the 7be 210pbxs ratio as an indicator of suspended sediment age or fraction new sediment in suspension reported here is a technique to use the 7be 210pbxs ratio as a measure of suspended sediment age or as an indicator of the fraction of the suspended sediment that is recently eroded from the landscape although both 7be and 210pbxs are delivered seasonally and stochastically to the landscape by precipitation the ratio of the two radionuclides varies substantially less the 7be 210pbxs ratios measured in three different catchments decrease in the following manner precipitation c 16 suspended sediments in rivers 6 7 suspended sediments in estuaries 4 6 sediment collected in sediment traps in the estuary c 1 surface sediment of the estuary c 0 5 decreases in the 7be 210pbxs ratio in suspended sediments can be interpreted to be the result of increased age of the sediment since 7be decays faster than 210pb alternatively a decrease in the 7be 210pbxs ratio in suspended sediments can be interpreted to be the result of dilution of newly tagged 7be rich sediment by 7be dead sediment for example by erosion of soil below the 7be enriched surface layer or by resuspension of 7be dead bottom sediment presented here is a model which uses the 7be 210pbxs ratio in suspended sediments to determine the time since the particles were tagged by precipitation derived radionuclides i e the age of the suspended sediment in addition an alternative model is presented to determine the fraction of the sediment that is newly tagged these two models are applied to three catchments old woman creek ohio weeks bay alabama and south slough oregon and yield similar findings at all three sites sediment ages increase from 0 in newly tagged material to 50 80 days in rivers to about 80 100 days in the estuaries to about 200 days in the sediment traps to about 300 days on surface bottom sediments alternatively the percentage new sediment decreases from 100 per cent in newly tagged material to about 35 50 per cent in rivers to 25 35 per cent in the estuary to less than 10 per cent in the sediment traps to 1 4 per cent on the surface of the bottom sediments

ar ar and i xe ages and thermal histories of three unusual metal rich meteorites abstract portales valley sombrerete and northwest africa nwa 176 are three unrelated meteorites which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies measured 39 ar 40 ar ages of these meteorites are 4477 11 ma and 4458 16 ma two samples of portales valley 4541 12 ma and 4524 13 ma respectively ma million years all one sigma errors the ar ar data for portales valley show no evidence of later open system behavior suggested by some other chronometers measured 129 i 129 xe ages of these three meteorites are 4559 9 0 5 ma 4561 9 1 0 ma and 4544 ma respectively relative to shallowater 4562 3 0 4 ma from stepwise temperature release data we determined the diffusion characteristics for ar and xe in our samples and calculated approximate closure temperatures for the k ar and i xe chronometers adopting results and interpretations about these meteorites from some previous workers we evaluated all these data against various thermal cooling models we conclude that portales valley formed 4560 ma ago cooled quickly to below the i xe closure temperature then cooled deep within the parent body at a rate of 4 c ma through k ar closure we conclude that sombrerete formed 4562 ma ago and cooled relatively quickly nwa 176 likely formed and cooled quickly 4544 ma ago or later than formation times of most meteorite parent bodies for all three meteorites the ar ar ages are in better agreement with i xe ages and preferred thermal models if we increase these ar ar ages by 20 ma such age corrections would be consistent with probable errors in 40 k decay parameters in current use as suggested by others the role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system

sm nd chronology and petrogenesis of mesosiderites we have obtained sm nd data from four mesosiderite silicate clasts including three clasts with a variety of textures from the vaca muerta type 1a mesosiderite and one gabbroic clast from the mt padbury mesosiderite the gabbroic vaca muerta pebble 12 and basaltic pebble 16 yield identical 147 sm 143 nd ages of 4 48 0 19 ae and 4 48 0 09 ae respectively while the highly recrystallized pebble 5 gives an age of 4 42 0 02 ae mt padbury yields an age of 4 52 0 04 ae all clasts show a correlation of 142 nd 144 nd with 147 sm 144 nd and provide unequivocal evidence for the presence of live 146 sm at the time of their formation calculated initial 146 sm 144 sm values range from 0 004 pebble 5 to 0 006 pebbles 12 16 and mt padbury and are generally consistent with the 147 sm 143 nd ages however discordance of whole rock leach and residue data and some disagreement between 146 sm 142 nd relative ages and 147 sm 143 nd absolute ages indicate small but significant disturbances to the sm nd systematics the ranges of ages and initial 146 sm 144 sm and 143 nd 144 nd values suggest that each of these silicate clasts underwent a separate protracted evolution on its parent body prior to mixing with metal textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite pebble 12 is extremely lree depleted probably a result of several melt extraction events whereas pebbles 5 and 16 and mt padbury have nearly chondritic sm nd with bulk ree concentrations higher than chondrites by factors of 5 to 15 in general the sm nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation preservation of diverse old ages and the presence of 146 sm imply that metal silicate mixing did not seriously alter the sm nd isotopic memories of these clasts we present a model for metal silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing we show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature depth of burial grain size and diffusion parameters application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing then the clasts must have been buried no deeper than 1 10 m in regolith during the initial high temperature cooling phase in order to prevent the sm nd systems from being extensively reset in order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal silicate mixing and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at 600 700 c the data from this study point to the following history for the mesosiderite parent body 1 differentiation of a silicate parent body within the first 50 m y of solar system history to form diverse parent magmas 2 emplacement of primitive and differentiated mafic magmas near the planetary surface and extensive differentiation in the near surface environment 3 formation and reworking of regolith breccias through impact gardening at the near surface of the body 4 mixing of molten fe ni metal with the regolith followed by rapid cooling 100 150 m y after the origin of the solar system 5 slow cooling from temperatures of 700 c to produce the observed nickel diffusion profiles in the iron phase 6 mild impact metamorphism and brecciation to obtain the much younger k ar ages and 7 recent collisions or perturbations sending mesosiderite fragments into earth crossing trajectories

determining the times and distances of particle transit in a mountain stream using fallout radionuclides abstract targeting of erosion and pollution control programs is much more effective if the time for fine particles to be transported through a watershed the travel distance the proportions of old and new sediment in suspension and the rate of erosion of the landscape can be estimated in this paper we present a novel technique for tracing suspended sediment in a mountain stream using fallout radionuclides sorbed to sediment atmospherically delivered 7 be 210 pb and 137 cs accumulate in the snowpack are released with its melting and sorb to fine particulates a portion of which are carried downslope into stream channels the half life of cosmogenic 7 be is short 53 4 days thus sediment residing on the stream bed should contain little of the radionuclide the different signatures of newly delivered sediment from the landscape with its 7 be tag and older untagged sediment from the channel is the basis for the tracing the total flux of such radionuclides compared to the inventory in the soil permits estimates of the rates of erosion of the landscape fine suspended particulates in the gold fork river id are transported downstream through the drainage in one or more steps having lengths of tens of kilometers length of the step decreases from about 60 km near the peak of the hydrograph to about 12 km near baseflow the percent of sediment in suspension that is new i e recently delivered from the landscape ranges from 96 to 12 the remaining sediment is resuspended older channel sediment residence times for particulates range from 1 6 days early in the hydrograph at the upper site to 103 days late in the hydrograph at the lowest elevation location rates of erosion of fine sediment calculated from the flux of radionuclides average 0 0023 cm year the long distance transport of fine particles suggests that delivery through the gold fork drainage to the basin outlet is fairly rapid once particles reach the channel and perhaps is also rapid in similar and smaller basins

beryllium 7 in soils and vegetation along an arid precipitation gradient in owens valley california 1 beryllium 7 is a potentially powerful tracer of atmospheric deposition and recent sediment transport but the quantity and distribution of 7be on arid landscapes have not been described we measured 7be in soil vegetation and dust in owens valley california and describe its distribution in aridisols and mollisols to evaluate its potential as a sediment tracer in desert environments beryllium 7 in vegetation and the upper few cm of soil is low but detectable 20 becquerels bq m 2 surface inventories of 7be at sites on the valley floor vary by a factor of five between the end of the rainy season april and the end of the dry season november in mollisols live grasses hold 50 bq 7be m 2 which is on the order of half of the total springtime surface inventory we find that within site variability at the 5 m scale is 5 to 22 1 relative standard error and can be explained by localized rain shadowing and erosion but between site variability at the km scale can be explained by differences in rainfall our alpine site has more than triple the inventory that is predicted from the rainfall 7be flux relationship that we generate using our springtime soil measurements and previously reported deposition data dust deposition does not appear to contribute significantly to 7be inventories but anomalously high 7be on the eastern flank of the sierras may be explained by a higher altitude air mass source and better scavenging efficiency of snow

formation of mesosiderites by low velocity impacts as a natural consequence of planet formation mesosiderites are an enigmatic class of stony iron meteorites composed of roughly equal amounts of pyroxenitic and basaltic silicates and metallic iron nickel olivine is rare we suggest here that during the period of planet formation mesosiderites originated by the low velocity collisions of large metallic core fragments with the surface of a differentiated asteroid size body relative velocities of the asteroids native to this region were low 1 km s 1 because it was distant from massive protoplanets however a few differentiated asteroids were destroyed by high velocity collisions with interlopers perturbed by protoplanets into highly eccentric orbits these collisions reduced mantles and crusts to small silicate fragments but left cores in the form of large durable metal fragments mesosiderite like pyroxenite basalt metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid the metal contents of other regolithic regions remained low most of the olivine in mesosiderite and howardite breccias we attribute to the mantles of the disrupted parent bodies the low 12 20 mg g 1 olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths

