to sea ice are introduced and conclusions are drawn on the origin of the impurity characteristics of marine ice including the still unresolved question of its anomalously low salinity

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

u pb zircon and geochemical constraints on the age origin and evolution of paleozoic arc magmas in the oyu tolgoi porphyry cu au district southern mongolia abstract uranium pb zircon ages are linked with geochemical data for porphyry intrusions associated with giant porphyry cu au systems at oyu tolgoi to place those rocks within the petrochemical framework of devonian and carboniferous rocks of southern mongolia in this part of the gurvansayhan terrane within the central asian orogenic belt the transition from devonian tholeiitic marine rocks to unconformably overlying carboniferous calc alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation radiogenic nd and pb isotopic compositions e nd t range from 3 1 to 7 5 and 206 pb 204 pb values for feldspars range from 17 97 to 18 72 as well as low high field strength element hfse contents of most rocks mafic rocks typically have 2 are consistent with magma derivation from depleted mantle in an intra oceanic volcanic arc the late devonian and carboniferous felsic rocks are dominantly medium to high k calc alkaline and characterized by a decrease in sr y ratios through time with the carboniferous rocks being more felsic than those of devonian age porphyry cu au related intrusions were emplaced in the late devonian during the transition from tholeiitic to calc alkaline arc magmatism uranium pb zircon geochronology indicates that the late devonian pre to syn mineral quartz monzodiorite intrusions associated with the porphyry cu au deposits are 372 ma whereas granodiorite intrusions that post date major shortening and are associated with less well developed porphyry cu au mineralization are 366 ma trace element geochemistry of zircons in the late devonian intrusions associated with the porphyry cu au systems contain distinct th u and yb gd ratios as well as hf and y concentrations that reflect mixing of magma of distinct compositions these characteristics are missing in the unmineralized carboniferous intrusions high sr y and evidence for magma mixing in syn to late mineral intrusions distinguish the late devonian rocks associated with giant cu au deposits from younger magmatic suites in the district

zircon and apatite as tools to monitor the evolution of fractionated i type granites from the central great xing an range ne china abstract it is widely accepted that granitic melts can be highly evolved however the effective records of this process are usually obscure especially for the exsolution and evolution of volatiles we present a comprehensive investigation on zircon and apatite from less to highly fractionated granites from the central xing an range ne china in order to reveal the mineral records for the evolution processes of granitic melt petrographic whole rock and mineralogical geochemical investigations indicate that the three granites are co magmatic and experienced different degrees of fractionation the variations of zircon morphology textures and geochemical compositions reflect the progressive evolution of granitic melts from volatile undersaturated to oversaturated conditions the significantly decreased raman spectra intensities imply that the zircon crystals experienced increased radiation damages and hydrothermal alteration during the fractionation of granitic magma correspondingly apatite textures transform from fluid inclusion free to fluid inclusion rich stage with increasing f cl ratios during the magmatic evolution implying that the segregation of isolated fluid phase in the fractionating magma accordingly we conclude that the textural and compositional evolution of zircon and apatite can be regarded as efficient tools for recording granitic magma evolution

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

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

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

ocean driven heating of europa s icy shell at low latitudes on jupiter s icy moon europa enigmatic chaos terrain where the icy crust is cut by a jumble of ridges and cracks occurs most commonly at lower latitudes simulations of convection in the ocean underlying europa s icy crust suggest that ocean dynamics can control an enhanced flow of heat to europa s equatorial surface and hence geological activity

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

high rates of inflation during a noneruptive episode of seismic unrest at semisopochnoi volcano alaska in 2014 2015

