excess argon in mount st helens plagioclase as a recorder of magmatic processes excess argon in plagioclase crystals from young 4000 ybp tephra layers from mount st helens washington illustrates the importance of argon isotopes in understanding magmatic processes 40ar 39ar step heating identifies two distinct argon reservoirs in the plagioclase a high cl k low ca k reservoir with atmospheric 40ar 36ar and a low cl k higher ca k reservoir with variable 40ar 36ar the first is probably glass whereas the second is true plagioclase felsic dacite plagioclases have little or no excess argon but those from more mafic layers have significantly higher 40ar 36ar indicating a non atmospheric source these variations are seen within a layer that contains both mafic and felsic dacite and are inconsistent with either xenocrystic or restitic origin for plagioclase the magma chamber exhibits long term open system degassing behavior punctuated by short term fluctuations from influx of high 40ar 36ar basalt the relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase

integrating the uranium series and elemental diffusion geochronometers in mixed magmas from volcan quizapu central chile magma mixing is a ubiquitous process in silicic magmatic systems we present pre and post mixing time scales for a single magma mixing event associated with the 1846 1847 eruption of volcan quizapu we compare and contrast 226 ra 230 th crystal residence ages between the mixed lavas of the 1846 1847 effusive eruption and the largely homogeneous plinian dacite eruption of 1932 both eruptions are dominated by the same dacite magma whereas the ear lier eruption contains evidence for a single episode of mafic recharge and mixing the use of uranium series mineral separate dating on a mixed magma requires deconvolution of different crystal populations we present a four component mixing model for the crystal separates to obtain dacite derived plagioclase model ages as well as andesite derived recharge plagioclase ages plagioclase model ages average 3 6 ka for dacite derived plagioclase from the mixed 1846 1847 eruption 1 4 kyr older than average plagioclase model ages of the 1932 eruption suggesting that magma mixing and reheating led to some resorption of young crystal rims in the 1846 1847 magmatic episode and or that some growth of plagioclase rims occurred during the repose time of 86 years the andesite derived plagioclase residence ages are younger than the corresponding dacite derived plagioclase residence ages but they are still of the order of thousands of years in addition we use mg diffu sion in plagioclase to constrain time scales of magma mixing we show that the 1846 1847 eruption was triggered by large volume mafic recharge magma that mixed and mingled with the resident dacite within probably days to weeks of the ensuing eruption

reconciling heterogeneity in crystal zoning data an application of shared characteristic diagrams at chaos crags lassen volcanic center california isotope trace element and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses these zoning data are used to infer crystal residence times magma mixing and other magmatic processes in many magmatic systems we present the shared characteristic diagram scd as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework an example from chaos crags in the cascade arc produces three main results 1 anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite 2 single crystal 87sr 86sr data from previous studies tepley et al 1999 are consistent with more shared history between crystals than in anorthite profiles this difference reflects a more homogeneous distribution of 87sr 86sr than the intensive parameters controlling plagioclase composition 3 the chaos crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole rock analyses that indicate only simple binary mixing the inconsistency between 87sr 86sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature water controlled zoning in plagioclase from chaos crags

phenocryst crystallization during ascent of alkali basalt magma at rishiri volcano northern japan abstract phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers a fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions on the basis of their large sizes however this assumption is not always true as demonstrated here for a porphyritic alkali basalt kutsugata lava from rishiri volcano northern japan all phenocryst phases in the kutsugata lava plagioclase olivine and augite have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals rarely a core region with distinct composition is present in all phenocryst phases phenocrysts excluding this core are occasionally in direct contact with each other forming crystal aggregates the equilibrium liquidus temperature of plagioclase the dominant phase 35 vol in the kutsugata lava can never exceed the estimated magmatic temperature unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent this suggests that most phenocrysts in the kutsugata lava were formed by decompression of the magma during ascent in a conduit rather than by cooling during residence in a magma reservoir in the magma chamber before eruption probably located at depth of more than 7 km only cores of the phenocrysts were present and the magma was nearly aphyric 4 wt in the magma chamber and liquidus temperatures of phenocryst phases were significantly suppressed large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution low ascent rate of the kutsugata magma which is suggested by pahoehoe lava morphology and no association of pyroclastics gave sufficient time for crystallization furthermore the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size which characterizes the porphyritic nature of the kutsugata lava alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