mesosiderite formation on asteroid 4 vesta by a hit and run collision collision and disruption processes of protoplanetary bodies in the early solar system are key to understanding the genesis of diverse types of main belt asteroids mesosiderites are stony iron meteorites that formed by the mixing of howardite eucrite diogenite like crust and molten core materials and provide unique insights into the catastrophic break up of differentiated asteroids however the enigmatic formation process and the poorly constrained timing of metal silicate mixing complicate the assignment to potential parent bodies here we report the high precision uranium lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry to reveal an initial crust formation 4 558 5 2 1 million years ago and metal silicate mixing at 4 525 39 0 85 million years ago the two distinct ages coincide with the timing of the crust formation and a large scale reheating event on the eucrite parent body probably the asteroid vesta this chronological coincidence corroborates that vesta is the parent body of mesosiderite silicates mesosiderite formation on vesta can be explained by a hit and run collision 4 525 4 million years ago that caused the thick crust observed by nasa s dawn mission and explains the missing olivine in mesosiderites howardite eucrite diogenite meteorites and vestoids mesosiderite meteorites may originate from a hit and run impact on the parent asteroid of eucrite meteorites probably vesta as mesosiderite zircon u pb ages are found to coincide with those for eucrites

beryllium 7 wet deposition variation with storm height synoptic classification and tree canopy state in the mid atlantic usa short lived fallout isotopes such as beryllium 7 7be are increasingly used as erosion and sediment tracers in watersheds 7be is produced in the atmosphere and delivered to the earth s surface primarily in precipitation however relatively little has been published about the variation in 7be wet deposition caused by storm type and vegetation cover our analysis of precipitation throughfall and sediments in two forested headwater catchments in the mid atlantic usa indicates significant variation in isotope deposition with storm type and storm height individual summer convective thunderstorms were associated with 7be activity concentrations up to 5 0 bq l 1 in precipitation and 4 7 bq l 1 in throughfall while single event wet depositional fluxes reached 168 bq m 2 in precipitation and 103 bq m 2 in throughfall storms originating from the continental usa were associated with lower 7be activity concentrations and single event wet depositional fluxes for precipitation 0 7 1 2 bq l 1 and 15 8 65 0 bq m 2 and throughfall 0 1 0 3 bq l 1 and 13 5 98 9 bq m 2 tropical systems had relatively low activity concentrations 0 2 0 5 bq l 1 in precipitation and 0 2 1 0 bq l 1 in throughfall but relatively high single event depositional fluxes due to large rainfall volumes 32 8 67 6 bq m 2 in precipitation and 25 7 134 bq m 2 in throughfall the largest sources of 7be depositional variation were attributed to storm characteristics including precipitation amount and maximum storm height 7be activity associated with fluvial suspended sediments also exhibited the highest concentration and variability in summer 175 1450 bq kg 1 we conclude the dominant source of variation on event level 7be deposition is storm type our results illustrate the complex relationships between 7be deposition in precipitation and throughfall and demonstrate event scale relationships between the 7be in precipitation and on suspended sediment copyright 2015 john wiley sons ltd

variations in the magnetic properties of meteoritic cloudy zone iron and stony iron meteorites form the widmanstatten pattern during slow cooling this pattern is comprised of several microstructures whose length scale composition and magnetic properties are dependent upon cooling rate here we focus on the cloudy zone a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties we present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim x ray photoemission electron microscopy x peem is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites slow cooling rates the iab iron meteorites and the pallasites intermediate cooling rates and the iva iron meteorites fast cooling rates the proportions of magnetic phases within the cloudy zone are also characterised using mossbauer spectroscopy we present the first observations of the magnetic state of the cloudy zone in the mesosiderites showing that for such slow cooling rates tetrataenite islands grow larger than the multi domain threshold creating large scale regions of uniformmagnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis for the most rapidly cooled iva meteorites the time available for fe ni ordering is insufficient to allow tetrataenite formation again leading to behaviour that is unsuitable for paleomagnetic analysis the most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates 2 500c myr which produces islands that are just right both in size and degree of fe ni order

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

production of hybrid granitic magma at the advancing front of basaltic underplating inferences from the sesia magmatic system south western alps italy abstract the permian sesia magmatic system of the southwestern alps displays the plumbing system beneath a permian caldera including a deep crustal gabbroic complex upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts whereas granites define a distinct more radiogenic cluster sri 0 708 and 0 710 respectively afc computations starting from the best mafic candidate for a starting melt show that nd and sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of 30 of partially depleted crust and 15 30 gabbro fractionation trace elements of the deep crustal gabbro cumulates require a further 60 fractionation of the andesitic basalt and loss of 40 of silica rich residual melt the composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the mafic complex during the growth of the mafic complex progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping at each cycle of mafic intrusion and incorporation residual and anatectic melts are produced in roughly constant proportions because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the permo carboniferous province of europe and elsewhere

energy constrained open system magmatic processes iv geochemical thermal and mass consequences of energy constrained recharge assimilation and fractional crystallization ec rafc 1 a wealth of geochemical and petrological data provide evidence that the processes of fractional crystallization assimilation and magma recharge replenishment dominate the chemical signatures of many terrestrial igneous rocks previous work spera and bohrson 2001 bohrson and spera 2001 has established the importance of integrating energy species and mass conservation into simulations of complex magma chamber processes an extended version of the energy constrained formulation energy constrained recharge assimilation fractional crystallization ec rafc tracks mass and compositional variations of melt cumulates and enclaves in a magma body undergoing simultaneous recharge assimilation and fractional crystallization spera and bohrson 2002 because many ec rafc results are distinct from those predicted by extant rafc formulations the primary goal of this paper is to present a range of geochemical and mass relationships for selected cases that highlight issues relevant to modern petrology among the plethora of petrologic problems that have important well documented analogues in nature are the geochemical distinctions that arise when a magma body undergoes continuous versus episodic recharge the connection between erupted magmas and associated cumulate bodies the behavior of recharge fractionation dominated systems rfc thermodynamic conditions that promote the formation of enclaves versus cumulates and the conditions under which magma bodies may be described as chemically homogeneous investigation of the effects of continuous versus episodic recharge for mafic magma undergoing rafc in the lower crust indicates that the resulting geochemical trends for melt and solids are sensitive to the intensity and composition of recharge suggesting that ec rafc may be used as a tool to distinguish the nature of the recharge events compared to the record preserved in melts the geochemical and mass characteristics of solids associated with particular rafc events may record a more complete view of the physiochemical history of an open system magma body the capability of ec rafc to track melts and solids creates a genetic link that can be compared to natural analogues such as layered mafic intrusions and flood basalts or mafic enclaves and their intermediate composition volcanic or plutonic hosts the ability to quantify chemical and volume characteristics of solids and melts also underscores the need for integrated field petrologic and geochemical studies of igneous systems while it appears that a number of volcanic events or systems may be characterized by continuous influx or eruption of magma steady state systems reports describing compositional homogeneity for products that represent eruptions of more than one event are relatively rare in support of this ec rafc results indicate that very specific combinations of recharge conditions bulk distribution coefficients and element concentrations are required to achieve geochemical homogeneity during cooling of a magma body undergoing rafc in summary critical points are that ec rafc provides a method to quantitatively investigate complex magmatic systems in a thermodynamic context it predicts complex nonmonotonic geochemical trends for which there are natural analogues that have been difficult to model and finally ec rafc establishes the link between the chemical and physical attributes of a magmatic system application of ec rafc promises to improve our understanding of specific tectonomagmatic systems as well as enhance our grasp of the essential physiochemical principles that govern magma body evolution

petrogenesis of mesosiderites i origin of mafic lithologies and comparison with basaltic achondrites abstract new petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites combined with existing petrologic and trace element data have served as a basis for interpretation of the petrogenesis of mesosiderite clasts compared with the basaltic achondrites the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrillite and more commonly contain augite as a late magmatic phase their pyroxenes tend to be more mgo rich and have lower fe mn ratios which are positively correlated with the fe mg ratio some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase the basaltic clasts commonly show super chondritic eu sm ratios and slight lree depletions the cumulate gabbro clasts are extremely depleted in lil have very high eu sm ratios and high lu sm ratios all of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt cumulate gabbro metal source region near the parent body surface reducing agents p c s from the metallic fraction reduce feo out of the silicate melt and result in the positive correlation of fe mn with fe mg observed in pyroxene analyses and the modal abundances of tridymite merrillite and augite xenocrystic plagioclase is a natural consequence of remelting a cumulate gabbro rich source as are the unique ree patterns observed

metamorphism isotopic ages and composition of lower crustal granulite xenoliths from the cretaceous salta rift argentina crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the cretaceous salta rift at quebrada de las conchas provincia salta argentina were investigated to get information about the age and the chemical composition of the lower crust most of the xenoliths have a granitoid composition with quartz plagioclase garnet rutile k feldspar as major minerals the exceedingly rare mafic xenoliths consist of plagioclase clinopyroxene garnet hornblende all xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca 900 c and pressures of ca 10 kbar the sm nd mineral isochron ages are 95 1 10 4 ma 91 5 13 0 ma 89 0 4 2 ma granitoid xenoliths and 110 7 23 6 ma mafic xenolith these ages are in agreement with the age of basanitic volcanism ca 130 100 and 80 75 ma and are interpreted as minimum ages of metamorphism lower crustal temperature at the time given by the isochrons was above the closure temperature of the sm nd system 600 700 c the sm nd and rb sr isotopic signatures 147sm 144nd 0 1225 0 1608 143nd 144ndt0 0 512000 0 512324 87rb 86sr 0 099 0 172 87sr 86srt0 0 708188 0 7143161 and common lead isotopic signatures 206pb 204pb 18 43 18 48 207pb 204pb 15 62 15 70 208pb 204pb 38 22 38 97 of the granitoid xenoliths are indistinguishable from the isotopic composition of the early paleozoic metamorphic basement from nw argentina apart from the lower 208pb 204pb ratio of the basement the sm nd depleted mantle model ages of ca 1 8 ga from granitoid xenoliths and early paleozoic basement point to a similar proterozoic protolith time constraints the well equilibrated granulite fabric p t conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca 90 ma and to a prominent thermal anomaly in the lithosphere the composition of the xenoliths is similar to the composition of the early paleozoic basement in the andes of nw argentina and northern chile a thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths

