mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

rapid releases of metal salts and nutrients following the deposition of volcanic ash into aqueous environments deposition of volcanic ash into aqueous environments leads to dissolution of adsorbed metal salts and aerosols increasing the bioavailability of key nutrients volcanogenic fertilization events could increase marine primary productivity leading to a drawdown of atmospheric co2 here we conduct flow through experiments on unhydrated volcanic ash samples from a variety of locations and sources measuring the concentrations and fluxes of elements into de ionized water and two contrasting ocean surface waters comparisons of element fluxes show that dissolution of adsorbed surface salts and aerosols dominates over glass dissolution even in sustained low ph conditions these surface ash leachates appear unstable decaying in situ even if kept unhydrated volcanic ash from recent eruptions is shown to have a large fertilization potential in both fresh and saline water fluorine concentrations are integral to bulk dissolution rates and samples with high f concentrations display elevated fluxes of some nutrients particularly fe si and p bio limiting micronutrients are released in large quantities suggesting that subsequent biological growth will be limited by macronutrient availability importantly acidification of surface waters and high fluxes of toxic elements highlights the potential of volcanic ash leachates to poison aqueous environments in particular large ph changes can cause undersaturation of caco3 polymorphs damaging populations of calcifying organisms deposition of volcanic ash can both fertilize and or poison aqueous environments causing significant changes to surface water chemistry and biogeochemical cycles

effects of regolith megaregolith insulation on the cooling histories of differentiated asteroids the cooling histories and in particular the core cooling rates of differentiated asteroids are calculated employing a variety of thermal conductivity structures in order to simulate the potential insulating effects of regolith and megaregolith brecciated impact rubble layers the regolith is assumed to have thermal conductivity 0 005 the unbrecciated mantle conductivity and its thickness in our most typical model is assumed to be 0 003 the parent body radius the megaregolith if present is assumed to have thermal conductivity 0 1 that of unbrecciated mantle we find that a combination of relatively thick megaregolith and regolith can potentially reduce the core cooling rate by more than a factor of 10 below the rate predicted by models with conventional thermal conductivity structure for any given measured cooling rate the radius implied for the parent asteroid can potentially be reduced by a factor of 5 from the radius predicted by models with conventional thermal conductivity structure realistically the regolith and megaregolith thicknesses required to engender these extreme effects were probably seldom generated except perhaps within the largest asteroids where even if generated they would quickly be reduced by compaction and sintering nonetheless our results for models with plausible regolith and megaregolith thicknesses indicate that most of the iron meteorites and the pallasites probably formed in relatively small r less than 50 km parent bodies we have parameterized our results so that effects of intermediate configurations can readily be calculated the parameterization also facilitates evaluation of additional variables e g the initial temperature the concentrations of long lived radioactive heat sources etc albeit these variables are far less crucial than the thermal conductivity structure our results indicate that the insulation of even a thin regolith causes metallalographic cooling rates to be relatively uniform and practically independent of depth within a given asteroid stochastic differences in cratering regolith production histories may have engendered radically different cooling rates from asteroids of similar radius or similar cooling rates from asteroids of different radius

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

hydrometeor enhanced tephra sedimentation constraints from the 18 may 1980 eruption of mount st helens 1 uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits in this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the mount st helens 18 may 1980 msh80 eruption in addition we collate all the data needed to model distal ash sedimentation from the msh80 eruption cloud four particle size subpopulations were present in distal fallout with modes at 2 2 4 2 5 9 and 8 3 settling rates of the coarsest subpopulation closely matched predicted single particle terminal fall velocities sedimentation of particles 100 m was greatly enhanced predominantly through aggregation of a particle subpopulation with modal diameter 5 9 0 2 19 3 m mammatus on the msh80 cloud provided a mechanism to transport very fine ash particles with predicted atmospheric lifetimes of days to weeks from the upper troposphere to the surface in a matter of hours in this mechanism ash particles initiate ice hydrometeor formation high in the troposphere subsequently the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits this work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons

low temperature phase decomposition in iron nickel metal of the portales valley meteorite the portales valley meteorite provides an opportunity to investigate and compare the microstructure in fe ni metal of the metallic particles in the chondritic portion and in the metal veins the low temperature phase decomposition of fe ni metal was investigated using scanning electron microscopy transmission electron microscopy and atomic force microscopy the microstructure is formed as the portales valley meteorite cooled from high temperatures and includes the outer taenite rim the cloudy zone clear taenite and martensite martensite in turn decomposes into a fine admixture of fcc rods in a bcc matrix the width of the island phase of the cloudy zone in the metal particles of the chondritic portion and the metal veins can be used to estimate a low temperature cooling rate the microstructural evidence indicates that the chondritic portions and the metal veins in the portales valley meteorite cooled together as a mixture with a cooling rate of roughly 6 5 k ma

