u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

an analytical study for 1 dimensional steady flow in volcanic conduits the effects of magma properties viscosity and volatile content and geological conditions radius and length of conduit on the dynamics of explosive eruptions are analytically investigated on the basis of a simplified model for 1 dimensional steady flow in a volcanic conduit the conduit flow is characterized by bubbly flow before magma fragmentation and by gas pyroclast flow after magma fragmentation the features of the flow are largely determined by the balance of viscous effects of the bubbly flow and expansion of the gas pyroclast flow under a given boundary condition those effects are described by two non dimensional numbers and the parameter represents the ratio of effects of wall friction and gravity in the bubbly flow the length of the bubbly flow region decreases as increases the parameter represents the ratio of pressures at the vent and at fragmentation level the length of the gas pyroclast flow region decreases as increases three regimes of conduit flow are defined according to when is smaller than a certain value determined by initial volatile content and length of conduit the conduit flow is composed mainly of the gas pyroclast flow region regime 1 on the other hand when 10 the conduit flow is composed mainly of the bubbly flow region regime 3 regime 2 is the intermediate regime between regimes 1 and 3 the flow variables such as level of fragmentation and magma discharge rate are sensitive to magmatic properties and geological conditions in regime 2 a graphical method to estimate the effects of these parameters is also proposed

early permian peak metamorphism recorded in u pb system of black slates from the ogcheon metamorphic belt south korea and its tectonic implication abstract we report pb pb whole rock and uraninite chemical th u total pb isochron method chime ages of carbonaceous black slates from the ogcheon metamorphic belt south korea the pb isotopic data of whole rock samples yield 207 pb 206 pb ages of 283 33 and 291 13 ma for two outcrops from the northeastern part of the belt the uraninite chime age is estimated at 283 26 and 281 27 ma for the northeastern and the middle part of the belt respectively all of the above ages are identical within error ranges and represent the timing of peak metamorphism after the late precambrian intraplate rifting on the other hand the 207 pb 206 pb whole rock age for the southwestern part of the belt is estimated at 194 27 ma probably representing the timing of contact thermal metamorphism associated with the intrusion of jurassic granitic plutons rb sr isotopic data of the black slates do not define any meaningful isochron the early permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the silurian devonian or the triassic time instead it corroborates the probability that the two zones in the ogcheon belt the ogcheon metamorphic belt and the taebaegsan zone were separated from each other before the development of major structural framework in the former our data do not support an idea that the ogcheon belt corresponds to the continuation of the triassic collision belt in east central china

nondestructive chemical dating of young monazite using xrf 1 design of a mini probe age data for samples from the central alps and comparison to u pb tims data the analytical capabilities of an improved mini xrf instrument for trace element analysis are explored to obtain u th pb ages for single monazite grains as small as 50 m the detection limits for u th and pb with counting times of 10 min are 10 ppm counts collected for 40 min are sufficient to date single monazite grains as young as 15 ma the precision of the age data depends essentially on the counting statistics of the main x ray peak npb l 1 2 used to analyze for lead the age uncertainty is proportional to the age of the grain the total contents of th and u and the squared integration time the reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central alps for which the chemical ages are compared to mass spectrometric u pb tims ages we show that populations of analyses obtained using the new instrument on single grains as young as 25 30 ma agree within error with the tims data for monazite separates with complex age patterns such as polymetamorphic samples yielding discordant isotopic ages the statistical analysis of chemical age data obtained from populations of 26 50 grains is advantageous in that it may provide additional insight into the reasons for the discordancy chemical th u pb dating of monazite by the xrf method offers considerably improved accuracy and precision compared with chemical dating by electron microprobe which typically becomes impractical for samples less than 100 200 ma in age conversely the minimum grain size required for the new instrument 50 m is well above the spatial resolution of the electron microprobe 2 m a companion paper scherrer et al this volume describes further developments towards microanalysis by xrf with improved spatial resolution

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

monazite geochronology of uhp and hp metamorphism deformation and exhumation nordoyane western gneiss region norway u th pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of uhp and hp plates 820 c 39 kbar 780 c 18 kbar metamorphosed during the late silurian early devonian collision between baltica and laurentia the upper plate contains a microdiamond bearing kyanite garnet graphite gneiss and associated kyanite eclogites independently indicating probable diamond forming conditions in situ dating of monazite in the microdiamond sample using the shrimp ii at the geological survey of canada yielded ages of 415 6 8 ma for those included in garnet and 398 6 ma for those in the matrix these ages compare to 408 0 5 6 and 397 5 4 4 ma determined using the electron microprobe at the university of massachusetts both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100 950 ma and scattered grains with ages of 900 500 ma but no ages of 1680 1650 ma equivalent to the local baltica basement were found agreement between the two techniques allowed evaluation of monazite age domains 198 analyses from the microdiamond rock and a kyanite garnet sillimanite mylonite produced from it using a combination of high resolution element imaging and trace element analysis of u th pb and y this comparison yielded three mean ages of 407 0 2 1 ma 394 8 2 3 ma and 374 6 2 7 ma combining this geochronology with previous p t estimates we propose that the uhp unit reached its maximum depth of 125 km at a maximum age of 407 ma when monazite was included in garnet and experienced 65 km of exhumation at an average rate of 10 9 mm year during top southeast thrusting that brought it into contact with the hp unit following these events both units were exhumed together at an average rate of 3 8 mm year until reaching a depth of 37 km at 395 ma where these rocks experienced extensive re equilibration and top west and left lateral shearing after 395 ma these units continued to be exhumed at an average rate of 0 8 to 1 4 mm year until 375 ma the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite the exhumation histories of these units record a change in mechanism from syncollisional exhumation through late to post orogenic collapse that was a consequence of plate reorganization