afc3d a 3d graphical tool to model assimilation and fractional crystallization with and without recharge in the r environment abstract afc3d is an original graphical free software developed in the framework of the r scientific environment and dedicated to the modelling of assimilation and fractional crystallization without afc and with afc r recharge facilitating the search for the solutions of the equations originally proposed by depaolo 1981 1985 and first solved in a graphical way by aitcheson and forrest 1994 the software presented here allows a graphical 3d representation of mass of assimilated crust mass of original magma as a function of r rate of crustal assimilation rate of fractional crystallization and recharge rate of magma replenishment rate of assimilation for each element isotope finding a coherent set of r parameter triples in a mostly automated way mathematically optimized solutions are derived which can and should then be discussed and evaluated from a geological and petrological point of view by the end user the presented contribution presents the software and a series of models published in the literature which are discussed as case studies of application and whose solutions are sometimes enhanced based on the results provided by the software

chronology and petrology of silicates from iie iron meteorites evidence of a complex parent body evolution iie iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of h chondrites however these silicates show a wide range of alteration ranging from netschaevo and techado the inclusions of which are little altered to highly differentiated silicates like those in kodaikanal weekeroo station and colomera which have lost metal and sulfur and are enriched in feldspar we find these inclusions to show varying degrees of shock alteration we made 39ar 40ar age determinations of watson techado miles colomera and sombrerete watson has an ar ar age of 3 677 0 007 gyr similar to previously reported ages for kodaikanal and netschaevo we suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event the space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other iies 39ar 40ar ages inferred for the other four meteorites analyzed are considerably older than watson and are techado 4 49 0 01 gyr miles 4 405 0 012 gyr colomera 4 470 0 010 gyr and sombrerete 4 541 0 0012 gyr these ages are in fair agreement with previously reported rb sr isochron ages for colomera and weekeroo station although several mechanisms to form iie meteorites have been suggested it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by 0 8 gyr we suggest that those iies with older isotopic ages are a product of partial melting and differentiation within the parent body followed by mixing of silicate and metal while both were relatively hot netschaevo and watson may have formed by this same process or by impact mixing 4 5 gyr ago but their isotopic ages may have been subsequently reset by shock heating kodaikanal apparently is required to have formed more recently in which case impact melting and differentiation seems the only viable process we see no compelling reasons to believe that iie silicate and metal derived from different parent bodies or that the parent body of iies was the same as that of h chondrites

ar ar ages and thermal histories of enstatite meteorites abstract compared with ordinary chondrites there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies in this study we report 39ar 40ar dating results for five el chondrites khairpur pillistfer hvittis blithfield and forrest five eh chondrites parsa saint marks indarch bethune and reckling peak 80259 three igneous textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies zaklodzie queen alexandra range 97348 and queen alexandra range 97289 and three aubrites norton county bishopville and cumberland falls several ar ar age spectra show unusual 39ar recoil effects possibly the result of some of the k residing in unusual sulfide minerals such as djerfisherite and rodderite and other age spectra show 40ar diffusion loss few additional ar ar ages for enstatite meteorites are available in the literature when all available ar ar data on enstatite meteorites are considered preferred ages of nine chondrites and one aubrite show a range of 4 50 4 54 ga whereas five other meteorites show only lower age limits over 4 35 4 46 ga ar ar ages of several enstatite chondrites are as old or older as the oldest ar ar ages of ordinary chondrites which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies or were metamorphosed to lower temperatures compared to other chondrite types many enstatite meteorites are brecciated and or shocked and some of the younger ar ar ages may record these impact events although impact heating of ordinary chondrites within the last 1 ga is relatively common for ordinary chondrites only bethune gives any significant evidence for such a young event

estimating surface subsurface sediment mixing in karst settings using 7be isotopes this study shows that the cosmogenic radionuclide beryllium 7 can be used to track sediment movement through caves the activities of beryllium 7 and cesium 137 were measured in two different karst settings at both surface and subsurface sites before and after storm runoff events at one site 7be enriched sediment was detected up to 1 5 km along a stream conduit after a moderate storm event however the activity of 137cs was too variable to show a meaningful pattern the percentages of surface sediment that was found ranged from 0 to 52 along the entire 3 km cave stream and from 33 to 52 along the upper 1 5 km at the other site as much as 96 of the sediment initially discharged at the spring during a storm event was fresh surface material that had traveled into and through the cave stream moreover during the 4 day runoff event approximately 23 of the total suspended sediment flux was estimated to originate from surface erosion with 78 being reworked sediment from within the cave the data in this study shows that cosmogenic radionuclides with multi year half lives are too long lived to track sediment origins in the caves whereas 7be with a 53 2 day half life can be used to track movement of sediment along cave streams

sediment yield of the lower tana river kenya is insensitive to dam construction sediment mobilization processes in a semi arid tropical river system dam construction in the 1960s to 1980s significantly modified sediment supply from the kenyan uplands to the lower tana river to assess the effect on suspended sediment fluxes of the tana river we monitored the sediment load at high temporal resolution for 1 year and complemented our data with historical information the relationship between sediment concentration and water discharge was complex at the onset of the wet season discharge peaks resulted in high sediment concentrations and counterclockwise hysteresis while towards the end of the wet season a sediment exhaustion effect led to low concentrations despite the high discharge the total sediment flux at garissa c 250 km downstream of the lowermost dam between june 2012 and june 2013 was 8 8 mt yr 1 comparison of current with historical fluxes indicated that dam construction had not greatly affected the annual sediment flux we suggest that autogenic processes namely river bed dynamics and bank erosion mobilized large quantities of sediments stored in the alluvial plain downstream of the dams observations supporting the importance of autogenic processes included the absence of measurable activities of the fall out radionuclides 7be and 137cs in the suspended sediment the rapid lateral migration of the river course and the seasonal changes in river cross section given the large stock of sediment in the alluvial valley of the tana river it may take centuries before the effect of damming shows up as a quantitative reduction in the sediment flux at garissa many models relate the sediment load of rivers to catchment characteristics thereby implicitly assuming that alterations in the catchment induce changes in the sediment load our research confirms that the response of an alluvial river to external disturbances such as land use or climate change is often indirect or non existent as autogenic processes overwhelm the changes in the input signal copyright 2015 john wiley sons ltd

the use of short lived radionuclides to quantify transitional bed material transport in a regulated river we investigate the use of the short lived fallout radionuclide beryllium 7 7be t1 2 53 4 days as a tracer of medium and coarse sand 0 25 2 mm which transitions between transport in suspension and as bed load and evaluate the effects of impoundment on seasonal and spatial variations in bed sedimentation we measure 7be activities in approximately monthly samples from point bar and streambed sediments in one unregulated and one regulated stream in the regulated stream our sampling spanned an array of flow and management conditions during the annual transition from flood control in the winter and early spring to run of the river operation from late spring to autumn sediment stored behind the dam during the winter quickly became depleted in 7be activity this resulted in a pulse of dead sediment released when the dam gates were opened in the spring which could be tracked as it moved downstream measured average sediment transport velocities 30 80 metres per day m d 1 exceed those typically reported for bulk bed load transport and are remarkably constant across varied flow regimes possibly due to corresponding changes in bed sand fraction results also show that the length scale of the downstream impact of dam management on sediment transport is short c 1 km beyond this distance the sediment trapped by the dam is replaced by new sediment from tributaries and other downstream sources copyright 2006 john wiley sons ltd

39ar 40ar ages and thermal history of mesosiderites determinations of 39ar 40ar ages for plagioclase separates of three clasts from the vaca muerta and mount padbury mesosiderites give similar results ages increase monotonically by 0 2 0 3 ga when plotted against increasing 39ar release from the samples and give average values of 3 95 ga when combined with revised ages for mesosiderite analyses previously reported nineteen analyses of thirteen mesosiderites give an average ar ar age of 3 94 0 10 ga the average age at 90 39ar release is 4 13 0 14 ga and the ages at 10 39ar release range over 3 32 3 97 ga it seems unlikely that the 39ar 40ar ages were produced by direct impact heating but they probably are independent evidence for slow cooling deep within the mesosiderite parent body we conclude that the sloped age plateaus and the significant range in ar ar closure temperatures calculated for these mesosiderites 75 340 c are due to significant differences in ar diffusion properties among samples from the ar diffusion data we estimate an approximate mesosiderite cooling rate of 0 2 c ma which is similar to the preferred cooling rate of 0 1 0 5 c ma derived by others from ni diffusion and metal textures the 39ar 40ar ages seem incompatible with very slow cooling rates of 0 2 c ma unless the closure temperatures for ar diffusion and formation of metal cooling textures on the parent body substantially overlap any possible role of fragmentation of the mesosiderite parent body in producing the younger ar ar ages and the very slow cooling rates is unclear