a geochemical study of the winonaites evidence for limited partial melting and constraints on the precursor composition abstract the winonaites are primitive achondrites which are associated with the iab iron meteorites textural evidence implies heating to at least the fe ni fes cotectic but previous geochemical studies are ambiguous about the extent of silicate melting in these samples oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites here we analysed a suite of winonaites for modal mineralogy and bulk major and trace element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite iab parent asteroid modal mineralogy and geochemical data are presented for eight winonaites textural analysis reveals that for our sub set of samples all except the most primitive winonaite northwest africa 1463 reached the fe ni fes cotectic however only one tierra blanca shows geochemical evidence for silicate melting processes tierra blanca is interpreted as a residue of small degree silicate melting our sample of winona shows geochemical evidence for extensive terrestrial weathering all other winonaites studied here fortuna queen alexander range 94535 hammadah al hamra 193 pontlyfni and nwa 1463 have chondritic major element ratios and flat ci normalised bulk rare earth element patterns suggesting that most of the winonaites did not reach the silicate melting temperature the majority of winonaites were therefore heated to a narrow temperature range of between 1220 the fe ni fes cotectic temperature and 1370 k the basaltic partial melting temperature silicate inclusions in the iab irons demonstrate partial melting did occur in some parts of the parent body ruzicka and hutson 2010 thereby implying heterogeneous heat distribution within this asteroid together this indicates that melting was the result of internal heating by short lived radionuclides the brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact which allowed the preservation of the largely unmelted winonaites despite major element similarities to both ordinary and enstatite chondrites trace element analysis suggests the winonaite parent body had a carbonaceous chondrite like precursor composition the parent body of the winonaites was volatile depleted relative to ci but enriched compared to the other carbonaceous classes the closest match are the cm chondrites however the specific precursor is not sampled in current meteorite collections

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

isotopic fractionation of germanium in iron meteorites significance for nebular condensation core formation and impact processes abstract the siderophile and volatile nature of germanium allow ge isotopes to be used to investigate the early history of planetesimals by tracing processes of core formation and impact through the study of magmatic and non magmatic iron meteorites germanium isotopic compositions were measured using a hexapole collision cell mc icpms with an external reproducibility of 0 06 amu iron meteorites display heavy ge isotopic compositions 74ge 74ge 70ge 0 27 to 1 92 with respect to a jmc ge standard which contrast with the light isotopic compositions of the studied terrestrial materials aldrich ge standard 74ge 1 68 1 72 0 22 sphalerite 74ge 0 69 all the data fall on the theoretical mass fractionation line indicating no isotopic anomalies the samples from magmatic irons iia iib iiiab iic have relatively homogeneous ge isotopic compositions of 74ge 1 77 0 22 2 for a large range of ge contents 36 189 ppm in contrast the samples of the non magmatic groups iab iie have lower and more variable ge isotopic compositions with 74ge 1 15 0 20 for iab group ge 254 493 ppm 74ge 0 27 to 0 43 for iie old group ge 63 69 ppm and 74ge 1 40 0 22 for the watson sample ge 50 ppm from the iie young group except for iie samples no within group correlation of 74ge with ir or ge contents is observed demonstrating that fractional crystallization or crystal segregation do not fractionate ge isotopes the lack of inter group correlation between the ge isotopic composition of magmatic irons and ni content an indicator of redox processes suggests that if ge isotopes are fractionated by redox induced diffusion during metal silicate segregation isotopic equilibration towards the initial composition must also occur during isotopic exchange at high temperature thus the average 74ge value of 1 77 0 22 should be representative of their parent body precursors the lack of correlation between the ge isotopic composition and ge ni ratios and the high ge ni ratio of iia samples ge ni in ci chondrites may reflect the high proportion of metal condensates present in the parent body before metal silicate differentiation these metal phases would carry the ge isotopic signature of a ge enriched gas phase of the solar nebula for the iab non magmatic group the light and homogeneous ge isotopic composition may result from condensation from a 70ge enriched vapour subsequent to partial melting within a porous parent body the origin of the heating event 26al decay or impact on the surface of the chondritic body cannot be distinguished in contrast it can be demonstrated that impact events are involved in the formation of iie irons the negative 74ge ge correlation with increasing 74ge values from the old group to the watson sample of the young group would result from non rayleigh evaporation of 70ge during impact with gas melt back reaction the high 74ge of the watson sample young group is consistent with successive impacts on the surface of the iie parent body the very light ge isotopic composition of the old less impacted iie irons compared to other studied iron meteorites suggests a light ge isotopic composition for the iie parent body