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

petrogenesis and assembly of the don manuel igneous complex miocene pliocene porphyry copper belt central chile the 4 0 3 6 ma don manuel igneous complex dmic central chile provides a window into igneous processes involved in magma genesis associated with porphyry style copper mineralization this study uses petrographic petrological geochemical and isotopic data to examine the evolution of magmas from the mid to lower crustal source region to shallow emplacement the data provide evidence for progressive oxidation of magma during differentiation and ascent fractionation of cl from s through degassing and the late stage near solidus removal of cl from the system magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns hydrous derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3 5 and 5 km intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing these crystal cargoes represent samples of crystal mush entrained from different depths as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite testifying to magma mingling during ascent sulfur and chlorine contents of apatite within the different dmic units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine the hypabyssal nature of the dmic affords a detailed integrated record of magmatic differentiation processes occurring within trans crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry style copper mineralization

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

testing and parameterizing a conceptual model for solute transport in a fractured granite using multiple tracers in a forced gradient test 1 a cross hole tracer test involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients bromide and pentafluorobenzoate and a weakly sorbing solute tracer lithium ion was conducted in a fractured granite near an underground nuclear test cavity in central nevada the test was conducted to 1 test a conceptual radionuclide transport model for the site and 2 obtain transport parameter estimates for predictive modeling the differences between the responses of the two nonsorbing tracers when normalized to injection masses are consistent with a dual porosity transport system in which matrix diffusion is occurring the large concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggests that diffusion occurs primarily into matrix pores not simply into stagnant water within the fractures the relative responses of the tracers at late times suggest that the diffusion accessible matrix pore volume is possibly limited to only half the total volume of the flow system implying that the effective retardation factor due to matrix diffusion may be as small as 1 5 for nonsorbing solutes in the system the lower end of the range of possible sorption kd values deduced from the lithium response is greater than the upper 95 confidence bound of kd values measured in laboratory sorption tests using crushed granite from the site this result suggests that the practice of using laboratory sorption data in field scale transport predictions of cation exchanging radionuclides such as 137cs and 90sr should be conservative for the shoal site

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

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

linking petrology and seismology at an active volcano many active volcanoes exhibit changes in seismicity ground deformation and gas emissions which in some instances arise from magma movement in the crust before eruption an enduring challenge in volcano monitoring is interpreting signs of unrest in terms of the causal subterranean magmatic processes we examined over 300 zoned orthopyroxene crystals from the 1980 1986 eruption of mount st helens that record pulsatory intrusions of new magma and volatiles into an existing larger reservoir before the eruption occurred diffusion chronometry applied to orthopyroxene crystal rims shows that episodes of magma intrusion correlate temporally with recorded seismicity providing evidence that some seismic events are related to magma intrusion these time scales are commensurate with monitoring signals at restless volcanoes thus improving our ability to forecast volcanic eruptions by using petrology

u series evidence for crustal involvement and magma residence times in the petrogenesis of parinacota volcano chile in this study we present th u disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the nevados de payachata volcano which erupted through 70 km of continental crust in the central volcanic zone of the andes 18 s 69 w both lavas and mineral separates were analyzed by mass spectrometry for 238u 230th disequilibria the lavas are characterized either by 230th enrichment or depletion relative to its parent nuclide 238u mineral separates are used to derive u th isochron ages and these ages compare favorably with inferred stratigraphic ages or k ar ages although in one case the u th age is significantly older than the stratigraphic age despite relatively constant sr nd and pb isotope ratios the lavas display inverse trends in 230th 238u versus ce yb or ba hf diagrams these trends cannot be interpreted by simple two component mixing rather there must be three and perhaps four components involved in the genesis of the parinacota lavas a mantle wedge a slab fluid and a lower crustal component can be identified a sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence the existence of binary arrays can be explained by variable amounts of crustal material the process of crust mantle interaction must have been short enough to preserve u th disequilibrium 300 ka

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

occurrence and origin of submarine plunge pools at the base of the us continental slope high resolution bathymetric data from the new jersey and californian continental margins show a marked depression running along parts of the base of the continental slope detailed analysis reveals that the depressions are a series of discrete plunge pools with associated downslope topographic ramparts we have used new bathymetric data to create our own data base of over 150 examples and systematically analyse plunge pool morphology and location previous observations of plunge pools have been sparse plunge pools are up to 1100 m wide and 75 m deep with a mean diameter of 400 m and a mean depth of 21 m plunge pools only occur where there are sharp decreases in slope of more than 4 and are well developed where changes in slope exceed 15 we propose plunge pools can be created by two mechanisms firstly they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment laden density flows that causes the deposition of bedload and the creation of a sediment bar this bar then defines the downslope margin of a pool secondly the impact of high momentum sediment laden density flows can excavate a depression as has been observed for subaerial snow avalanches sediment deposited downslope of these impact pools is very poorly sorted and partly derived from erosion within the pool both mechanisms influence whether turbidity currents are generated from high density sediment laden density flows influence whether depositional flows are channelised and have implications for base of slope facies models