thermal and shock history of mesosiderites and their large parent asteroid to elucidate the geological evolution of the mesosiderite stony iron meteorites and their 3 4 3 8 gy 40ar 39ar ages we have investigated their shock and thermal histories we have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 gpa shock stage s1 s2 three mesosiderites contain a tiny fraction 0 1 overall of mineral fragments that were shocked to shock stage s3 s6 levels the uniformity of shock features within all fragments shows that these fragments were not shocked in situ our shock data for mesosiderites the absence of evidence of 3 6 3 9 gy impact melt the shock history of impact heated ordinary chondrites and the difficulty in quantitatively removing ar in an impact event all suggest that the mesosiderite parent body did not suffer a major impact event 3 6 3 9 gy ago metallographic cooling rates of 0 03 c my at 400 c were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and fe ni p phase diagram cooling rates of 0 01 c my at 425 325 c were estimated from published compositional data for kamacite grains in four mesosiderites these two techniques and four other semiquantitative metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite we infer that cooling rates at 400 c were 0 02 0 03 c my and certainly less than 0 5 c my the inferred cooling rate is too slow to allow ar closure before 4 gy all of the shock thermal and age data for mesosiderites are consistent with slow cooling at depth 1 my after metal and silicate were mixed around 4 4 gy ago thermal models indicate that the mesosiderites probably cooled in an asteroid some 200 400 km in radius

depth and areal extent of sheet and rill erosion based on radionuclides in soils and suspended sediment sheetwash and rilling are two important mechanisms of soil erosion by runoff the relative contribution of each mechanism has been a vexing question because measuring thin sheet erosion is difficult fortuitously various fallout radionuclides have distinct distributions in the soil column thus different depths of erosion produce suspended sediment with unique radionuclide signatures those signatures can be used to estimate the depth and areal extent of sheet and rill erosion we developed a model to execute multiple mass balances on soil and radionuclides to quantify these erosion mechanisms radionuclide activities 7be 137cs 210pb in the soil of a 6 03 ha agricultural field near treynor iowa and in suspended sediment washed off the field during thunderstorm runoff were determined by gamma spectroscopy using the model we examined 15 5 million possible combinations of the depth and areal extent of rill and sheet erosion the best solution to the mass balances corresponded to rills eroding 0 38 of the basin to a depth of 35 mm and sheetwash eroding 37 of the basin to a depth of 0 012 mm rill erosion produced 29 times more sediment than sheet erosion

suspended sediment dynamics in a southeast asian mountainous catchment combining river monitoring and fallout radionuclide tracers summary soil erosion is intense in mountainous tropical regions where heavy storms result in the supply of large quantities of sediment to rivers the origin and dynamics of suspended sediment were analysed in a catchment located in northern laos during the first erosive flood of the rainy season in may 2012 the catchment was equipped with 4 successive gauging stations draining areas ranging 0 2 11 6 km2 fallout radionuclides beryllium 7 7be unsupported pb 210 210pbxs and cesium 137 137cs were determined on rainfall overland flow stream water suspended sediment soil surface and subsurface samples with n 3 19 75 75 65 and 14 respectively assumptions underpinning the 7be labelling method were validated by implementing experiments in the laboratory i e rainwater 7be sorption to soil particles and in the field i e 7be 210pbxs activity ratio evolution in rainwater and related overland flow during a natural storm event radionuclide analyses provided a way to quantify variations in sediment dynamics and origin throughout the flood 1 a proportion of recently eroded sediment labelled by 7be and referred to as fresh sediment ranging between ca 8 35 in suspended loads 2 higher contributions of fresh sediment at the beginning of the flood rising stage 3 a progressive dilution of fresh sediment by particles remobilised from the riverbed channel 4 the dominance of particles originating from the soil surface ca 70 80 of total sediment load in upper parts and a much larger contribution of subsurface material ca 64 at the downstream station the original contribution of 7be labelled particles derived from collapsed riverbanks to sediment export was also demonstrated this pilot study supports the use of fallout 7be and 210pbxs in tropical catchments to constrain sediment dynamics it also puts forward the need to better characterize the sources of sediment in order to avoid possible misinterpretations

fallout 210pb as a soil and sediment tracer in catchment sediment budget investigations a review increasing anthropogenic pressures coupled with climate change impacts on natural resources have promoted a quest for innovative tracing techniques for understanding soil redistribution processes and assessing the environmental status of soil resources among the different existing tracers the fallout component of the radioisotope lead 210 also termed unsupported or excess lead 210 pb 210 ex when referring to its presence in soil or sediment arguably offers the broadest potential for environmental applications due to its origin and relatively long half life for more than five decades pb 210 ex has been widely used for dating sediments to investigate sedimentation processes and since the 1990s to provide information on the magnitude of soil and sediment redistribution the aim of this review is to provide a comprehensive evaluation and discussion of the various applications of pb 210 ex as a tracer in terrestrial and aquatic environments with particular emphasis on catchment sediment budget investigations this paper summarizes the state of the art related to the use of this tracer the main assumptions the requirements including the need for accurate analytical measurements and for parallel validation and the limitations which must be recognised when using this fallout radionuclide as a soil and sediment tracer lessons learned and current and future research needs in the environmental and radiochronological application of pb 210 ex are also presented and discussed c 2014 elsevier b v all rights reserved

energy constrained recharge assimilation and fractional crystallization ec raχfc a visual basic computer code for calculating trace element and isotope variations of open system magmatic systems 1 volcanic and plutonic rocks provide abundant evidence for complex processes that occur in magma storage and transport systems the fingerprint of these processes which include fractional crystallization assimilation and magma recharge is captured in petrologic and geochemical characteristics of suites of cogenetic rocks quantitatively evaluating the relative contributions of each process requires integration of mass species and energy constraints applied in a self consistent way the energy constrained model energy constrained recharge assimilation and fractional crystallization ec ra fc tracks the trace element and isotopic evolution of a magmatic system melt solids undergoing simultaneous fractional crystallization recharge and assimilation mass thermal and compositional trace element and isotope output is provided for melt in the magma body cumulates enclaves and anatectic i e country rock melt theory of the ec computational method has been presented by spera and bohrson 2001 2002 2004 and applications to natural systems have been elucidated by bohrson and spera 2001 2003 and fowler et al 2004 the purpose of this contribution is to make the final version of the ec ra fc computer code available and to provide instructions for code implementation description of input and output parameters and estimates of typical values for some input parameters a brief discussion highlights measures by which the user may evaluate the quality of the output and also provides some guidelines for implementing nonlinear productivity functions the ec ra fc computer code is written in visual basic the programming language of excel the code therefore launches in excel and is compatible with both pc and mac platforms the code is available on the authors web sites http magma geol ucsb edu and http www geology cwu edu ecrafc as well as in the auxiliary material

open system magma chamber evolution an energy constrained geochemical model incorporating the effects of concurrent eruption recharge variable assimilation and fractional crystallization ec e raχfc significant petrogenetic processes governing the geochemical evolution of magma bodies include magma recharge including formation of quenched inclusions or enclaves heating and concomitant partial melting of country rock with possible contamination of the evolving magma body assimilation and formation and separation of cumulates by f ractional crystallization rafc although the importance of modeling such open system magma chambers subject to energy conservation has been demonstrated the effects of concurrent removal of magma by eruption and or variable assimilation involving imperfect extraction of anatectic melt from wall rock have not been considered in this study we extend the ec rafc model to include the effects of eruption and variable amounts of assimilation ax this model called ec e 0racfc tracks the compositions trace elements and isotopes temperatures and masses of magma body liquid melt eruptive magma cumulates and enclaves within a composite magmatic system undergoing simultaneous eruption recharge assimilation and fractional crystallization the model is formulated as a set of 4 t i s coupled nonlinear differential equations where the number of trace elements radiogenic and stable isotope ratios modeled are t i and s respectively solution of the ece 0racfc equations provides values for the average temperature of wall rock ta mass of melt within the magma body mm masses of cumulates mct enclaves men and wall rock m o a and the masses of anatectic melt generated m a and assimilated cm a in addition t trace element concentrations and i s isotopic ratios in melt and eruptive magma cm em dm cumulates cct em dm enclaves cen er d o r and anatectic melt ca ea d o a as a function of magma temperature tm are also computed input parameters include the user defined equilibration temperature teq a factor describing the efficiency of addition of anatectic melt c from country rock to host magma the initial temperature and composition of pristine host melt tom c o m em d o m recharge melt t o r c o r er dor and wall rock t o a c o a ea d o a distribution coefficients dm dr da and their temperature dependences dhm dhr dha latent heats of transition melting or crystallization for wall rock dha pristine magma dhm and recharge magma dhr as well as the isobaric specific heat capacity of assimilant cp a pristine cp m and recharge cp r melts the magma recharge mass and eruptive magma mass functions mr tm andme tm respectively are specified a priori mr tm and me tm are modeled as either continuous or episodic step like processes melt productivity functions which prescribe the relationship between melt mass fraction and temperature are defined for end member bulk compositions characterizing the local geologic site ec e 0racfc has potential for addressing fundamental questions in igneous petrology such as what are intrusive to extrusive ratios i e for particular magmatic systems and how does this factor relate to rates of crustal growth how does i e vary temporally at single long lived magmatic centers what system characteristics are most profoundly influenced by eruption what is the quantitative relationship between recharge and assimilation in cases where the extraction efficiency can be shown to be less than unity what geologic criteria are important and