on the relationship between eruption intensity and volcanic plume height insights from three dimensional numerical simulations abstract height of plumes generated during explosive volcanic eruptions is commonly used to estimate the associated eruption intensity i e mass eruption rate mer in order to quantify the relationship between plume height and mer we performed a parametric study using a three dimensional 3d numerical model of volcanic plumes for different vent sizes the results of five simulations indicate that the flow pattern in the lower region of the plume systematically changes with vent size and hence with mer for mers 7 kg s 1 the flow in the lower region has a jet like structure the jet like regime for mers 10 8 kg s 1 the flow shows a fountain like structure the fountain like regime the flow pattern of plumes with 4 10 7 kg s 1 8 kg s 1 shows transitional features between the two flow regimes within each of the two flow regimes the plume height increases as the mer increases whereas plume heights remain almost constant or even decrease as mer increases in the transitional regime as a result the jet like and fountain like regimes show distinct relationships of plume height and mer the different relationships between the two regimes reflect the fact that the efficiency of entrainment of ambient air in the jet like regime is substantially lower than that in the fountain like regime it is suggested that in order to estimate eruption intensity from the observed plume heights it is necessary to take the different flow regimes depending on mer into account

analytical perils and progress in electron microprobe trace element analysis applied to geochronology background acquisition interferences and beam irradiation effects electron probe microanalysis epma of accessory minerals such as monazite xenotime and thorite for minor and trace element concentrations and geochronology requires consideration of beam irradiation effects increasing heat and charge as higher current densities and lengthy counting times are employed and requires careful detailed assessment of background intensities and interferences a carbon coat 250 a thickness is generally inadequate for prevention of absorbed current s uctuation and beam damage when using the high current densities applied for high precision e g 200 na focused beam beam irradiation effects include element mobility in monazite resulting in p loss relative to ree coating materials of higher electrical and thermal conductivity are indicated and use of gold 100 a is strongly suggested systematic compositional and therefore age variability can result simply from analytical effects requiring evaluation of all aspects of data acquisition the spectra relevant to measurement of y th pb and u are complex especially in ree bearing minerals acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences as well as curvature background intensities can be extracted directly from scan data by regression x ray mapping allows delineation of domains guiding background acquisition and detailed quantitative analysis minor substitution or s uorescence of unexpected elements can compromise analyses one documented effect being the s uorescence of k in monazite adjacent to or hosted by k feldspar or micas this effect clearly evident within 10 micrometers from k feldspar can result in erroneous u concentrations leading to misinterpretation of rim ages as younger overgrowths absorption edges associated with th also become relevant to the measurement of u at high th concentrations because background intensity is sensitive to variation in average atomic number backgrounds must be acquired from each identi ed domain with particular attention being paid to th variation misapplication of background intensities can result in large age discrepancies for example application of backgrounds obtained from a high th domain in monazite to measurement of a low th domain 1 3 of the amount in the high th domain results in an overestimation of the um background intensity of 0 008 cps na and an overall increase in age of 70 m y

transmission electron microscope study of polyphase and discordant monazites site specific specimen preparation using the focused ion beam technique electron microprobe emp u th pb dating on polyphase and discordant monazites from polymetamorphic granulites of the andriamena unit north central madagascar reveals inconsistent chemical ages to explain these drastic variations transmission electron microscopy tem foils were prepared directly from thin sections by using the focused ion beam technique the most important result of the tem study is the demonstration of the presence of small 50 nm pb rich domains where large variations in emp ages occur we suggest that radiogenic pb was partially reincorporated in monazite during the recrystallization at 790 ma because the excited volume of emp is 4 m3 u th pb dating yielded various apparent older ages without geological significance in addition tem analysis of the foils revealed the presence of an 150 nm wide amorphous zone along the grain boundary of monazite and its host quartz this fe si al rich phase may have formed as a result of fluid activity at 500 ma and the phase s amorphous state may be due to the irradiation from u and th decay in the monazite this demonstrates for the first time the enormous potential of the tem investigations on site specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data

the decompression of volcanic jets in a crater during explosive volcanic eruptions during explosive volcanic eruptions fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10 100 atm and at the speed of sound of the mixture we show that for magmatic volatile contents no in the range 0 03 no 0 06 this has the approximate value 0 95 0 05 nort 12 where r is the gas constant and t is the eruption temperature this speed is nearly independent of the vent pressure and vent radius by assuming there is negligible mixing with the air we have modelled the decompression of such jets following eruption from the vent for free decompression into the atmosphere the velocity of the decompressed jet has the approximate value 1 85 0 05 nort 12 this is nearly independent of the eruption rate for decompression into a crater the process is more complex at low eruption rates with low vent pressure the material becomes underpressured as it rises in the crater a shock then forms in the crater this recompresses the magma volatile mixture which then issues from the crater as a relatively slow subsonic jet at atmospheric pressure as the eruption rate increases such shocks move towards the top of the crater and ultimately cannot form in the crater instead the material issues from the crater as a high speed supersonic jet the upward thrust provided by the crater walls on this high pressure jet then increases the upward velocity above that of a freely decompressing jet the presence of a crater may therfore cause collapse in relatively small eruptions whereas it may promote formation of buoyant eruption columns at higher eruption rates if a crater grows through erosion during an eruption column collapse will typically ensue even if the mass flux remains steady