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

ocean circulation beneath the ronne ice shelf the intimate thermal contact between the base of antarctic ice shelves and the underlying ocean enables changes in climate to have a rapid impact on the outflow of ice from the interior of antarctica1 2 furthermore water modified by passage under ice shelves particularly in the weddell sea is believed to be an important constituent of antarctic bottom water3 a water mass that can be observed as far north as 50 n in the deep oceans4 antarctic bottom water is both cold and oxygen rich and plays an important part in the cooling and ventilation of the world s oceans because of the difficulty in gaining access the oceano graphic regime beneath ice shelves is very poorly sampled5 by successfully drilling through the ice however we were able to obtain oceanographic data from beneath the largest antarctic ice shelf the ronne filchner ice shelf in the southern weddell sea we find that our data agree well with the predictions of a relatively simple oceanographic plume model of sub ice shelf circulation6 this model can therefore be used with some confidence to investigate the links between climate changes ice shelf melting and bottom water production

chromite silicate chemistry of the neoarchean sittampundi complex southern india implications for subduction related arc magmatism the neoarchean layered anorthositic complex at sittampundi in southern india is known for its chromitite layers that are mostly associated with anorthosite an 90 100 the chromitites contain feal rich chromites concentrated in layers between amphibole rich layers with a dominant mineralogy of amphibole spinel plagiocase sapphirine the chromite rich layers contain only amphibole and plagioclase mineral compositions illustrated by x ray composition maps and profiles show subtle chemical differences the chrome spinels are of refractory grade with cr2o3 and al2o3 contents varying between 34 40 wt and 23 28 wt the chromite compositions are noticeably different from those in layered igneous intrusions of the bushveld stillwater type the existence of original highly calcic plagioclase feal rich chromite and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra subduction zone arc setting in terms of mineralogy and field relations the sittampundi chromitites are remarkably similar to anorthosite hosted chromitites in the neoarchean fiskensset anorthositic complex greenland we propose that the sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone this melting process produced hydrous aluminous basalt which fractionated at depth to give rise to a variety of high alumina basalt compositions from which the anorthositic complex with its cumulate chromite rich and amphibole rich layers formed within the magma chamber of a supra subduction zone arc c 2011 elsevier b v all rights reserved

zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system this study documents the chemical and textural responses of zircon in the elba igneous complex with particular reference to the 7 to 7 8 ma old monte capanne pluton in relation to its coeval volcanic counterpart capraia using bse imaging and quantitative electron microprobe analyses the monte capanne pluton displays multiple field and geochemical evidence for magma mixing the samples we have investigated including monzogranitic mafic enclave and dyke samples display similar zircon textures and are associated with an extremely large range of trace and minor element hf y hree th u compositions which contrast with relatively simple textures and zoning patterns in zircons from a capraia dacite we have used a relatively simple textural classification patchy zoning homogenous cores oscillatory zoning and unzoned zircon as the basis for discussing the chemical composition and chemical variation within zircons from the monte capanne pluton based on these data and other works dini et al 2004 in lithos 78 101 118 2004 it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the monte capanne magma chamber in particular mixing was responsible for the development of the patchy zoning texture in the zircon cores which was associated with reactions between other accessory phases including monazite apatite allanite which we infer to have significantly affected the th distribution in zircon zircons from the mc pluton displaying homogeneous cores have chemical affinities with zircons in the coeval capraia volcanic system consistent with the participation of a capraia like mantle end member during mixing further zircon growth in the mc pluton produced the oscillatory zoning texture which records both long term crystal fractionation and transient recharge with both silicic and mafic magmas events in a hybrid magma chamber it is inferred that hf and the th u ratio cannot be used alone to infer magmatic processes due to their dependency on temperature nor are they a diagnostic feature of xenocrystic grains this study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system

rongbuk re visited geochronology of leucogranites in the footwall of the south tibetan detachment system everest region southern tibet abstract the hermit s gorge area adjacent to the rongbuk glacier on the north side of mount everest is a critical location to establish the timing and duration of movement along the ductile strand of the south tibetan detachment system stds a low angle north dipping normal fault that bounds the upper part of the greater himalayan sequence monazite from four leucogranite samples in the hermit s gorge that bracket the timing of ductile fabric development has been dated using both u th pb id tims and la mc icpms results suggest that the earliest 16 4 ma leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc silicates whereas structurally higher sills and dykes that post date fabric development are slightly younger all within uncertainty of one another at 15 6 to 15 4 ma field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the stds as being on going at 16 4 ma but had ceased prior to 15 6 ma while brittle faulting along the stds is younger than 15 4 ma combined with data from the everest massif and surrounding region ages of granite crystallization and ductile shearing propagated up structural section and northward with time