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

magmatic evolution of the skye igneous centre western scotland modelling of assimilation recharge and fractional crystallization the skye igneous centre forming part of the british tertiary magmatic province developed over a 7 myr period 61 54 ma and is characterized by a complex suite of lavas hypabyssal and intrusive rocks of picritic to granitic composition the intrusion of magma from mantle to crust at 2 10 3 km 3 yr 6 mt yr advected magmatic heat of roughly 0 2 gw averaged over the period of magmatism supporting an excess heat flux of about 130 mw m 2 or about twice the present day average continental heat flow the volume of new crust generated at skye 15 000 km 3 spread over the present day area of skye corresponds to 9 km of new crust the geochemical evolution of the skye magmatic system is constrained using the energy constrained recharge assimilation and fractional crystallization ecrafc model to understand variations in the sr and pb isotopic and sr trace element composition of the exposed magmatic rocks with time the character composition and specific enthalpy of both assimilant and recharge magma appears to change systematically up section suggesting that the magma reservoirs migrated to progressively shallower levels as the system matured the model of the magma transport system that emerges is one in which magma batches are stored initially at lower crustal levels where they undergo rafc evolution residual magma from this stage then migrates to shallower levels where mid crustal wall rock is assimilated the recharge magma at this level is characterized by an increasingly crustal signature for some of the stratigraphically youngest rocks the data suggest that the magma reservoirs ascended into and interacted with upper crustal torridonian metasediments

sediment dynamics during the rainy season in tropical highland catchments of central mexico using fallout radionuclides tropical regions are affected by intense soil erosion associated with deforestation overgrazing and cropping intensification this land degradation leads to important on site e g decrease in soil fertility and off site e g reservoir siltation and water pollution impacts this study determined the mean soil particle and sediment residence times in soils and rivers of three subcatchments 3 12 km2 with contrasted land uses i e cropland forests and rangelands draining to a reservoir located in highlands of the transvolcanic mexican belt calculations were based on rainfall amount and river discharges as well as on fallout radionuclide measurements be 7 cs 137 and pb 210 conducted on rainfall precipitated samples soil sampled in the catchments and suspended sediment collected by automatic samplers in the river during most storms recorded throughout the 2009 rainy season calculations using a radionuclide two box balance model showed that the mean residence time of particles in soils ranged between 5000 1500 and 23 300 7000 years in contrast sediment residence time in rivers was much shorter fluctuating between 50 30 and 200 70 days the shortest mean residence times were measured in a hilly catchment dominated by cropland and rangelands whereas they were the longest in an undulating catchment dominated by forests and cropland calculation of the be 7 excess pb 210 in both rainfall and sediment allowed gaining insight on sediment dynamics throughout the rainy season the first heavy storms of the year exported the bulk of the sediment stock accumulated in the river channel during the previous year then during the rainy season the two steeper catchments dominated by cropland and rangelands reacted strongly to rainfall sediment was indeed eroded and exported from both catchments during single heavy storms on several occasions in 2009 in contrast the agro forested catchment with gentler slopes exported sediment at a constant and low rate throughout the rainy season overall land cover and flood type clearly proved to exert more control on sediment export than slope steepness and rainfall erosivity our results show the priority of stabilising old gully systems to prevent their extension by regressive erosion to cropland and to concentrate the implementation of on site erosion control measures in cropland and rangeland of the most reactive catchments

petrogenesis of early neogene magmatism in the northern puna implications for magma genesis and crustal processes in the central andean plateau of uolm 2 and mm group rocks assuming the same endnew compositional data and petrogenetic models are presented for member compositions the modelling suggests genesis of the mm pre upper miocene volcanism in the northern puna of argentina magmas at higher pressure than the uolm 2 centres 10 kbar 22 s 24 s two phases of volcanism produced small dome vs 7 kbar which may reflect the influence of crustal thickening in complexes of mainly silicic andesite to low sio2 rhyolite the the plateau region by the mid miocene the felsic dome complexes upper oligocene lower miocene phase uolm 20 17 ma of this study are compositionally similar to the large volume calderaproduced two distinct groups of rocks the uolm 1 group is sourced felsic ignimbrites that dominated volcanism in the region metaluminous and mainly andesitic with isotopic compositions like from 10 to 2 ma and our results suggest that there is no fundamental those of the recent arc sr srt 0 706 ndt 3 the difference in magma genesis between them the differences in the uolm 2 group is more silicic and peraluminous and has isotopic volumes and the mode of eruption reflect changes in the stress and compositions indicating a substantial crustal contribution sr thermal regime with time srt 0 713 ndt 8 the mid miocene phase mm 15 12 ma produced rocks similar in composition to those of the uolm 2 group sr srt 0 710 ndt 7 but with higher incompatible element contents ratios of ba nb and zr

39ar 40ar dating of mesosiderites evidence for major parent body disruption 4 ga ago 39ar 40ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of ar by one or more thermal events less than 4 0 ga ago three samples gave release patterns of constant age whereas the other samples typically showed 200 400 ma increases in 39ar 40ar age with increasing gas extraction temperature these stepwise releases indicate ages of 3 4 3 8 ga 1 ga 109 a averaged ages for individual meteorites are 3 45 3 82 ga no substantial evidence exists in the temperature releases for 39ar 40ar ages older than 4 0 ga and the ar ages appear to be in contradiction to 244pu fission track ages previously reported for a few mesosiderites the ar ages probably date the event that caused disturbance in previously reported rb sr sm nd and pb pb ages of estherville and other mesosiderites data on release of ar as a function of extraction temperature were used in arrhenius plots to estimate activation energies for ar diffusion of 18 52 kcal mol these values were then used to calculate closure temperatures against ar loss of 2 340 c and 180 600 c for assumed cooling rates of 1 c ma and 1 c a respectively the ar data suggest that for certain assumed models of mesosiderite formation the 39ar 40ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of 1 c ma models which call for metal silicate mixing 4 5 ga ago followed by cooling at 1 c ma from temperatures 1150 c seem unlikely single event models which produce the silicate metal mixture at high temperatures rapidly cool the objects to temperatures of 500 c followed by cooling of 1 c ma and are not consistent with the ar data unless the thermal event occurred 500 c and left them deeply buried to undergo slow cooling

thermal history of mesosiderites quantitative constraints from compositional zoning and fe mg ordering in orthopyroxenes abstract we have derived mathematical relations to calculate cooling rates from the extent of compositional zoning developed during cooling across the interface of a natural diffusion couple these relations were used to calculate the high temperature cooling rates of the mesosiderites lowicz and clover springs from the available data on compositional zoning across core overgrowth interface of orthopyroxene grains we have also determined the cation ordering in four selected orthopyroxene crystals from bondoc and estherville mesosiderites with very high precision and used these data to calculate their low temperature cooling rates the compositional zoning of orthopyroxene crystals reflects extremely rapid cooling rates at least 1 c 100 years in the temperature range 850 1150 c however simultaneous consideration of both metallographic and cation ordering data for estherville within the framework of either an asymptotic or an exponential cooling model requires a cooling rate of l c ma near 250 c the cation ordering data for bondoc for which no metallographic data are yet available are suggestive of even slower cooling rate which implies excavation from a somewhat greater depth in the parent body however within the limits of their uncertainties the measured site occupancies of the orthopyroxene crystals from bondoc can be reconciled with a cooling rate similar to that of estherville the calculated cooling rates at both high and low temperatures have been used to develop a thermal evolution model of mesosiderites the suggested model is not incompatible with an asteroidal parent body for these meteorites further it is shown that the closure temperature of ar ar age must be tied to the slow cooling rate below 500 c

beryllium 7 and lead 210 chronometry of modern soil processes the linked radionuclide accumulation model lrc abstract soil systems are known to be repositories for atmospheric carbon and metal contaminants but the complex processes that regulate the introduction migration and fate of atmospheric elements in soils are poorly understood this gap in knowledge is attributable in part to the lack of an established chronometer that is required for quantifying rates of relevant processes here we develop and test a framework for adapting atmospheric lead 210 chronometry 210 pb half life 22 years to soil systems we propose a new empirical model the linked radionuclide accumulation model lrc aka lark that incorporates measurements of beryllium 7 7 be half life 54 days to account for 210 pb penetration of the soil surface during initial deposition a process which is endemic to soils but omitted from conventional 210 pb models e g the constant rate of supply crs model and their application to sedimentary systems we validate the lrc model using the 1963 1964 peak in bomb fallout americium 241 241 am half life of 432 years as an independent corroborating time marker in three different soils we locate a sharp 241 am weapons horizon at disparate depths ranging from 2 5 to 6 cm but with concordant ages averaging 1967 4 via the lrc model similarly at one site contaminated with mercury hg t we find that the lrc model is consistent with the recorded history of hg emission the close agreement of pb am and hg behavior demonstrated here suggests that organo metallic colloid formation and migration incorporates many trace metals in universal soil processes and that these processes may be described quantitatively using atmospheric 210 pb chronometry the 210 pb models evaluated here show that migration rates of soil colloids on the order of 1 mm yr 1 are typical but also that these rates vary systematically with depth and are attributable to horizon specific processes of leaf litter decay eluviation and illuviation we thus interpret 210 pb models to quantify i exposure of the soil system to atmospheric aerosol deposition in the context of ii organic carbon assimilation colloid production and advection through the soil profile the behavior of some other elements such as cs diverges from the conservative colloid behavior exemplified by pb and am and in these cases the value of empirical 210 pb chronometry models like lrc and crs is as a comparator rather than as an absolute chronometer we conclude that 210 pb chronometry is valuable for tracing colloidally mediated transport of pb and similarly refractory metals as well as the mobile pool of carbon in soils