3 d numerical simulations of eruption column collapse effects of vent size on pressure balanced jet plumes abstract buoyant columns or pyroclastic flows form during explosive volcanic eruptions in the transition state these two eruption styles can develop simultaneously we investigated the critical condition that separates the two eruption styles referred to as the column collapse condition by performing a series of three dimensional numerical simulations in the simulation results we identify two types of flow regime a turbulent jet that efficiently entrains ambient air jet type and a fountain with a high concentration of the ejected material fountain type hence there are two types of column collapse jet type and fountain type which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse mdr cc is larger or smaller than that for the generation of a fountain mdr jf for a given exit velocity temperature controls the relative magnitude of mdr cc relative to mdr jf and hence the type of collapse for given magma properties e g temperature and water content the column collapse condition is expressed by a critical value of the richardson number ri cc g 0 l 0 w 0 2 where g 0 is the source buoyancy l 0 is the vent radius and w 0 is the exit velocity when the jet type collapse occurs at the column collapse condition ri cc is independent of the exit velocity when the fountain type collapse occurs at the column collapse condition on the other hand ri cc decreases as the exit velocity increases as the exit velocity exceeds the sound velocity a robust flow structure with a series of standing shock waves develops in the fountain which suppresses entrainment of ambient air and enhances column collapse

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

simple tests of a semi implicit semi lagrangian model on 2d mountain wave problems abstract the fully compressible 3d nonhydrostatic semi implicit semi lagrangian mc2 mesoscale compressible community model described by tanguay et al has been modified in order to incorporate orography through the gal chen and somerville transformation of the vertical coordinate by denis in this study a 2d version of the model is tested against classical solutions covering various mountain wave regimes for continuously stratified flows a close inspection of the propagation of the vertical momentum flux is performed to asses the accuracy and stability of the numerical method the study emphasizes also the fact that a correct representation of forced hydrostatic gravity waves is reliable for courant numbers less than 0 5 this limitation may be less severe as the flow becomes more nonhydrostatic furthermore the sensitivity of the isothermal reference state for flows with realistic static stability and over steep slope mountain is explored

nondestructive chemical dating of young monazite using xrf 2 context sensitive microanalysis and comparison with th pb laser ablation mass spectrometric data a newly developed xrf microprobe at the institute of mineralogy and petrology university of bern switzerland has been applied for precise chemical th u pb dating of individual monazite grains separated from pb free polished petrographic thin sections the nondestructive nature of the xrf measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis empa chemical dating by xrf microprobe analysis and isotopic 208pb 232th dating by laser ablation plasma ionisation multi collector mass spectrometry la pimms analysis as an example the 2 precision achieved with the xrf microprobe for well characterised reference material monazite fc 1 tims age 54 3 1 ma xrf age 55 3 2 6 ma doubly polished to 30 m in thickness is below 5 after 90 min integration time 50 kv 30 ma at a spatial resolution of 90 m at 38 m spatial resolution the uncertainty is 35 for the same integration time the sample characteristics are 200 300 ppm of pb xrf 3 8 5 1 wt of th empa and 0 4 1 4 wt u empa combined with an electron microprobe and conventional optical microscopy the xrf microprobe is thus a competitive low cost and nondestructive alternative to more costly isotopic methods the xrf microprobe is easy to use and maintain

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

particle fallout thermal disequilibrium and volcanic plumes we develop a model of a volcanic eruption column that includes the effects of fallout of pyroclasts and thermal disequilibrium we show that clast fallout with no thermal disequilibrium has only a small effect upon the column however disequilibrium changes column behaviour significantly and can even induce collapse our results may explain the lower plume heights and less widely dispersed fallout of cone forming eruptions contrasted with sheet forming eruptions the model also predicts that the transition from the gas thrust to the convective region in a column results in an inflection in dispersal curves that some features of the stratigraphy common to many fall deposits may result from column velocity structure and that there may exist a region near the volcanic vent in which maximum pyroclast size does not decrease significantly with distance

an improved u th pb age calculation for electron microprobe dating of monazite abstract determination of u th pb ages using the electron probe microanalyser epma is an inexpensive alternative method for dating monazite the method is rapid and reliable both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization the main limitation of the method is its rather poor precision i e 45 to 120 ma for ages ranging from 300 to 3000 ma calculated on each spot the precision is limited by pb content and by counting statistics which cannot give precision better than 2 for individual determinations on u th and pb even at high levels a procedure that uses the new th pb vs u pb diagram to improve the calculated precision on u th pb ages gives results to within 5 to 15 ma for ages ranging from 300 to 3000 ma with complex polygenetic monazite in which either the points show large scattering indicated by a large mswd or the regression line exhibits a slope very different from neighbour theoretical isochrons the procedure must be applied separately on homogeneous domains only this makes it possible to distinguish between events separated by a gap of 20 to 60 ma according to the range of ages concerned i e 300 to 3000 ma several examples are given to illustrate these systematics