petrologic diversity in mount st helens dacites during the last 4 000 years implications for magma mixing mount st helens has explosively erupted dacitic magma discontinuously over the last 40 000 years and detailed stratigraphic data are available for the past 4 000 years during this last time period the major element composition of the dacites has ranged from mafic 62 64 wt sio2 to felsic 65 67 wt sio2 temperature has varied by about 150 c 770 920 c and crystallinity has ranged between 20 and 55 water content of these dacites has also fluctuated greatly although the source for the dacitic magmas is probably partial melting of lower crustal rocks there is strong physical evidence such as banded pumices thermal heterogeneities in single pumices phenocryst disequilibrium contrasts between compositions of glass inclusions and host matrix glass and amphibole reaction rims that suggests that magma mixing has been prominent in the dacitic reservoir indeed we suggest that the variations in major and trace element abundances in mount st helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas magma mixing has also controlled the composition of andesites erupted at mount st helens and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite or possibly rhyodacite and basalt the flux of the felsic endmember to the reservior appears to have been relatively constant whereas the flux of basalt may have increased in the past 4 000 years as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period

the isotopic and chemical evolution of mount st helens isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the mt st helens volcano suggest three different episodes 1 40 000 2500 years ago eruptions of dacite with end 5 esr 10 variable 18o 206pb 204pb 18 76 ca sr 60 rb ba 0 1 la yb 18 2 2500 1000 years ago eruptions of basalt andesite and dacite with end 4 to 8 esr 7 to 22 variable 18o thought to represent melting of differing mantle crust reservoirs 206pb 204pb 18 81 18 87 variable ca sr rb ba la yb and high zr 3 1000 years ago to present day eruptions of andesite and dacite with end 6 esr 13 18o 6 variable206pb 204pb ca sr 77 rb ba 0 1 la yb 11 none of the products exhibit eu anomalies and all are lree enriched there is a strong correlation between87sr 86sr and differentiation indices these data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet but not feldspar followed by intrusion of this crustal reservoir by mantle derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re equilibration of pb isotopic compositions than other components suggesting that pb is being transported via a fluid phase the nd and sr isotopic compositions lie along the mantle array and suggest that the mantle underneath mt st helens is not as depleted as morb sources there is no indication of seawater involvement in the source region

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

re examination of crystal ages in recent mount st helens lavas implications for magma reservoir processes u series data for recent mount st helens lavas suggest that crystallization preceded eruption by more than 0 5 ka but are complicated by possible evidence of crystal recycling and or addition of radium to the liquid after crystallization we report new ion and electron microprobe trace and major element data for plagioclase and pyroxene in these recent mount st helens lavas and use these data to reassess 226ra 230th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates revised 226ra 230th model crystallization ages are 2 4 ka for plagioclase with the exception of the 1982 dacite and 0 15 5 7 ka for pyroxene in contrast to previous interpretations no late stage addition of ra to the liquid after precipitation of the minerals is required the variability of ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re equilibration of xenocrysts with a new host liquid ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases ra th ages have not been significantly modified by ra diffusion into or out of the crystals high 226ra ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal liquid chemical equilibrium one possibility is that of growth entrapment of surface enrichments during rapid crystallization which could lead to less discrimination between ra and ba than predicted by calculated bulk partition coefficients 226ra 230th crystal ages for the castle creek andesite and basalt that are younger than 230th 238u ages of the same crystals could be explained by mixing of crystals into melts with different 230th 232th ratios by combinations of older and younger crystal growth within the same magma or for the basalt by diffusion of ra into crystals at high magmatic temperatures average plagioclase ages in most of the samples of 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano this observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