statistical modeling of a former arctic ocean ice shelf complex using antarctic analogies 1 geophysical mapping and coring of the central arctic ocean seafloor provide evidence for repeated occurrences of ice sheet ice shelf complexes during previous glacial periods several ridges and bathymetric highs shallower than present water depths of 1000m show signs of erosion from deep drafting armadas of icebergs which originated from thick outlet glaciers and ice shelves mapped glacigenic landforms and dates of cored sediments suggest that the largest ice shelf complex was confined to the amerasian sector of the arctic ocean during marine isotope stage mis 6 however the spatial extent of ice shelves can not be well reconstructed from occasional groundings on bathymetric highs therefore we apply a statistical approach to provide independent support for an extensive mis 6 ice shelf complex which previously was inferred only from interpretation of geophysical and geological data specifically we assess whether this ice shelf complex comprises a likely source of the deep draft icebergs responsible for the mapped scour marks the statistical modeling is based on exploiting relations between contemporary antarctic ice shelves and their local physical environments and the assumption that arctic ocean mis6 ice shelves scale similarly analyzing ice thickness data along the calving front of contemporary ice shelves a peak over threshold method is applied to determine sources of deep drafting icebergs in the arctic ocean mis6 ice shelf complex this approach is novel to modeling arctic paleoglacial configurations predicted extreme calving front drafts match observed deep draft iceberg scours if the ice shelf complex is sufficiently large

zircon textures and composition refractory recorders of magmatic volatile evolution zircon textures and composition have been used to infer magmatic processes including closed system fractional crystallization magma mixing or replenishment and country rock assimilation here we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution we present field whole rock chemical textural mineral chemical and u pb age data from evolved fine to coarse grained granite intrusions on melville peninsula nunavut canada zircon forms two main populations in these granites type 1 and type 2 zircon type 1 zircon is present in all samples but predominant in fine grained granite crystals are euhedral and inclusion rich and show periodic fine scale oscillatory zoning comparatively low concentrations of u 2 200 ppm and hf 1 6 wt high zr hf 40 62 and pervasive alteration type 2 zircon is predominant in coarse grained granite crystals form overgrowths on type 1 zircon and individual crystals they are subhedral and inclusion poor and show weak irregular large scale oscillatory zoning high u up to 7 250 ppm and hf 1 5 2 0 wt low zr hf 37 44 and only local alteration compatible trace element concentrations and zr hf change sharply across the boundary of type 1 to type 2 zircon 207pb 206pb ages preclude a significant hiatus between crystallization of the two types we argue against magmatic versus hydrothermal crystallization country rock assimilation or magma mixing as causes for the crystallization of type 1 and type 2 zircon we propose instead that type 1 zircon formed from volatile undersaturated magmas and that type 2 zircon formed from volatile saturated magmas magmas fractionated by volatile driven filter pressing into crystal rich mush and crystal poor magma crystal rich mush with abundant type 1 zircon crystallized to fine grained granite volatile rich magma crystallized to type 2 zircon and coarse grained granite while type 1 zircon was pervasively altered by exsolving magmatic volatiles type 2 zircon was only locally affected by subsolidus hydrothermal alteration

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

regime of the filchner ronne ice shelves antarctica seismic radio echo twerle balloon and oceanographic data of the filchner ronne ice shelves are analysed a large thin area in the central ronne ice shelf is found to differ from the morphology of the ross and filchner ice shelves this area is partly filled by basal saline ice which can be attributed to regional regelation caused by oceanic circulation

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

uranium series accessory crystal dating of magmatic processes complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals e g zircon allanite although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples it emerged relatively recently that magmatic crystallization can occur over extended durations this discovery arose from applying high spatial resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks the emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from 1 000 to many hundreds of thousands of years accessory mineral dating of volcanic rocks and cognate plutonic xenoliths opens new research avenues for crystal age fingerprinting that correlates pyroclastic deposits lavas and plutonic rocks by using characteristic age dis