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

a three dimensional method for calculating independent chemical u pb and th pb ages of accessory minerals abstract previous theoretical considerations on the chemical u th total pb dating method failed to distinguish between thorogenic and uranogenic lead however it can be shown that the data points are located on a plane in the three dimensional tho2 pbo uo2 space the calculation of the best fit plane yields a slope in the tho2 pbo and uo2 pbo coordinate projections and an initial pbo value from the two slopes th pb and u pb ages can be calculated independently the method described in this paper is applied to monazites from the hercynian g4 granite of the fichtelgebirge germany th pb and u pb ages were calculated at 323 20 ma and 304 15 ma respectively the intercept value close to zero indicates that no significant amounts of common lead are present in the monazites studied

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

fluid dynamics in explosive volcanic vents and craters abstract explosive volcanic jets can transition to buoyant plumes or collapse to form pyroclastic density currents depending on their ability to entrain and heat the ambient air recent one dimensional 1d analysis shows that fluid acceleration through volcanic vents and craters changes the velocity and pressures within these jets sufficiently enough to be a first order control on plume dimensions and therefore air entrainment and column stability koyaguchi et al 2010 these 1d studies are only applicable to craters and vents with angles of less than about 30 to vertical using analytical formulations and numerical simulations this study describes 2d effects of shallowly dipping vents and craters on volcanic eruptions the effect of vents on acceleration and expansion of eruptive mixtures of ash and gas is described as a force imparted on the fluid by the vent wall the wall force fw this force is a measure of the momentum coupling between an eruption and the solid earth that takes place in the vent rapid divergence of supersonic eruptive fluid within shallowly dipping vents occurs via prandtl meyer expansion which results in different pressure and velocity fields than those predicted by 1d analysis this expansion decreases fw and the vertical acceleration experienced by the eruptive fluid in the vent for jets predicted by 1d analysis to exit the vent at supersonic velocities and at atmospheric pressure this decrease in fw will cause an increase in the predicted plume area decreasing column stability the complex 2d shape of volcanic vents can change jet structure presence and location of shock waves and preclude the development of jets that exit the vent supersonically with no internal standing shock waves i e perfectly expanded or pressure balanced jets these significant complications in jet structure and increase in plume radius may result in changes to air entrainment plume stability and tephra distribution

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

on the rise of turbulent plumes quantitative effects of variable entrainment for submarine hydrothermal vents terrestrial and extra terrestrial explosive volcanism 1 the maximum height reached by a turbulent plume rising in a stratified environment is a key tool to estimate the flux released at its source particularly for large scale flows because flux can often be very hard to measure directly this height is known to be mainly controlled by the stratification of the ambient fluid source buoyancy flux and the efficiency of turbulent mixing between the plume and the external fluid the latter effect has been only superficially explored in spite of its fundamental control on the dynamics here we show that commonly used one dimensional models incorporating a constant entrainment coefficient do not provide satisfying predictions we propose a new model allowing for variable entrainment which gives excellent predictions of maximum heights reached by laboratory plumes in stratified environments we then apply our formalism to natural plumes produced by explosive volcanic eruptions under terrestrial paleo martian and venusian conditions and by submarine hydrothermal activity at mid ocean ridges source mass discharge rates deduced from maximum volcanic column heights for terrestrial eruptions are found to be greater than those estimated with the commonly used constant entrainment parameterization by a factor of 2 in the paleo martian atmosphere existing models overestimate plume heights by 14 27 in the current atmosphere of venus the maximum height reached by a volcanic plume is also found to be smaller than previously estimated for large eruption rates the source heat flux released by the tag field atlantic ocean deduced from several submarine hydrothermal plumes is found greater by a factor 3 with our model

effects of the crater on eruption column dynamics 1 during explosive eruptions a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow we systematically investigate how the condition that separates these two eruption styles column collapse condition depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow flow inside a crater and eruption column dynamics the results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate and water content nf strongly depend on crater shape depth d and opening angle when a crater is present the decompression and or compression of gas pyroclast mixture inside and just above the crater result in two distinct types of column collapse collapse with increasing hm side collapse and that with decreasing lm side collapse hm side collapse is caused by an increase in conduit radius during the waxing stage of an eruption lm side collapse is associated with a decrease in magma chamber pressure during the waning stage the value of for hm side collapse varies by two orders of magnitude depending on crater shape for fixed nf this estimate also depends on assumed models for decompression into the atmosphere the value of for lm side collapse is 108 kg s 1 when d tan 102 m these results are consistent with the field observations from the st helens 1980 and pinatubo 1991 eruptions