constraints on crystal storage timescales in mixed magmas uranium series disequilibria in plagioclase from holocene magmas at mount hood oregon abstract uranium series crystal ages interpreted within a textural and geochemical framework can provide insight into crystal storage timescales especially in cases where crystals may derive from multiple sources we report here 230 th 226 ra model ages of two distinct populations of plagioclase from low silica dacites from mount hood oregon a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge mixing and incorporation of plagioclase derived from mafic and silicic end member magmas we have measured trace element concentrations and 238 u 230 th 226 ra disequilibria in four plagioclase size fractions from the timberline 1500 a and old maid 215 a eruptive sequences after correction for groundmass and apatite contamination average 230 th 226 ra model ages of large 500 m plagioclase are 4 5 ka timberline and 5 5 ka old maid with ages of cores that are 10 ka in each case indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions these model ages are longer than timescales of repose between eruptions indicating that these crystals resided in the sub surface over multiple eruptions likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas remobilizing a fraction of the large plagioclase crystals during each eruptive event after correction for large plagioclase contamination small 226 ra ba relative to equilibrium with liquid proxies groundmass and mafic inclusion leading to 230 th 226 ra model ages that are 230 th 226 ra disequilibria require that the majority of crystals in the separate are young 226 ra ba could potentially be explained by rapid crystallization immediately prior to and or during mixing events consistent with evidence of rapid crystallization of rims rapid crystallization of mafic intrusions may trigger eruption at mount hood by producing a partially crystalline mafic magma capable of mixing with a reheated silicic crystal mush

the genesis of arc dacites the case of mount st helens wa throughout the last 35 ka mount st helens has been the most active volcano in the cascade arc but the origin of its voluminous dacites remains controversial these dacites were traditionally interpreted as a result of melting metabasaltic lower crust yet recent studies have challenged this view and suggested an origin dominated by differentiation of mafic magmas through assimilation fractional crystallization afc processes to address this discrepancy on the origin of dacites at mount st helens we conduct an interdisciplinary study using a combination of thermal and geochemical modeling our results show that 45 crystallization of a basaltic andesite parent reproduces the compositions of the dacites with a maximum of 20 30 assimilation of lower crustal lithologies amphibole textures and compositions support such a differentiation trend in a polybaric mush system combined with recent geophysical imaging and experimental data we suggest that mount st helens dacites are generated by 1 mantle derived arc magma evolving by afc to intermediate compositions in a lower crustal magma reservoir and 2 ascent of these magmas to a mid to upper crustal reservoir where they reach high crystallinity without significant further chemical differentiation and are subject to frequent recharge that leave a clear mixing mingling overprint

constraints on the nature of the subvolcanic reservoir at south sister volcano oregon from u series dating combined with sub crystal trace element analysis of plagioclase and zircon abstract we present sub crystal scale 238u 230th zircon ages and 238u 230th 226ra plagioclase ages of bulk mineral separates from the holocene 2 0 2 3 ka eruptions of the rock mesa rm and devil s hills dh rhyolites at south sister volcano oregon we link these age data with sub crystal trace element analyses of zircon and plagioclase to provide insight into the subvolcanic system at south sister as an example of a small volume continental arc volcano our results document the presence of coeval yet physically distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed zircons from the rm and dh dominantly record ages from 20 to 80 ka with some grains recording ages 350 ka whereas plagioclase records 230th 226ra ages of 2 3 6 8 ka rm and 4 0 9 6 ka dh 3 and a 238u 230th age of 10 34 ka dh 3 we interpret zircons with ages 50 ka within a heterogeneous magmatic system and coexisted as physically distinct dominantly liquid bodies prior to eruption thus we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous largely crystalline reservoir similarities in crystal age and chemical data to that at other young silicic systems e g mount st helens okataina caldera complex suggest that this model may be more generally applicable to silicic magmas

the extent of u series disequilibria produced during partial melting of the lower crust with implications for the formation of the mount st helens dacites the extent to which u series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for u th and ra we show that partial melting of mafic lower crust can only produce small disequilibria between 238u 230th and 226ra crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect consequently u series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent unless the timescales of silicic magma production within the crust are short compared to the half life of 226ra our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust we show that the 226ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and or the involvement of mantle derived basalts less than a few thousand years old application to mount st helens suggests that crystallisation of young mantle derived magma is likely to be the dominant process in the formation of these dacites