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

environmental chemistry of beryllium 7 in addition to the stable isotope 9be be is also formed as two cosmogenic isotopes of interest to earth scientists cosmogenic be is formed primarily in the stratosphere from cosmic ray spallation of oxygen and nitrogen but some is produced in the troposphere and in situ on the surface of the earth production of cosmogenic be was hypothesized by peters 1955 soon after naturally occurring 7be was identified in precipitation by arnold and al salih 1955 and essentially concurrently and independently by goel et al 1956 10be t1 2 1 5 106 yr was first detected by arnold 1956 in marine sediment cores the 10be 7be production ratio is approximately 0 5 lal and peters 1967 after its formation in the atmosphere cosmogenic be adsorbs electrostatically to aerosols that may be washed out by precipitation and delivered to ecosystems the amount of cosmogenic be that reaches the surface of the earth is a function of production rate cosmic ray intensity stratosphere troposphere mixing circulation and advection within the troposphere and efficiency of removal from the troposphere wet and dry deposition feely et al 1989 since the discovery of cosmic ray produced be numerous researchers have gathered data on its 1 production 2 fluxes to the oceans and terrestrial ecosystems 3 distribution and inventory in soils snow sediments and vegetation and 4 geochemical behavior in 1955 peters suggested that the relatively long half life of 10be could make it useful for quantifying tertiary sedimentation rates and other surficial processes the focus of this chapter is the short lived radionuclide 7be t1 2 53 12 0 07 days jaeger et al 1996 due to its short half life relative ease of measurement and well defined source term 7be serves as a useful tool for tracing and quantifying environmental processes on the

seasonal controls on meteoric 7be in coarse grained river channels cosmogenic 7be is a natural tracer of short term hydrological processes but its distribution in upland fluvial environments over different temporal and spatial scales has not been well described we measured 7be in 450 sediment samples collected from perennial channels draining the middle of the connecticut river basin an environment that is predominantly well sorted sand by sampling tributaries that have natural and managed fluctuations in discharge we find that the 7be activity in thalweg sediments is not necessarily limited by the supply of new or fine grained sediment but is controlled seasonally by atmospheric flux variations and the magnitude and frequency of bed mobilizing events in late winter 7be concentrations in transitional bedload are lowest typically 1 to 3 bq kg 1 as 7be is lost from watersheds via radioactive decay in the snowpack in mid summer however 7be concentrations are at least twice as high because of increased convective storm activity which delivers high 7be fluxes directly to the fluvial system a mixed layer of sediment at least 8 cm thick is maintained for months in channels during persistent low rainfall and flow conditions indicating that stationary sediments can be recharged with 7be however bed mobilizing rain on snowmelt events in late spring can reset 7be amounts and concentrations in the channel as previously buried old sediment with low 7be is mixed into the thalweg we conclude that given proper temporal and spatial sampling 7be is a valuable tracer of seasonal timescale mass transport and exchange in coarse grained fluvial systems copyright 2013 john wiley sons ltd

k ar dating of rocks on mars requirements from martian meteorite analyses and isochron modeling radiometric age dating of martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand martian history performing k ar age dating of igneous rocks on mars by robots however presents technical challenges some of these challenges can be defined by examining ar ar data acquired on martian meteorites and others can be evaluated through numerical modeling of simulated k ar isochrons like those that would be acquired robotically on martian rocks excess 40ar is present in all shergottites thus for martian rocks the slopes of k ar isochrons must be determined to reasonable precision in order to calculate reliable ages model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age issues addressed here are how many k ar analyses should be made of rocks thought to have the same age what range of k concentrations should these analyzed samples have and what analytical uncertainty in k ar measurements is desirable meteorite data also are used to determine the d a 2 diffusion parameters for ar in plagioclase and pyroxene separates of several shergottites and nakhlites these data indicate the required temperatures and times for heating similar martian rocks in order to extract ar quantitatively extracting radiogenic 40ar could be difficult and degassing cosmogenic ar from mafic phases even more so considering all these factors robotic k ar dating of martian rocks may be achievable but will be challenging

beryllium 7 the cinderella of fallout radionuclide sediment tracers current concern for the sustainable management of soil resources and the many adverse effects of increased sediment loads on streams and river systems has generated a need for more information on rates and patterns of soil loss and an improved understanding of catchment sediment budgets traditional measurement techniques are frequently unable to meet the requirements and fallout radionuclides have been shown to offer considerable potential for tracing the mobilization transfer redistribution and storage of soil and sediment within catchments and river systems caesium 137 and lead 210 have been the most widely used fallout radionuclide sediment tracers but their half lives mean that the information that they provide is in most instances limited to the medium term e g 25 120 years the need for information on shorter timescales has directed attention to the potential for using beryllium 7 which has a considerable shorter half life for tracing soil and sediment redistribution over short timescales e g days weeks to date however there have been relatively few attempts to exploit this potential this situation is likely to be in part a reflection of the emphasis of early work on 137cs and 210pb and the more recent nature of work using 7be however it may also reflect the limitations and constraints imposed by the nature of 7be fallout and the short half life of this radionuclide this contribution explores recent work in using 7be as a sediment tracer and the constraints imposed by its origin and behaviour attention is directed to fallout inputs the application of 7be to tracing soil and sediment redistribution on slopes and on river floodplains its use as a source fingerprint and other related applications the likely future potential of 7be as a sediment tracer is assessed copyright 2012 john wiley sons ltd

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

chronology and sources of lunar impact bombardment abstract the moon has suffered intense impact bombardment ending at 3 9 gyr ago and this bombardment probably affected all of the inner solar system basin magnetization signatures and lunar crater size distributions indicate that the last episode of bombardment at about 3 85 gyr ago was less extensive than previously thought we explore the contribution of the primordial mars crosser population to early lunar bombardment we find that mars crosser population initially decays with a 80 myr half life with the long tail of survivors clustering on temporarily non mars crossing orbits between 1 8 and 2 au these survivors decay with half life of about 600 myr and are progenitors of the extant hungaria asteroid group in the same region we estimate the primordial mars crosser population contained about 0 01 0 02 earth masses such initial population is consistent with no lunar basins forming after 3 8 gya and the amount of mass in the hungaria group as they survive longer and in greater numbers than other primordial populations mars crossers are the best candidate for forming the majority of lunar craters and basins including most of the nectarian system however this remnant population cannot produce imbrium and orientale basins which formed too late and are too large to be part of a smooth bombardment we propose that the imbrian basins and craters formed in a discrete event consistent with the basin magnetization signatures and crater size distributions this late impactor shower would be triggered by a collisional disruption of a vesta sized body from this primordial mars crossing population wetherill g w 1975 proc lunar sci conf 6 1539 1561 that was still comparable to the present day asteroid belt a 3 9 gya this tidal disruption lead to a short lived spike in bombardment by non chondritic impactors with a non asteroidal size frequency distribution in agreement with available evidence this body wetherill s object also uniquely matches the constraints for the parent body of mesosiderite meteorites we propose that the present day sources of mesosiderites are multi km sized asteroids residing in the hungaria group that have been implanted there soon after the original disruption of their parent 3 9 gyr ago

genesis of magmas from the tres virgenes volcanic complex baja california sur mexico abstract the late pleistocene tres virgenes volcanic complex tvvc baja california sur mexico is formed by 3 volcanic edifices el viejo el azufre and la virgen although the tvvc is emplaced in a tectonic environment where subduction was terminated its volcanic products are calc alkaline in addition to its typical arc calc alkaline nature we classified the tvvc rocks as adakitic typical arc calc alkaline rocks are widely described in baja california so we focus on the adakitic rocks relatively high silica adakitic rocks 57 64 sio2wt sr y 30 49 la yb 4 8 from the tvvc are constrained between 110 90 ka and could be produced when typical arc calc alkaline magmas partially melt the lower crust alternatively these high silica adakitic rocks could be formed by mixing between typical arc calc alkaline magmas and low silica adakitic rocks low silica adakitic rocks 50 55 sio2wt sr y 50 65 la yb 3 7 are produced when the metasomatized mantle wedge is partially melted with the exception of la virgen tefra a plinian eruption where typical arc calc alkaline magmas are involved mineral composition and lack of xenocrysts suggest mixing and assimilation are not dominant processes at the tvvc upper crust reservoir in contrast mineral textures in some rocks suggest that magma reservoirs were heated we propose that the low silica adakitic magmas are the heating mixing elements modifying the reservoir plumbing system

the geochemistry of a dying continental arc the incapillo caldera and dome complex of the southernmost central andean volcanic zone 28 s the pleistocene incapillo caldera and dome complex 5 570 m marks the southernmost siliceous center of the andean central volcanic zone 28 s where the steeply dipping 30 segment of the subducting nazca plate transitions into the chilean flatslab to the south the eruption of the incapillo caldera and dome complex began with a 3 1 ma effusive phase characterized by 40 rhyodacitic dome eruptions this effusive phase was terminated by an explosive caldera forming event at 0 51 ma that produced the 14 km3 incapillo ignimbrite distinctive and virtually identical chemical signatures of the domes and ignimbrites sio2 67 72 wt la yb 37 56 ba la 16 28 la ta 30 50 87sr 86sr 0 70638 0 70669 e nd 4 2 to 4 6 indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor the strong hree depletion sm yb 6 8 that distinguishes incapillo magmas from most of the large ignimbrites of the altiplano puna plateau can be explained by the extent and degree of partial melting at lower crustal depths 40 km in the presence of garnet at upper crustal depths this high pressure residual geochemical signature also common to adjacent late miocene pliocene pircas negras andesites was partially overprinted by shallow level assimilation and fractional crystallization processes energy constrained afc modeling suggests that incorporation of anatectic upper crustal melts into a fractionated adakite like dacitic host best explains the petrogenesis of incapillo magmas the diminution of the sub arc asthenospheric wedge during nazca plate shallowing left the incapillo magma chamber unreplenished by both mafic mantle derived and lower crustal melts and thus stranded at shallow depths within the andean crust based on its small size and distinctive high pressure chemical signature the incapillo caldera and dome complex provides an endmember model for an andean caldera erupting within a waning magmatic arc over a shallowing subduction zone