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

inflation detected at mount veniaminof alaska with campaign gps 1 in 2005 a repeat gps campaign was conducted at 12 sites on mt veniaminof volcano and the surrounding region a previous survey in 2002 provided initial locations for the sites the deformation that occurred in the three years between the two surveys consist of 2 4 cm of displacement caused by the northwest convergence of the pacific and north america plates and 0 5 1 cm displacements caused by a deep intrusion that may be associated with eruptive activity in 2003 2005 the site velocities are corrected for compression using a pre existing model for a locked subduction zone and the residuals modeled with volcanic sources in homogeneous and layered material although the data are fit equally well by several source models we nevertheless gain important information about the deformation source the best fitting models have depths that range from 3 to 17 km

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

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

fitful and protracted magma assembly leading to a giant eruption youngest toba tuff indonesia the paroxysmal eruption of the 74 ka youngest toba tuff ytt of northern sumatra produced an extraordinary 2800 km3 of nonwelded to densely welded ignimbrite and coignimbrite ashfall we report insights into the duration of ytt magma assembly obtained from ion microprobe u th and u pb dates including continuous age spectra over 50 of final zircon growth for pumices and a welded tuff spanning the compositional range of the ytt a relatively large subpopulation of zircon crystals nucleated before the penultimate caldera related eruption at 501 ka but most zircons yielded interior dates 100 300 ka thereafter zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of 100 to 500 ka final zircon growth is evident as thin rim plateaus that are in th u chemical equilibrium with hosts and that give crystallization ages within tens of ka of eruption the longevity and chemical characteristics of the ytt zircons as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge is especially favored at the cool and wet eutectoid conditions that characterize at least half of the ytt wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals repeated magma recharge may have contributed to the development of compositional zoning in the ytt but considered together with limited allanite quartz and other mineral dating and geospeedometry regular perturbations to the magma reservoir over 400 ka did not lead to eruption until 74 ka ago

ion microprobe dating of zircons from active dayingshan volcano tengchong se tibetan plateau time scales and nature of magma chamber storage abstract the rare occurrences of active volcanoes on the tibetan plateau provide a unique opportunity to investigate time scales of magma chamber processes for post collisional potassic volcanism this research utilizes high spatial resolution u th disequilibrium dating methods to date zircons from the active dayingshan volcano tengchong se tibetan plateau which erupted last during the holocene zircon crystal ages can be used to provide constraints on the magma chamber storage time measured zircon interiors yielded an isochron age of 87 5 6 5 ka while zircon outermost rims yielded an isochron age of 58 13 ka the calculated zircon crystal storage time for the most recent eruption is 48 ka at least two separate late pleistocene zircon crystallization events have occurred where the older age population represents antecrysts remobilized from an earlier magmatic episode but protracted crystallization cannot be ruled out regardless the similarity in zircon age populations between dayingshan volcano and maanshan volcano 13 km apart in the tengchong volcanic field suggests a synchronous thermal history of the magma reservoir possibly in an extensive interconnected magma chamber beneath the tengchong volcanic field

late quaternary caldera forming eruptions in the eastern aleutian arc alaska late quaternary calderas have been identified at 12 of 40 volcanic centers in the eastern aleutian arc and sufficient radiocarbon dates and geologic information have now been obtained to either date or constrain the timing of the climactic caldera forming eruptions at least eight major caldera forming events each characterized by estimated eruption volumes of more than 10 km 3 occurred at seven different volcanic centers in the holocene and as many as six of these had estimated eruption volumes of more than 50 km 3 eruptions of similar magnitude formed two other calderas in wisconsin time the dating of these hitherto little known events adds significantly to the previously existing chronology of large prehistoric eruptions this refined chronology is important in understanding eruption induced climate changes in assessing volcanic hazards and in developing a tephrochronology for northwestern north america