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

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

crystal size distributions csd in three dimensions insights from the 3d reconstruction of a highly porphyritic rhyolite growth histories and residence times of crystals in magmatic systems can be revealed by studying crystal sizes size distributions and shapes in this contribution serial sectioning has been employed on a sample of porphyritic rhyolite from a permo carboniferous laccolith from the halle volcanic complex germany to reconstruct the distribution of felsic phenocrysts in three dimensions in order to determine their true shapes sizes and three dimensional size distributions a model of all three phenocryst phases quartz plagioclase k feldspar with 217 crystals and a larger model containing 1599 k feldspar crystals was reconstructed in three dimensions the first model revealed a non touching framework of crystals in three dimensions suggesting that individual crystals grew freely in the melt prior to quenching of the texture however crystal shapes are complex and show large variation on a zingg diagram intermediate over long axis plotted against short over intermediate axis they often do not resemble the crystallographic shapes expected for phenocrysts growing unhindered from a melt indicating complex growth histories in contrast the three dimensional size distribution is a simple straight line with a negative slope stereologically corrected size distributions from individual sections compare well with stereologically corrected size distributions obtained previously from the same sample however crystal size distribution csd data from individual sections scatter considerably it is shown that csds can be robustly reproduced with a sampling size of greater than 200 crystals the kind of shape assumed in stereological correction of csds however has a large influence on the calculation and estimation of crystal residence times

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

reappraisal of uranium series isotope data in kamchatka lavas implications for continental arc magma genesis uranium series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones however interpretations of u th ra data suggest very different timescales of slab dehydration here we present new u th ra data from kamchatka along with a number of alternative models for production of radioactive disequilibrium variations in 226ra 230th and 231pa 235u activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c 6000 years the association of the largest 226ra excesses with high sr th in the most primitive lavas suggests that ra th fractionation is controlled by slab dehydration less than 10 ka ago we show that u th data can be explained by dynamic melting of a recently ka metasomatized mantle wedge dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231pa excess because 238u 230th disequilibrium is inferred to be controlled by partial melting there is no requirement for multi stage slab dehydration commencing 150 ka we suggest that ra th disequilibria constrain the timing of slab dehydration whilst u th fractionation is dominated by partial melting at least at the kamchatka arc

experimental determination of ra mineral melt partitioning for feldspars and 226ra disequilibrium crystallization ages of plagioclase and alkali feldspar radium and other trace element partition coefficients have been experimentally determined for plagioclase and alkali feldspar in equilibrium with silicate melts in anorthitic plagioclase alkalis e g na k rb cs and the heavy earth alkalis e g ba ra are incompatibles in albitic plagioclase na is compatible whereas ca and sr are always compatible in plagioclase draplagioclase melt was determined to be 0 017 0 006 an91 0 025 0 009 an81 and 0 47 0 08 an34 for three different an contents the ratio dra dba increasing with decreasing an component in alkali feldspar k rb ba ra and sr are compatible whereas na cs and ca are incompatible draalkali feldspar melt was determined to 2 54 0 04 for a or75ab21an4 composition the ratio dra dba is 0 55 0 01 for plagioclase dra and xan are related as rt lndra kj mol 52 54 5 75 xan 4 25 1 51 and dra dba exp 9 47 6 21 xan 13 66 1 52 rt kj mol allowing calculation of dra from the an content of magmatic plagioclase for alkali feldspar we propose the equation dra dba exp 16 06 0 58 15 52 0 67 xor rt kj mol the experimentally determined partition coefficients are then used to recalculate th ra ba model ages leading to generally shorter feldspar residence times the variations encompass 20 to 90 shorter crystallization ages with respect to the originally calculated ones in two cases feldspar crystallization ages increase slightly for several cases the measured plagioclase and melt have either never been in equilibrium or the th ra ba system has been disturbed after crystallization

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

holocene geomagnetic secular variation recorded by volcanic deposits at mount st helens washington a compilation of paleomagnetic data from volcanic deposits of mount st helens is presented in this report the database is used to determine signature paleomagnetic directions of products from its holocene eruptive events to assign sampled units to their proper eruptive period and to begin the assembly of a much larger database of paleomagnetic directions from holocene volcanic rocks in western north america the paleomagnetic results from mount st helens are mostly of high quality and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates the muddy river andesite s paleomagnetic direction however indicates that it is more likely part of the pine creek eruptive period rather than the castle creek period in addition the two fingers andesite flow is more likely part of the middle kalama eruptive period and not part of the goat rocks period the paleomagnetic data from mount st helens and mount hood document variation in the geomagnetic field s pole position over the last 2 500 years a distinct feature of the new paleosecular variation psv record similar to the fish lake record oregon indicates a sudden change from rapid clockwise movement of the pole about the earth s spin axis to relatively slow counterclockwise movement at 800 to 900 years b p