using radiometric fingerprinting and phosphorus to elucidate sediment transport dynamics in an agricultural watershed the major goals of this study were to determine stream bed sediment erosion deposition rates sediment age percent new sediment and suspended sediment origin during two storm events of contrasting magnitudes 11 9 mm over 5 h and 58 9 mm over 39 h using fallout radionuclides excess lead 210 210pbxs and beryllium 7 7be and link the nature and type of sediment source contributions to potential phosphorus p off site transport the study was conducted in cropland dominated and mixed land use subwatersheds in the non glaciated pleasant valley watershed 50 km2 in south central wisconsin fine sediment deposition and erosion rates on stream beds varied from 0 76 to 119 29 mg cm 2 day 1 at sites near the watershed outlet and 1 72 to 7 72 mg cm 2 day 1 at sites in the headwaters respectively during the two storm events the suspended sediment age ranged from 123 12 to 234 33 days during the smaller storm event however older sediment was more prevalent p 0 037 in the streams during the larger event with suspended sediment age ranging from 226 9 to 322 114 days during the small and large storm event percent new sediment in suspended sediment ranged from 5 3 2 1 to 21 0 2 9 and 5 3 2 7 to 6 7 5 7 respectively in the cropland dominated subwatershed upland soils were the major source of suspended sediment whereas in the mixed land use subwatershed both uplands and stream banks had relatively similar contributions to suspended sediment in stream suspended and bed sediment p levels ranged from 703 193 to 963 84 mg kg 1 during the two storm events the p concentrations in suspended and bed sediment were reflective of the dominant sediment source upland or stream bank or mixed overall sediment transport dynamics showed significant variability between subwatersheds of different land use characteristics during two contrasting storm events copyright 2014 john wiley sons ltd

use of fallout 137cs in investigations of overbank sediment deposition on river floodplains abstract river floodplains have been recognised as important sinks for storing suspended sediment and associated contaminants mobilised from the upstream catchment however information on contemporary rates of overbank sedimentation is difficult to obtain using conventional methods measurements of the 137cs content of floodplain sediments provide an alternative approach to obtaining estimates of medium term rates ca 40 years of overbank sediment deposition the 137cs approach requires only a single site visit and minimum sample preparation furthermore it is capable of providing information on spatial patterns of sediment deposition on floodplains which is needed to improve our understanding of the processes involved in overbank flow and sediment deposition this paper reviews the basis for using 137cs measurements in floodplain sedimentation studies and presents improved procedures for interpreting 137cs profiles in floodplain sediments and for obtaining estimates of sedimentation rates from single measurements of the total 137cs inventories of bulk sediment cores the results obtained from a case study undertaken on the floodplain of the river stour dorset uk are presented

feasibility of afc models for the petrogenesis of calc alkaline magma series one of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation afc the equations governing these processes are well known and widely applied the power and attraction of the model are that with the minimum of constraints petrogenetic schemes can be erected for essentially any group of rocks these models are usually based on the observed variations in a small number of trace elements and or isotopic systems in this paper we have generated well constrained afc models for the variscan calc alkaline querigut complex in the french pyrenees our models are based on geological constraints on the identities of the parent magma and candidate assimilants and the sm nd and rb sr isotope systematics of the rocks the modelling is successful in mimicking the isotope array for the actual rocks however the results are massively inconsistent with those produced using the major and trace element concentrations our major conclusion is that afc modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and the proposed assimilant with due respect for crystal liquid partition coefficients in so far as they are known the results should then be accepted only if the isotope based model parameters are consistent with those derived from major and trace element variations

olivine coronas metamorphism and the thermal history of the morristown and emery mesosiderites abstract coronas are present on all millimeter sized mineral clasts of olivine in the emery and morristown mesosiderites and are a manifestation of high temperature t 850 1100 c metamorphism these coronas formed by reaction and diffusion between olivine and a mesosiderite like matrix assemblage the bulk composition of the coronas can be approximated by a mixture of 10 25 wt olivine and as 90 75 wt metal free matrix except for p and cr which are significantly enriched in coronas phosphorus and cr diffused relatively rapidly to coronas and were derived from a large volume of matrix most likely from metal that was originally enriched in these elements prior to metamorphism the coronas in both meteorites show a similar zone sequence but are systematically thicker in emery 800 m wide than in morristown 350 m wide suggesting that emery experienced more grain growth and more intensive metamorphism than morristown textural relationships suggest that corona formation and high temperature metamorphism occurred largely after intensive millimeter scale brecciation and after or during metal silicate mixing a local equilibrium model can explain many features of the coronas but chemical equilibrium was maintained only on a very small scale overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high temperature metamorphism the compositional interface between core and overgrowth plagioclase is extremely sharp suggesting that cooling rates were 0 1 c y at the peak temperature of metamorphism consistent with high temperature metamorphism occurring in a near surface region of the parent body

eruption triggered mixing of extra caldera basalt and rhyolite complexes along the east gallatin washburn fault zone yellowstone national park wy usa abstract though mixing and commingling of magmas is common mixing between rhyolite and basalt magmas is not commonly preserved in volcanic rocks the presence of at least four mixed magma complexes at yellowstone national park suggests that mingling is not due to random intersections of feeder dikes geochemical analyses also show that though these magmas appear to be commingled there is mixing between the two disparate end members our model combines previous work on the grizzly lake gardner river crystal spring and appolinaris spring mixed magma complexes with results from new analyses recent mixing experiments and regional structural geology coeval extensional tectonism as seen in the east gallatin washburn fault zone is also present in other areas of basalt and rhyolite mixing mingling e g iceland the central portions or core of the complexes contain increased concentrations of emulsion rock occasional basaltic pillows in a rhyolite matrix net veining and mixed magma with highly variable geochemistry sio 2 ranges from 50 to 78 wt phenocrysts have been transferred between mafic and felsic portions of the complexes and suggest that these mixed magmas did not have enough time or energy e g heat to thoroughly mix into complete hybrid intermediate magmas this implies that mixing occurred during eruption furthermore analyses at the micron scale suggest that zones of chaotic mixing between basalt and high silica rhyolites may be more complete than previously thought during mixing of high silica rhyolites and basalts with greater than 4 wt mgo the temperature of the rhyolitic magmas was approximately 850 c with a viscosity between 1 10 6 and 3 10 6 pas the basalt was approximately 1070 c with a viscosity of 2 10 2 to 9 10 3 pas prior to mixing mixing of these two extreme end members may have required decompression of the lower basaltic magma chamber during eruption of the overlying rhyolitic magma chamber into through structurally weakened zones of the upper crust the decompression induced mixing was facilitated by coeval extensional tectonics and structures in the norris mammoth corridor extra caldera rhyolites are some of the first volcanics following the collapse of the current caldera and also some of the youngest volcanics in the yellowstone plateau and require further understanding of volcanism in the yellowstone plateau

suspended sediment sources and transport distances in the yellowstone river basin the activity of fallout radionuclides 7 be 1 3 7 cs and 2 1 0 pb was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the yellowstone river basin samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of soda butte creek to billings montana a 423 km long reach of channel the inventory of radionuclides in soil increases with precipitation and is highest in the headwaters the activity of radionuclides in suspended sediment decreases downstream and more activity is observed earlier than later in the flood hydrograph the radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment at sites high in the drainage soils contribute 50 to the suspended load and this value decreases to 11 26 downstream fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream these estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure this study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size

diverse eucritic pebbles in the vaca muerta mesosiderite abstract seven 5 cm basaltic pebbles from the vaca muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe three additional pebbles were only examined petrographically two pebbles are impact melt breccias four are similar texturally to main group eucrites or some clasts in polymict eucrites three are similar to cumulate eucrites and one is a fragmental breccia similar to certain polymict eucrites cumulate pebbles have extremely low ree concentrations and high eu sm ratios reflecting the virtual absence of intercumulus liquid siderophile interelement ratios in the pebbles are similar to those in vaca muerta metal except for anomalously low ir higher silica and merrillite in the pebbles relative to eucrites may reflect reduction of feo in the silicates by p from mesosiderite metal pebbles were annealed following incorporation into the vaca muerta host impacts were the dominant heat source the occurrence of 20 impact melt breccias among the pebbles and 35 40 melt breccias among mesosiderite whole rocks indicate that mesosiderites were more extensively impact melted than howardites we suggest three alternative models to account for this 1 mesosiderites were derived from a separate parent body that experienced a higher meteoroid fluence than the basaltic achondrite parent body 2 the low velocity accretion of large metallic core fragments from disrupted differentiated asteroids led to metal enrichment of a few regions of the regolith and heat retention in these mesosideritic regions and 3 mesosideritic regolith regions were less likely to be reduced to fine particles by extensive shock than howarditic regions

maximum and minimum water contents of granitic melts generated in the crust a reevaluation and implications abstract recent results of studies aimed to determine the water solubilities and the liquidus temperatures in synthetic and natural granitic systems are used to constrain the minimum water contents of granitic melts generated in the crust as a function of pressure temperature melt composition and water activity it is concluded that melts generated in the crust at high temperature and low water activities such as a type granites contain higher water contents than previously assumed up to 20 at 800 900 c and 3 7 kbar as a consequence the melt fraction produced at a given amount of available water can be up to 20 lower the minimum water content of melts generated by partial melting of crustal protoliths especially quartzofeldspathic rocks can be predicted adequately from the liquidus curves in granitic systems this water content can be higher if the melt composition departs from a minimum or eutectic quartzofeldspathic composition melts generated from quartzofeldspathic rocks at low temperature 750 c and below are necessarily water rich 5 wt h 2 o and have approximately the same viscosity or even slightly lower viscosities than melts generated at higher temperatures viscosities in the range 10 4 3 10 5 4 pa s recent experimental data also show that the value of the positive d p d t slope of the liquidus curves for fixed water contents above 800 c is lower than predicted by previous models thus adiabatic decompression produces a higher amount of melt than previously assumed at high temperature the formation of melt by adiabatic decompression can be up to 50 higher than assumed