evidence for initial excess 231pa in mid ocean ridge zircons abstract a limiting factor in the accuracy and precision of u pb zircon dates is accurate correction for initial disequilibrium in the 238u and 235u decay chains the longest lived and therefore most abundant intermediate daughter product in the 235u isotopic decay chain is 231pa t1 2 32 71 ka and the partitioning behavior of pa in zircon is not well constrained here we report high precision thermal ionization mass spectrometry tims u pb zircon data from two samples from ocean drilling program odp hole 735b which show evidence for incorporation of excess 231pa during zircon crystallization the most precise analyses from the two samples have consistent th corrected 206pb 238u dates with weighted means of 11 9325 0 0039 ma n 9 and 11 920 0 011 ma n 4 but distinctly older 207pb 235u dates that vary from 12 330 0 048 ma to 12 140 0 044 ma and 12 03 0 24 to 12 40 0 27 ma respectively if the excess 207pb is due to variable initial excess 231pa calculated initial 231pa 235u activity ratios for the two samples range from 5 6 1 0 to 9 6 1 1 and 3 5 5 2 to 11 4 5 8 the data from the more precisely dated sample yields estimated dpazircon duzircon from 2 2 3 8 and 5 6 9 6 assuming 231pa 235u of the melt equal to the global average of recently erupted mid ocean ridge basaltic glasses or secular equilibrium respectively high precision id tims analyses from nine additional samples from hole 735b and nearby hole 1105a suggest similar partitioning the lower range of dpazircon duzircon is consistent with ion microprobe measurements of 231pa in zircons from holocene and pleistocene rhyolitic eruptions schmitt 2007 schmitt 2011 the data suggest that 231pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions and excess initial 231pa may be more common in zircons than acknowledged the degree of initial disequilibrium in the 235u decay chain suggested by the data from this study and other recent high precision datasets leads to resolvable discordance in high precision dates of cenozoic to mesozoic zircons minor discordance in zircons of this age may therefore reflect initial excess 231pa and does not require either inheritance or pb loss

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

evidence for neoarchean hydrous arc magmatism the anorthosite bearing mayville intrusion western superior province canada abstract the neoarchean 2742 8 0 8 ma megacrystic anorthosite bearing mayville layered intrusion of the bird river greenstone belt in the superior province of canada is well preserved and well exposed given its good preservation we studied the mayville intrusion through a combination of extensive field and petrographic observations mineral chemistry and whole rock major and trace element and sm nd and u th pb isotopic analyses to unravel its petrogenesis and geodynamic setting and determine whether its amphibole is of igneous origin and indicative of hydrous magmatism the mayville intrusion intrudes pillow basalts and gabbros of the mayville assemblage which was erupted onto and emplaced into the mesoarchean phase of the maskwa lake tonalite trondhjemite granodiorite ttg batholith some of the anorthosites and leucogabbros of the mayville intrusion have undergone extensive shearing and hydrothermal alteration and had their sm nd and u th pb isotope systems reset despite metamorphism and deformation affecting the mayville intrusion it has largely retained its primary igneous minerals and textures and trace element and sm nd and pb isotope compositions abundant interstitial magmatic amphibole oikocrysts in the anorthosites and leucogabbros exhibit whole grain optical continuity and well preserved igneous grain boundaries with magmatic plagioclase indicating that the mayville intrusion crystallized from a hydrous magma the mayville intrusion has variably positive end 2743 ma values 0 7 to 3 9 that are indicative of its derivation from a depleted mantle source the anorthosites and leucogabbros of the mayville intrusion crystallized from a hydrous ca and al rich tholeiitic parental magma that fractionated from a hydrous primitive arc tholeiitic parental magma these lithologies were derived by partial melting of a depleted shallow hydrous sub arc mantle wedge source the parental magmas to the gabbros leucogabbros and anorthosites of the anorthosite to leucogabbro zone subsequently experienced minor contamination by mesoarchean ttgs maskwa lake ttg batholith the field relationships and trace element geochemistry of the mayville intrusion and its proximity to the continental arc maskwa lake ttg batholith indicate that it formed in a chilean style continental back arc geodynamic setting the findings of this study demonstrate that hydrous arc magmatism and modern style plate tectonic processes have been operating since at least the late archean