oxygen isotope relationships in iron meteorites iron meteorites with oxygen bearing phases can be classified in terms of their oxygen isotopic abundances these iron meteorite classes are isotopically similar to various stony meteorite classes which may indicate a common origin the group iab and iiicd irons may be related to the winonaites group iie irons may be related to h chondrites group iva irons may be related to l or ll chondrites

gas aerosol ash interaction in volcanic plumes new insights from surface analyses of fine ash particles the reactions occurring between gases aerosols and silicate ash particles in volcanic eruption plumes remain poorly understood despite the fact that they are at the origin of a range of volcanic environmental atmospheric and health effects in this study we apply x ray photoelectron spectroscopy xps a surface sensitive technique to determine the chemical composition of the nearsurface region 2 10 nm of nine ash samples collected from eight volcanoes in addition atomic force microscopy afm is used to image the nanometer scale surface structure of individual ash particles isolated from three samples we demonstrate that rapid acid dissolution of ash occurs within eruption plumes this process is favoured by the presence of fluoride and is believed to supply the cations involved in the deposition of sulphate and halide salts onto ash afm imaging also has permitted the detection of extremely thin 10 nm coatings on the surface of ash this material is probably composed of soluble sulphate and halide salts mixed with sparingly soluble fluoride compounds the surface approach developed here offers promising aspects for better appraising the role of gas aerosol ash interaction in dictating the ability of ash to act as sinks for various volcanic and atmospheric chemical species as well as sources for others c 2007 elsevier b v all rights reserved

revised model calculations for the thermal histories of ordinary chondrite parent bodies recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model miyamoto et al 1981 for the thermal evolution and internal structure of ordinary chondrite parent bodies model calculations assumed a heat source term derived from the decay of 26al justification is provided differences from the previous model include varying the thermal diffusivity parameter with increasing temperature and decreasing porosity using variable physical and thermal parameters to provide end member models and incorporating a shortened thermal history of 60 ma obtained from new pb pb chronology of phosphates rather than 100 ma times of isotopic closure in chondrite phosphates overlap the thermal model estimates and postmetamorphic cooling rates from the model approximately coincide in both trend and magnitude with metallographic and fission track cooling rate data model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1 4 to 3 1 ma after calcium aluminum inclusion cai formation model calculations also predict that both the h and the l chondrite parent asteroids consisted of 80 equilibrated and 20 unequilibrated chondritic material and that their original radii ranged from 80 to 95 km

injection of gases into the stratosphere by explosive volcanic eruptions 1 explosive eruptions can inject large amounts of volcanic gases into the stratosphere these gases may be scavenged by hydrometeors within the eruption column and high uncertainties remain regarding the proportion of volcanic gases which eventually reach the stratosphere these are caused by the difficulties of directly sampling explosive volcanic eruption columns and by the lack of laboratory studies in the extreme parameter regime characterizing them using the nonhydrostatic nonsteady state plume model active tracer high resolution atmospheric model atham we simulated an explosive volcanic eruption we examined the scavenging efficiency for the climatically relevant gases within the eruption column the low concentration of water in the plume results in the formation of relatively dry aggregates more than 99 of these are frozen because of their fast ascent to low temperature regions consideration of the salinity effect increases the amount of liquid water by one order of magnitude but the ice phase is still highly dominant consequently the scavenging efficiency for hcl is very low and only 1 is dissolved in liquid water however scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process the salinity effect increases the total scavenging efficiency for hcl from about 50 to about 90 the sulfur containing gases so2 and h2s are only slightly soluble in liquid water however these gases are incorporated into ice particles with an efficiency of 10 to 30 despite scavenging more than 25 of the hcl and 80 of the sulfur gases reach the stratosphere because most of the particles containing these species are lifted there sedimentation of the particles would remove the volcanic gases from the stratosphere hence the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them which is in turn dependent on the volcanic and environmental conditions