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

large eddy simulation of gas particle kinematic decoupling and turbulent entrainment in volcanic plumes abstract in the framework of the iavcei international association of volcanology and chemistry of the earth interior intercomparison study on volcanic plume models we present three dimensional 3d numerical simulations carried out with the ashee ash equilibrium eulerian model the ashee model solves the compressible balance equations of mass momentum and enthalpy of a gas particle mixture and is able to describe the kinematic decoupling for particles characterized by stokes number i e the ratio between the particle equilibrium time and the flow characteristic time lower than 0 2 or particles smaller than about 1 mm the computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a large eddy simulation approach and is thus suited to analyze the role of particle non equilibrium in the dynamics of turbulent volcanic plumes the two reference scenarios analyzed correspond to a weak mass eruption rate 1 5 10 6 kg s and a strong volcanic plume mass eruption rate 1 5 10 9 kg s in absence of wind for each scenario we compare the 3d results averaged in space and time with theoretical results obtained from integral plume models such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid scale turbulence model and the influence of gas particle non equilibrium processes on the large scale plume dynamics we thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant of the order of 20 but still lower than that typically associated with input data and integral model approximations in the weak plume case 3d results are consistent with the predictions of integral models in the jet and plume regions with an entrainment coefficient around 0 10 in the plume region in the strong plume case the self similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable with an average value of 0 24 for both cases integral model predictions diverge from the 3d plume behavior in the umbrella region the presented analysis of 3d numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies in addition high resolution 3d runs allow reproduction of observable quantities such as infrasound signals which can be useful for constraining eruption dynamics during real events

marginal stability of atmospheric eruption columns and pyroclastic flow generation explosive volcanic eruptions frequently generate fall and flow deposits simultaneously which can be attributed to a marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes this behavior is reproduced by laboratory experiments and numerical simulations ten well documented eruptions are used to test theoretical models of explosive eruptions three types of deposits fall flow and composite deposits made of intercalated flow and fall units are observed in these eruptions estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase using the simple assumptions that 1 the mass fraction of gas in the mixture is equal to the initial volatile content of magma and 2 jet expansion outside the vent is unconstrained by crater dimensions theoretical predictions are not consistent with the data agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts which lowers the gas content of the erupted mixture the effective amount of continuous gas phase carrying pyroclasts in suspension depends on the size distribution of pyroclasts in coarse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flow a new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations for given mass flux and initial dissolved volatile content changes of the size distribution of pyroclasts may have a strong effect on atmospheric column behavior

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

late quaternary pteropods in the south china sea carbonate preservation and paleoenvironmental variation abstract pteropod fossil assemblages were studied in four sediment cores selected from the south china sea scs in order to investigate carbonate preservation change through late quaternary glacial interglacial cycles these cores were selected at water depths w d below near and or above the present aragonite compensation depth acd i e s050 37kl 2695 m w d s049 8kl 1040 m w d and scs 12 543 m w d and rc14 79 706 m w d respectively the abundance of the aragonitic shells down core reveals an increase in pteropod preservation during glacial times in the cores which are below the present acd 1000 m w d carbonate dissolution increasesin interglacials as in the pacific open ocean even at the water depths above the present lysocline where higher carbonate contents was found in interglacials a carbonate preservation maximum however exists at the last deglaciation at sites shallower than the present pteropod lysocline 800 m w d carbonate dissolution shows little influence on the glacial interglacial change in carbonate preservation the glacial presence of pteropods on the northern continental slope reveals drastic acd fluctuations through glacial interglacial cycles and sinking of the acd to below or near 2700 m present water depths in the northern scs during glacial periods pteropod faunal composition indicates a western pacific open ocean origin and variations in fossil assemblages correlate well with sea surface temperature change as most of the straits around the scs emerged due to lower glacial sea level subtropical temperate water influence from the western pacific open ocean increased during glacial periods through the remaining water passageway of the bashi strait in the northeastern scs hence decreases in both the tropical warm water group and bathypelagic species as well as increases of subtropical temperate water species provide further evidence that glacial interglacial contrast in faunal composition is related to the lowered sea level which altered the configuration of the glacial scs

electron microprobe age mapping of monazite high resolution x ray maps of th u pb and y in monazite can be used to construct age maps which reveal the continuous spatial distribution of ages in a single grain of monazite the age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique the insights it can provide into monazite geochronology in general and its limitations compared to electron microprobe emp quantitative dating and other in situ geochronologic techniques age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques and they are a valuable aid for rigorously locating in situ analytical points age mapping also can be used to better understand the behavior of the u th pb system in monazite during metamorphism deformation and s uid circulation events age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale like a now healed fracture not visible in back scattered electron bse images or young domains less than 5 m in width located inside an older core in both cases using age maps as a template for locating in situ analysis points will minimize the peril of age mixing and erroneous geological interpretations in addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization and thus even more powerful applications of the monazite geochronometer the agemap program is available in a windows version and can be downloaded from the internet at the following address http www geo umass edu probe agemap

smith river allochthon a southern appalachian peri gondwanan terrane emplaced directly on laurentia the smith river allochthon is in a southern appalachian belt of metaclastic rocks that has traditionally been considered to be of peri laurentian origin new th u pb monazite ages confirm that the allochthon was involved in an early cambrian tectonothermal event indicate that it was exotic with respect to adjacent laurentian rocks and indicate that it is likely of gondwanan origin the allochthon may form a new link between the appalachians and the pampean terrane of western south america in addition its position in the orogen has implications for recent models of the opening of the iapetus