provenance and dispersal of glacial marine surface sediments in the weddell sea and adjoining areas antarctica ice rafting versus current transport abstract mineralogical and granulometric properties of glacial marine surface sediments of the weddell sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from antarctic sources the silt fraction shows marked spatial differences in quartz contents in the sand fractions heavy mineral assemblages display low mineralogical maturity and are dominated by garnet green hornblende and various types of clinopyroxene cluster analysis yields distinct heavy mineral assemblages which can be attributed to specific source rocks of the antarctic hinterland the configuration of modern mineralogical provinces in the near shore regions reflects the geological variety of the adjacent hinterland in the distal parts of the study area sand sized heavy minerals are good tracers of ice rafting granulometric characteristics and the distribution of heavy mineral provinces reflect maxima of relative and absolute accumulation of ice rafted detritus in accordance with major iceberg drift tracks in the course of the weddell gyre fine grained and coarse grained sediment fractions may have different origins in the central weddell sea coarse ice rafted detritus basically derives from east antarctic sources while the fine fraction is discharged from weak permanent bottom currents and or episodic turbidity currents and shows affinities to southern weddell sea sources winnowing of quartz rich sediments through intense bottom water formation in the southern weddell sea provides muddy suspensions enriched in quartz the influence of quartz rich suspensions moving within the weddell gyre contour current can be traced as far as the continental slope in the northwestern weddell sea in general the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice rafted detritus beyond the shelves of the study area

computing marine ice thickness at an ice shelf base the freezing of sea water to the base of an ice shelf can give rise to large patches of accumulated ice a phenomenon known as marine ice in this study a numerical method is presented for calculating the thickness of the marine ice layer using an ice shelf ocean model the present day modeling paradigm of ice shelf ocean interaction usually involves the fixed specification of the ice shelf geometry while the ocean circulation in the cavity beneath the ice shelf evolves freely this approach relies on several assumptions such as steady state ice shelf thickness and ice shelf flow fields in order to make reasonable quantitative estimates of the thermodynamic exchange processes occurring at the ice shelf base this paper discusses the impact of these and other assumptions on the estimation of the thickness of the marine ice layer model simulation results are presented for an idealized ice shelf ocean configuration as a demonstration of the feasibility of the numerical method a sensitivity analysis is given so as to quantify the relative uncertainty in the marine ice thickness that arises from uncertainties in the model input parameters these being principally the ice shelf flow field the basal accumulation rate and the ice shelf thickness field

numerical models of the filchner ronne ice shelf an assessment of reinterpreted ice thickness distributions recent radio echo soundings of the filchner ronne ice shelf by the german antarctic expedition 1983 1984 suggest that previous ice thickness measurements may have misinterpreted an internal radio echo reflecting horizon as the true ice shelf bottom this and the analysis of total ice thickness from surface altimetry suggests that a previously defined thin ice region comprising approximately 1 5 of the total ice shelf area may be underlain by a thick layer of possibly saline ice one possible way to verify the existence of such a layer is by measurement of its influence on the ice shelf flow regime here we evaluate this influence by conducting finite element simulations of two alternative ice thickness configurations we conclude that flow differences are sufficiently large to allow verification of the possible saline basal ice layer provided that surface strain rate measurements are conducted in certain key areas

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

accumulation and ice core studies on filchner ronne ice shelf antarctica accumulation rates in the eastern part of ronne ice shelf were determined by isotopic stratigraphy ebo the samples were taken from snow pits dug during the filchner i and ii operations in 1984 and 1986 in general the accumulation rate decreases towards the south the greatest decrease from 21 3 to 13 3 g cm 2 a 1 was observed between filchner station and measuring point 341 sited 270 km up stream of the ice edge the slbo values of the near surface layers vary between 25 and 29 0 the 1bo content in the more southerly part is progressively depleted in the direction of mollereisstrom paralleling a decrease in the accumulation rate near the ice edge the 1bo content decreases to the west a 100 m ice core drilled in 1984 at point 340 220 km from the ice edge probably goes back to a d 1460 it has been dated by isotopic stratigraphy the accumulation rate up stream of the drilling site was deduced from the sequence of annual layers using a simple ice flow model the accumulation shows strong variations over the last 200 years which may be caused in part by local variations in the accumulation on ronne ice shelf