mineralogy petrology chemistry and 39ar 40ar and exposure ages of the caddo county iab iron evidence for early partial melt segregation of a gabbro area rich in plagioclase diopside we describe coarse grained gabbroic material rich in plagioclase and diopside in the caddo county iab iron meteorite the gabbro is not a clast within a breccia but rather is an area with transitional margins located mainly at silicate metal boundaries only a few cm away from fine grained ultramafic silicates similar to winonaites medium grained orthopyroxene and olivine are found in transitional areas and show no shock disturbance of their textures a vein like region starting in an area of fine grained ultramafic silicate extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside these textures and mineralogies suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts from chondritic source materials compositional data on two mineralogically distinct samples of the gabbro rich portion of the inclusion were obtained by inaa compared to an average of iab silicate inclusions or winonaites the caddo county gabbro is enriched in the incompatible lithophile elements na ca sc ree and hf consistent with a melt origin for the gabbro the cosmogenic space exposure age of caddo county 5 1 ma is significantly younger than exposure ages of some other iab meteorites an 39ar 40ar age determination of the gabbroic material indicates a series of upward steps in age from 4 516 ga to 4 523 ga with a few high temperature ages up to 4 54 ga the older age could approximate the primary recrystallization age of silicates the stepped ar age spectrum may indicate differences in ar closure temperatures during slow cooling of 2 20 c myr in the parent body alternatively the younger ar ar ages may date a shock event which occurred while caddo county was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly misoriented rounded plagioclase domain

dhofar 378 martian shergottite evidence of early shock melting 1 shock heating of the dhofar 378 dho 378 martian shergottite produced melting vesiculation and flow of the plagioclase which upon cooling recrystallized into complex textures heating experiments on the similar zagami shergottite indicate that dho 378 was shock heated to 1000 1100 c and was cooled at 2 5 c h an 39ar 40ar analysis of dho 378 plagioclase indicates different ar diffusion domains and k ca ratios the lower temperature phase defines an ar ar isochron age of 141 32 ma the higher temperature phase released more 40ar but does not define an age the meteorite s thermal history was examined by constructing a generic model to compare cooling rates for objects of different sizes against fractional diffusion loss of ar for different cooling times using gas diffusion parameter values measured for dho 378 this model indicates that it is improbable that the major shock heating event occurred at the time that dho 378 was ejected from mars 3 ma ago rather we suggest that the time of shock heating is probably given by its ar ar age for dho 378 to cool sufficiently fast not to lose most of its 40ar 3 ma ago would require it to have been ejected into space as an impossibly small object larger and more reasonable mars ejection sizes indicate that dho 378 should have lost most of its 40ar on the basis of plagioclase texture and ar data we suggest that a major impact event 141 ma ago melted dho 378 plagioclase degassed most of its 40ar and deposited it in crater ejecta to cool a smaller and later impact ejected it into space 3 ma ago

rates of floodplain accretion in a tropical island river system impacted by cyclones and large floods abstract fluvial processes especially rates of floodplain accretion are less well understood in the wet tropics than in other environments in this study the caesium 137 137cs method was used to examine the recent historical sedimentation rate on the floodplain of the wainimala river in the basin of the rewa river the largest fluvial system in fiji and the tropical south pacific islands 137cs activity in the floodplain stratigraphy showed a well defined profile with a clear peak at 115 cm depth our measured accretion rate of 3 2 cm year 1 over the last ca 45 years exceeds rates recorded in humid regions elsewhere this is explained by the high frequency of tropical cyclones near fiji 40 since 1970 which can produce extreme rainfalls and large magnitude floods since the beginning of hydrological records large overbank floods have occurred every 2 years on average at the study site the biggest floods attained peak flows over 7000 m3 s 1 or six times the bankfull discharge concentrations of suspended sediments are very high max 200 500 g l 1 delivered mainly by channel bank erosion in the future climatic change in the tropical south pacific region may be associated with greater tropical cyclone intensities which will probably increase the size of floods in the rewa basin and rates of floodplain sedimentation

evolutionary history of the mesosiderite asteroid a chronologic and petrologic synthesis abstract recent petrologic chemical and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body mpb chronologic measurements indicate that initial melting of the mpb occurred shortly after accretion 4 56 ga ago at about the same time that the eucrites and angrites formed significant crustal remelting 4 47 ga ago probably associated with metal silicate mixing is responsible for forming abundant gabbroic clasts with extreme positive eu anomalies localized impact melting 4 5 3 9 ga ago produced quench textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type 4 mesosiderites collisional disruption and gravitational reassembly 3 9 ga ago is indicated by thorough ar degassing at this time metallographic cooling rates of 1 c ma were likely initiated by this event the history of higher temperature events measured by thermoluminescence was annealed out impact excavation and ejection of deeply buried breccias occurred 3 9 ga ago the 10 160 ma cosmic ray exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the mpb or from a 10 m size fragment it is unlikely that mesosiderites and hed howardite eucrite diogenite samples come from the same parent body there is no evidence for metal silicate mixing with attendant redox effects or major crustal remelting on the hed body bulk howardites have flat chondrite normalized rare earth element ree patterns whereas many bulk mesosiderites have ree patterns with positive eu anomalies and relative enrichments in light ree the nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic

use of fallout pb 210 measurements to investigate longer term rates and patterns of overbank sediment deposition on the floodplains of lowland rivers the potential for using fallout unsupported pb 210 210pb measurements to estimate rates of overbank sediment deposition on the floodplains of lowland rivers is explored a model which distinguishes the contribution from direct atmospheric fallout and the catchment derived input associated with the deposition of suspended sediment has been developed to interpret the fallout pb 210 inventories at floodplain sampling sites and to estimate average sediment accumulation rates over the past 100 years the approach has been successfully used to estimate rates of overbank sedimentation on the floodplains of the rivers culm and exe in devon u k a detailed investigation of the pattern of longer term sedimentation rates within a small reach of the floodplain of the river culm indicated a range of deposition rates between 0 07 and 0 59 g cm 2 a 1 which was in close agreement with estimates of current sedimentation rates obtained using sedimentation traps

energy constrained open system magmatic processes 3 energy constrained recharge assimilation and fractional crystallization ec rafc 1 geochemical data for igneous rock suites provide conclusive evidence for the occurrence of open system processes within thermally and compositionally evolving magma bodies the most significant processes include magma recharge with possible enclave formation and magma mixing assimilation of anatectic melt derived from wallrock partial melting and formation of cumulates by fractional crystallization rafc in this study we extend the energetically constrained assimilation and fractional crystallization ec afc model spera and bohrson 2001 bohrson and spera 2001 to include the addition of compositionally and thermally distinct recharge melt during simultaneous assimilation and fractional crystallization energy constrained recharge assimilation and fractional crystallization ec rafc tracks the trace element and isotopic composition of melt cumulates and enclaves during simultaneous recharge assimilation and fractional crystallization ec rafc is formulated as a set of 3 t i s coupled nonlinear differential equations where the number of trace elements and radiogenic and stable isotope ratios modeled are t i and s respectively solution of the ec rafc equations provides values for the average wallrock temperature ta mass of melt within the magma body mm mass of cumulates mct and enclaves men mass of wallrock involved in the thermal interaction mao mass of anatectic melt assimilated m a concentration of t trace elements and i s isotopic ratios in melt cm cumulates cct enclaves cen and anatectic melt ca as a function of magma temperature tm input parameters include the equilibration temperature teq the initial temperature and composition of pristine melt tmo cmo emo recharge melt tro cro ero and wallrock tao cao eao temperature dependent trace element distribution coefficients dm dr da heats of transition for wallrock ha pristine melt hm and recharge melt hr and the isobaric specific heat capacity of assimilant cp a pristine melt cp m and recharge melt cp r the magma recharge mass function mr tm is specified a priori and defines how recharge magma is added to standing magma the present ec rafc simulator incorporates a weak coupling to major element mass balance and phase relations based on laboratory experiments or gibbs energy minimization e g ghiorso 1997 ec rafc can be applied to a variety of magmatic systems including volcanic suites that show evidence of magma mixing layered mafic intrusions and granitoid plutons predictions for masses as well as compositions of magmatic products are part of the ec rafc solution the systems approach provides an opportunity to quantitatively assess the roles of assimilation fractional crystallization and magma recharge in magma evolution using trace element and isotopic constraints together with energy conservation

thermal history of mesosiderites revisited abstract the results of a fission track study of uranium rich merrillite crystals in a number of mesosiderites are clearly incompatible with cooling rates as low as 0 1 c myr which have previously been inferred for these meteorites classical metallographic techniques thus appear to give erroneous results when applied to this type of meteorites there appears to be no reason to advocate exotic scenarios to explain the thermal history of mesosiderites

mantle material in the main belt battered to bits the complete or near complete differentiation of a chondritic parent body is believed to result in an object with an fe ni core a thick olivine dominated mantle and a thin plagioclase pyroxene crust compositional groupings of iron meteorites give direct evidence that at least 60 chondritic parent bodies have been differentiated and subsequently destroyed a long standing problem has been that our meteorite collections and apparently our asteroid observations as well show a great absence of olivine dominated metal free mantle material while the basaltic achondrites hed meteorites represent metal free pyroxene dominated crustal samples the isotopic and geochemical evidence implies that this class is derived from only one parent body perhaps vesta thus the meteoritic and perhaps astronomical evidence also suggests a great absence of crustal material resulting from the collisional disruption of numerous parent bodies