denudation history of the high t p ryoke metamorphic belt southwest japan constraints from chime monazite ages of gneisses and granitoids chime chemical th u total pb isochron method monazite ages were determined for gneisses and granitoids from the eastern and western parts of the ryoke belt separated by about 500 km the monazite ages for the gneisses are concentrated between 102 and 98 ma and are interpreted as the time of monazite formation under lower amphibolite facies conditions the peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c 95 ma in both the eastern and western parts in the eastern part plutonism continued from c 95 ma to c 68 ma at intervals of 2 10 ma whereas in the western part it ceased at c 85 ma the chime monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts these lines of evidence are incompatible with a current view that the plutonometamorphism in the ryoke belt becomes younger towards the east the chime monazite ages coupled with available data on the depth at which the ryoke metamorphism took place and the emplacement of individual plutons show that the western part was eroded more rapidly about 1 5 mm year 1 than the eastern part about 0 8 mm year 1 over the time span from 91 to 85 ma the denudation rates agree well with those in active orogenic belts like the alps and himalayas

reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 2 application to the pinatubo 1991 eruption the granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic plinian phase of the 1991 eruption at pinatubo the tephra fall deposits are composed of two units layer c1 which corresponds to the first half of the climactic phase and layer c2 the second half the granulometric estimates of the expansion rate of the umbrella cloud for layers c1 and c2 are 7 1010 and 3 1010 m3 s respectively which agree with the observations of satellite images these estimates indicate that the magma discharge rate decreased from 9 108 to 3 108 kg s during the eruption the grain size distribution at the top of the eruption column is characterized by depletion of coarse clasts suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second particularly during the second half of the climactic phase the total amount of layers c1 and c2 is estimated to be 3 1012 kg and the very fine particles which did not deposit in the accessible on land area may occupy up to 60 of the total ejecta the effective duration of the eruption estimated by the present methods a few hours or less is substantially shorter than a previous estimate from the real time observation based on infrasonic data 10 hours this discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous plinian activity and generation of pyroclastic flows it is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time averaged features predicted by the present methods

numerical simulations of volcanic jets importance of vent overpressure journal of geophysical research vol 113 b02204 doi 10 1029 2007jb005133 2008 numerical simulations of volcanic jets importance of vent overpressure darcy e ogden 1 kenneth h wohletz 2 gary a glatzmaier 1 and emily e brodsky 1 received 24 april 2007 revised 5 october 2007 accepted 5 november 2007 published 29 february 2008 1 explosive volcanic eruption columns are generally subdivided into a gas thrust region and a convection dominated plume where vents have greater than atmospheric pressure the gas thrust region is overpressured and develops a jet like structure of standing shock waves using a pseudogas approximation for a mixture of tephra and gas we numerically simulate the effects of shock waves on the gas thrust region these simulations are of free jet decompression of a steady state high pressure vent in the absence of gravity or a crater our results show that the strength and position of standing shock waves are strongly dependent on the vent pressure and vent radius these factors control the gas thrust region s dimensions and the character of vertical heat flux into the convective plume with increased overpressure the gas thrust region becomes wider and develops an outer sheath in which the erupted mixture moves at higher speeds than it does near the column center the radius of this sheath is linearly dependent on the vent radius and the square root of the overpressure the sheath structure results in an annular vertical heat flux profile at the base of the convective plume which is in stark contrast to the generally applied gaussian or top hat profile we show that the magnitude of expansion is larger than that predicted from previous 1d analyses resulting in much slower average vertical velocities after expansion these new relationships between vent pressure and plume expansion may be used with observations of plume diameter to constrain the pressure at the vent citation ogden d e k h wohletz g a glatzmaier and e e brodsky 2008 numerical simulations of volcanic jets importance of vent overpressure j geophys res 113 b02204 doi 10 1029 2007jb005133 1 introduction 2 in large explosive volcanic eruptions the eruptive fluid issues from the vent as a high speed compressible gas with entrained solid particulates it is important to quantify the behavior of this gas thrust region because it provides a connection between the fluid dynamics in the conduit and that of the buoyant column if the eruptive fluid velocity is at or greater than sonic and vent pressure is higher than atmospheric pressure the dynamics will be complicated by the presence of standing shock waves that can drastically alter the distribution of the vertical heat flux necessary for eruption column stability the fluid dynamics and structure of a compressible jet issuing from a sonic nozzle into an ambient atmosphere of lower pressure are well known from experimental analytical and computational studies e g crist et al 1966 young 1975 norman et al 1982 figure 1 although application of compressible jet dynamics to explosive volcanic eruptions was first sug gested over 25 years ago by kieffer 1981 the concept has earth planetary sciences department university of california at santa cruz santa cruz california usa los alamos national laboratory los alamos new mexico usa copyright 2008 by the american geophysical union 0148 0227 08 2007jb005133 09 00 yet to be widely applied in modeling and analysis of explosive eruption columns 3 in this paper we present computational results that quantify the important effects of vent pressure on the fluid dynamics of volcanic jets and show that overpressured jets produce vertical heat flux profiles that are drastically different than those of pressure balanced jets note to avoid confusion here we use the physics convention and consider heat flux the thermal energy transfer per area per unit time j m 2 s 1 and heat flow the thermal energy transfer integrated over an entire area per time j s 1 in volcanology literature the term heat flux is often used to mean either of these things e g woods 1988 mastin 2007 the simulations shown here are time dependent though they assume a steady vent condition through these simulations we quantify the effects of vent pressure and radius on plume radius and heat flux distribution after expansion of the jet this may allow the prediction of major features of the eruptive structure we do not consider the effects of variations in conduit dynamics buoyancy or the presence of a crater in order to focus only on the effects of vent pressure and radius alone this study is not a complete picture of the complicated flow dynamics of a volcanic eruption rather the results presented here could be consid ered the simplest case to which one could compare the dynamics resulting from more complicated simulations and observations of high pressure volcanic jets b02204 1 of 18