on the occurrence trace element geochemistry and crystallization history of zircon from in situ ocean lithosphere we characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite evolved gabbro and metamorphosed ultramafic host rocks collected along present day slow and ultraslow spreading mid ocean ridges mors the geochemistry of 267 zircon grains was measured by sensitive high resolution ion microprobe reverse geometry at the usgs stanford ion microprobe facility three types of zircon are recognized based on texture and geochemistry most ocean crustal zircons resemble young magmatic zircon from other crustal settings occurring as pristine colorless euhedral type 1 or subhedral to anhedral type 2 grains in these grains hf and most trace elements vary systematically with ti typically becoming enriched with falling ti in zircon temperature ti in zircon temperatures range from 1 040 to 660 c corrected for a tio2 0 7 a sio2 1 0 pressure 2 kbar intra sample variation is typically 60 150 c decreasing ti correlates with enrichment in hf to 2 wt while additional hf enrichment occurs at relatively constant temperature trends between ti and u y ree and eu eu exhibit a similar inflection which may denote the onset of eutectic crystallization the inflection is well defined by zircons from plagiogranite and implies solidus temperatures of 680 740 c a third type of zircon is defined as being porous and colored with chaotic cl zoning and occurs in 25 of rock samples studied these features along with high measured la cl s ca and fe and low sm la n ratios are suggestive of interaction with aqueous fluids non porous luminescent cl overgrowth rims on porous grains record uniform temperatures averaging 615 26 c 2sd n 7 implying zircon formation below the wet granite solidus and under water saturated conditions zircon geochemistry reflects in part source region elevated hree coupled with low u concentrations allow effective discrimination of 80 of zircon formed at modern mors from zircon in continental crust the geochemistry and textural observations reported here serve as an important database for comparison with detrital xenocrystic and metamorphosed mafic rock hosted zircon populations to evaluate provenance

borehole evidence for a thick layer of basal ice in the central ronne ice shelf the ronne ice shelf floating between berkner island and the antarctic peninsula is the biggest ice shelf in antarctica it drains an estimated 1 2 106km2 of the antarctic ice sheet much of it resting on bedrock below sea level consequently the balance and dynamics of this ice shelf is of importance to antarctic glaciology especially with regard to the integration of this part of the cryo sphere into the global processes that control the climate of the earth extensive radio echo sounding res by robin and others revealed reflections in the central part of the ronne ice shelf at the relatively shallow depth of 100 200 m below surface1 the interpretation of these echoes which varied in strength was ambiguous and the possibility of internal reflecting horizons was thoroughly discussed but after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice it was decided to base a thickness map of the ronne ice shelf on these res echoes1 4 we now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped and that the shallow res reflections therefore do come from internal horizons

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

timing of incremental pluton construction and magmatic activity in a back arc setting revealed by id tims u pb and hf isotopes on complex zircon grains abstract the lifetimes and thermal histories of upper crustal plutons are increasingly determined using geochronology but complex growth of datable minerals in magmas impedes simple age interpretations careful field observation helps constrain zircon u pb dates in terms of timing of magma injection for example because relative ages of successive magma pulses must be honored we use id tims u pb zircon geochronology and field geology to construct timescales of incremental pluton assembly in the st jean du doigt sjdd bimodal layered intrusion brittany france field evidence suggests that early pulses were injected into a cold environment with little supersolidus interaction among successive magma pulses later injections occurred in a progressively hotter environment with protracted mafic and felsic magma interaction zircon dates show that early activity ca 347 ma predates the thermally mature episode by about 1 ma which terminated at ca 345 ma dates from samples displaying core rim zircon overgrowths span about 5 ma 351 346 ma which we interpret to represent two distinct crystallization events hf isotopic data from cores and rims are homogeneous precluding zircon inheritance from basement rocks these textures and dates could instead reflect zircon saturation fluctuations at the emplacement depth or antecrystic zircon grains recording pre emplacement magmatic growth