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

experimental study of radium partitioning between anorthite and melt at 1 atm we present the first experimental radium mineral melt partitioning data specifically between anorthite and a cmas melt at atmospheric pressure 226 ra disequilibria are an important chronometer of recent magmatic activity ion microprobe measurement of coexisting anorthite and glass phases produces a molar d ra 0 040 0 006 and d ra d ba 0 24 0 05 at 1400 c our results indicate that lattice strain partitioning models fit the divalent ca sr ba ra partition coefficient data of this study well supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values

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

crystal and magma residence at kilauea volcano hawaii 230th 226ra dating of the 1955 east rift eruption abstract previous estimates of crustal storage time of magmas at kilauea volcano hawaii range from a few years to a few thousand years leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation we present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226ra 230th disequilibria and apply this approach to the early phase of the 1955 east rift eruption at kilauea when fractionation of ra from ba a proxy for initial ra in the crystals during crystal growth is considered along with the effects of inclusions in the minerals the data are consistent with co precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000 300 400 a unless a significant fraction 30 of the crystals are remnants from an earlier batch of evolved magma in the system these data constrain the minimum magmatic residence time to be 550 yr considerably longer than most previous estimates of storage time at kilauea as well as those for some other basaltic systems for the temperature interval of augite plagioclase growth in the early 1955 magma a maximum constant cooling rate of 0 1 c yr 1 10 5 c h is derived from the minimum magmatic residence time of 550 yr the total magma storage time would be 2500 yr if this cooling rate applied to the entire thermal history of the magma although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr the disparate estimates of magma residence at kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma more work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and or of kilauean lavas in general

a combined geochronological approach to investigating long lived granite magmatism the shap granite uk abstract with the advent of more precise dating methods it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods here we attempt to reconcile a number of dating methods from the c 5 km2 caledonian shap granite northern england the results reveal a more complex and protracted evolution than indicated by application of any single dating method zircon u pb dates give a weighted mean age of 415 6 1 4 2 ma a mafic enclave dated at 412 2 2 ma revised rb sr feldspar age from davidson et al 2005 contains resorbed k feldspar and zircon crystals scavenged from the host crystal mush these ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 ma or later during acadian deformation previously reported re os ages on molybdenites associated with magmatic fluids have given ages of 405 2 1 8 2 ma selby et al 2008 and confirm the overlap of at least some magmatic activity with acadian deformation a similar emplacement age is supported by rb sr whole rock mineral and biotite k ar dates when adjusted for revised decay constants 402 3 ma and 401 7 ma respectively wadge et al 1978 the lower closure temperatures of these systems relative to the u pb system in zircon means that they are more likely to record the timing of final granite emplacement these data suggest that most zircons grew before final granite emplacement by about 10 ma on average we suggest that the majority of zircon crystals record pre emplacement magmatic activity within a deeper part of the system mafic enclaves and their scavenged cargo of crystals record the assembly of a mid crustal batholith where crystals remained at least locally mobile at 412 ma gravity data support the existence of an extensive 1500 km2 intrusive body originally at about 15 km depth beneath shap this batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss but episodic influxes of silicic magma between c 412 and 405 ma are thought to have enabled periods of rejuvenation these influxes are recorded by complex compositional zoning patterns within k feldspar megacrysts the shap granite itself is likely to represent a rejuvenated crystal slurry emplaced as a cylindrical cupola above the main magma body during acadian transpression this study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages

ra partitioning between phlogopite and silicate melt and 226ra ba 230th ba isochrons in this study we experimentally determine phlogopite melt partition coefficients of ra and other trace elements in a lamproitic system this work was achieved using an analytical technique la icp ms with low detection limits 0 01 fg permitting the measurement of the very low ra concentrations feasible in experiments 1 ppb d ra phlogopite melt was determined to 2 28 0 44 and 2 84 0 47 in two experiments the ratio d ra d ba is around 1 6 the compatibility of ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large xii coordinated interlayer site of phlogopite a re evaluation of d ra and d ra d ba for magmatic minerals containing appreciable ra yields d ra mineral melt ranging from 2 6 for phlogopite down to 2 3 10 5 for pyroxenes and d ra d ba mineral melt from 4 for leucite to 2 10 2 for orthopyroxene the influence of melt composition on d ra d ba is less than 10 all investigated minerals have different d ra d ba strongly fractionating ra from ba thus for magmatic systems 226 ra ba in the various minerals is not constant these minerals do not form a straight line in the 226 ra ba 230 th ba system at the time of crystallization and thus there is no 226 ra ba 230 th ba isochron at t 0 226ra 230th ba mineral dating is thus applicable only to model ages calculated from mineral glass pairs with known d ra