a three dimensional numerical simulation of spreading umbrella clouds 1 during explosive volcanic eruptions an eruption column buoyantly rises as a turbulent plume and an umbrella cloud spreads laterally as a gravity current at the neutral buoyancy level the source conditions of explosive eruptions such as mass discharge rates of magma at vents have been estimated from the field observations e g satellite images on the height of the eruption columns and the spreading rate of umbrella clouds on the basis of the one dimensional 1 d plume model and the gravity current model however these simplified models contain empirical constants entrainment coefficient of turbulent plume k and froude number of gravity current that should be justified we developed a time dependent three dimensional 3 d numerical model of eruption clouds to independently determine the values of these constants the 3 d model is designed to calculate quantitative features of turbulent mixing between an eruption cloud and the ambient air without any a priori empirical constants by applying a sufficiently fine grid size with a third order accuracy scheme it has reproduced the fundamental features of eruption clouds including eruption columns pyroclastic flows coignimbrite ash clouds and umbrella clouds the altitudes of the spreading umbrella clouds in the 3 d simulations are consistent with those of the neutral buoyancy level of eruption columns estimated by the 1 d plume model with the entrainment coefficient k 0 1 the relationship between the volumetric flow rate and the spreading rate of the umbrella cloud in the 3 d simulations is explained by the axisymmetrical gravity current model with 0 2 for eruption clouds in the tropical atmosphere and 0 1 for those in the midlatitude atmosphere on the other hand the total column height in the 3 d simulations strongly oscillates even for a constant mass discharge rate and its time average tends to be substantially greater than the column height estimated by the 1 d plume model with k 0 1 particularly for large scale eruption clouds in the midlatitude atmosphere we recommend a method to estimate the mass discharge rate from the observation of the column height or the spreading rate of umbrella cloud the method and the accuracy of the 3 d simulations are tested on the basis of the field data e g the total height of the eruption column the altitude and the spreading rate of the umbrella cloud and the mass discharge rate of the pinatubo 1991 eruption

a numerical study of turbulent mixing in eruption clouds using a three dimensional fluid dynamics model 1 the dynamics of eruption clouds in explosive volcanic eruptions are governed by entrainment of ambient air into eruption clouds by turbulent mixing we develop a new numerical pseudo gas model of an eruption cloud by employing three dimensional coordinates a third order accuracy scheme and a fine grid size in order to investigate the behavior of entrainment due to turbulent mixing the quantitative features of entrainment are measured by a proportionality constant relating the inflow velocity at the edge of the flow to the average vertical velocity i e the entrainment coefficient our model has successfully reproduced the quantitative features of entrainment observed in the laboratory experiments as well as fundamental features of the dynamics of eruption clouds such as the generation of eruption columns and or pyroclastic flows the value of the entrainment coefficient for eruption clouds is estimated from the column height and critical condition for column collapse by comparing results of our model with those of previous one dimensional models it is suggested that the value of the entrainment coefficient for an eruption cloud is approximately constant although the value estimated from the critical condition for column collapse k 0 07 is slightly smaller than that based on the column height k 0 1 this difference reflects the vertical change of flow structure in the eruption cloud the eruption cloud in the upper region exhibits a meandering instability which leads to efficient mixing whereas the cloud near the vent maintains a concentric structure with an inner dense core surrounded by an outer shear region our model is consistent with previous one dimensional models for steady eruption clouds supported by the laboratory experiments and it is also applicable to unsteady and transient features of actual eruption clouds

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

inter comparison of three dimensional models of volcanic plumes abstract we performed an inter comparison study of three dimensional models of volcanic plumes a set of common volcanological input parameters and meteorological conditions were provided for two kinds of eruptions representing a weak and a strong eruption column from the different models we compared the maximum plume height neutral buoyancy level where plume density equals that of the atmosphere and level of maximum radial spreading of the umbrella cloud we also compared the vertical profiles of eruption column properties integrated across cross sections of the plume integral variables although the models use different numerical procedures and treatments of subgrid turbulence and particle dynamics the inter comparison shows qualitatively consistent results in the weak plume case mass eruption rate 1 5 106 kg s 1 the vertical profiles of plume properties e g vertical velocity temperature are similar among models especially in the buoyant plume region variability among the simulated maximum heights is 20 whereas neutral buoyancy level and level of maximum radial spreading vary by 10 time averaging of the three dimensional 3d flow fields indicates an effective entrainment coefficient around 0 1 in the buoyant plume region with much lower values in the jet region which is consistent with findings of small scale laboratory experiments on the other hand the strong plume case mass eruption rate 1 5 109 kg s 1 shows greater variability in the vertical plume profiles predicted by the different models our analysis suggests that the unstable flow dynamics in the strong plume enhances differences in the formulation and numerical solution of the models this is especially evident in the overshooting top of the plume which extends a significant portion 1 8 of the maximum plume height nonetheless overall variability in the spreading level and neutral buoyancy level is 20 whereas that of maximum height is 10 this inter comparison study has highlighted the different capabilities of 3d volcanic plume models and identified key features of weak and strong plumes including the roles of jet stability entrainment efficiency and particle non equilibrium which deserve future investigation in field laboratory and numerical studies

numerical determination of the efficiency of entrainment in volcanic eruption columns 1 entrainment of air by turbulent mixing plays a central role in the dynamics of eruption clouds the amount of entrained air controls eruption styles and heights of eruption columns the efficiency of entrainment is quantified by the entrainment coefficient the ratio between mean inward radial velocity at the edge and the mean vertical velocity we directly determined the entrainment coefficient of eruption columns as a function of height on the basis of three dimensional numerical simulations the value of entrainment coefficient is similar to that for pure jets 0 07 just above the vent and approaches that for pure plumes 0 10 0 15 far from the vent between these two regions we identify a new transitional zone with a significantly small entrainment coefficient 0 05 this spatial variation in the entrainment coefficient is correlated with the change in vortical structure accompanying the addition of off source buoyancy in the transitional zone

the run out distance of large scale pyroclastic density currents a two layer depth averaged model abstract we have developed a new two layer pyroclastic density current pdc model that considers the effects of the entrainment and thermal expansion of ambient air in the upper dilute layer in order to investigate the dominant factors controlling the run out distance of large scale pdcs numerical simulations show a dilute current spreading radially from the collapsing eruption column edge and forming a thin dense basal current through particle settling i e a two layer pdc forms forward propagation of the two layer pdc stops owing to lift off of the dilute current and or deposition of the dense current and eventually the two layer pdc converges to a steady state our parametric study classifies the flow patterns of steady state two layer pdcs into one of two flow regimes according to the relative magnitude of the run out distances of the dilute and dense currents this relative magnitude critically depends on the ratio of the deposition speed at the base of the dense current d to the speed of particles settling from the dilute current to the dense current ws the run out distance of the whole two layer pdc is determined by that of the dilute current when d ws is large 4 10 3 otherwise when d ws is small 4 10 3 the run out distance of the dense basal current exceeds that of the dilute current the former increases as d ws decreases the run out distance of the dilute current is too short to explain some of the long run out distances of pdcs observed in the field e g the 1991 pinatubo and 2014 kelud eruptions it is suggested that the dense current traveling beyond the lift off point of the parent dilute current plays a significant role in the emplacement of pdcs with such long run out distances

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

chemical th u total pb dating by electron microprobe analysis of monazite xenotime and zircon from the archean napier complex east antarctica evidence for ultra high temperature metamorphism at 2400 ma abstract the chemical th u total pb isochron method chime of dating was carried out on monazite xenotime zircon and polycrase in six samples of ultra high temperature uht granulites and one of pegmatite from five localities in the napier complex enderby land east antarctica despite the chronological heterogeneity in many grains the chime ages overall show strong evidence for a major event near 2420 ma analyzed monazite and xenotime grains except monazite from mt riiser larsen have a narrow range of chime ages 2412 42 ma from beaver island and 2429 36 2421 18 and 2431 22 ma from reference peak these ages are indistinguishable from chime ages on zircon 2436 17 and 2420 20 ma and on central older domains of monazite 2404 54 ma from mt riiser larsen and on a zircon rim 2430 15 ma from mt cronus we conclude that the ca 2420 ma ages relate to the uht metamorphism but we do not exclude the possibility that lead loss has lowered the ages by some 50 ma this interpretation is consistent with a preponderance of ages obtained by other methods on the napier complex particularly the ca 2475 25 ma age for a generation of pegmatites coeval with the uht event and containing the uht mineral assemblage beryllian sapphirine khmaralite quartz events prior to 2500 ma and after 2420 ma have also left their imprint on the chime ages for example a 3646 ma chime age suggests that a zircon core is inherited whereas ca 2200 and 2000 ma chime ages in zoned monazite could be due to further lead loss during post uht activity the much younger chime age of 1094 67 ma for monazite from a pegmatite at zircon point casey bay confirms 1000 1100 ma lead loss ages reported for this region and could result from fluid activity in an outlier of rayner activity near the boundary between the rayner and napier complexes approximately 2500 ma ages have been reported from granulite facies but not uht rocks in the madras and nilgiri blocks in south india these blocks and the napier complex could have constituted a single structural unit by late archean time