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

isotopic composition of xenon and krypton in silicate graphite inclusions of the el taco campo del cielo iab iron meteorite noble gases extracted by temperature stepped pyrolysis from acid resistant silicate schreibersite and graphite residues of inclusions from the iab iron meteorite el taco campo del cielo have been analyzed for their isotopic composition the concentrations of spallogenic light gases agree with previous data on adjacent specimens nominal feldspar and pyroxene fractions have lost up to 80 of their 3he during or towards the end of the exposure of el taco to the cosmic radiation in olivine the 3ne21ne and nepne ratios are as low as 2 18 0 01 and 1 09 0 03 respectively presumably because the silicates were irradiated within a feni matrix under extremely heavy shielding argon kr and xe in the silicates are dominated by a trapped planetary component with 100 36ar132xe 700 and 1 7 84kr132xe 2 2 xenon released from the silicates at extraction temperatures 1000 c is isotopically close to ureilite xe while the high t xe from graphite and schreibersite is unlike any of the presently well established types of xe the isotopic composition of this xe can be generated as a mixture of mass fractionated u xe with light isotopes depleted by 2 2 0 1 per atomic mass unit and 244pu xe the fission component 10 in the case of 136xe is not due to in situ production it must have been added prior to incorporation of the xe into the el taco parent body and presumably underwent the mass fractionation process together with u xe from the silicates 127i n produced 128xe and radiogenic 129xe are released together with the same 128xe 129xe ratio in nominal olivine pyroxene and feldspar although the maximum release from pyroxene occurs about 300 c higher than from feldspar from this the cooling rate of silicates for the temperature range 1500 1200 c is deduced to be 100 c per million years the 80kr82kr ratio of 79 81br n produced 80 82kr is 2 86 0 04 in agreement with the value found in inclusions of the iab linwood but higher than observed in almost all stone meteorites

finite strain effects in experimental mullions 1986 abstract mullions are cylindrical structures developed at the interface between rocks of different competence shortened along the layering natural examples include phyllosilicate rich meta igneous sheets and massive sulfides in silicate country rocks in axial profiles they consist of rounded lobes convex into the less competent material which alternate with sharp cusps pointing toward the more competent material mullions have been produced experimentally in incompetent single layers of silicone putty viscosity l 2 9 x 104 pa s and power law exponent n 1 embedded in a strain rate softening host of plastilina silicone putty mixture h 5 5 10 pa s and n 7 1 the initiation development and changes in geometry of mullions in the two interfering interfaces of a single incompetent layer shortened along its length by pure shear are reported up to 10 bulk shortening bs gentle flexures develop first in one interface then by 17 bs in both these rounded irregularities become cuspate at 25 bs where a lobe develops on one contact a cusp is slightly preferred on the opposite side such antisymmetric mullions contrast with the symmetrical pinch and swell mullions predicted by smith but are like natural examples and those figured in the literature the amplitudes of the cusps increase while their wavelength range decreases with finite shortening it has been found that mature cusps distort but do not transgress the original contact even if the viscosity contrast is 1 1000 beyond 40 bs cusps either mature to flames or combine to horns and after 60 bs even the last surviving mullions cease to amplify dynamically and the layer only thickens uniformly most stages seen in the models can be matched by natural field examples

40ar 39ar dating of plagioclase grain size separates from silicate inclusions in iab iron meteorites and implications for the thermochronological evolution of the iab parent body abstract in order to better constrain the thermochronological evolution of the iab parent body we performed a 40ar 39ar age study on individual silicate inclusions of the iab irons caddo county campo del cielo landes and ocotillo in contrast to earlier studies several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the ar ar ages in nearly all inclusions we found significantly different ar ar ages among the separates caddo county 4 472 0 02 4 562 0 02 ga campo del cielo 2 4 362 0 04 4 442 0 03 ga landes 2 4 412 0 05 4 522 0 04 ga ocotillo 4 382 0 04 4 462 0 03 ga these ages were calculated using the new 40k decay constant published by mundil r renne p r min k and ludwig k r 2006 resolvable miscalibration of the 40ar 39ar geochronometer eos trans agu 87 fall meet suppl abstract v21a 0543 the ages did not systematically correlate with the respective grain size of the separate as expected i e smaller grains did not necessarily show younger ages due to later closure to ar diffusion or easier re opening of the system in the course of a reheating event compared to larger grains based on the large range of ar ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the iab parent body while some grains remained in a hot deep environment that allowed ar diffusion over an extended time period in some cases combined with grain coarsening others cooled significantly earlier near surface through the k ar blocking temperature these different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by benedix et al benedix g k mccoy t j keil k and love s g 2000 a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body meteorit planet sci 35 1127 1141 due to rapid cooling after the impact some of the age differences among the grains could be preserved based mainly on our caddo county ar ar age information the iab parent body was destroyed by impact and reassembled between 4 5 and 4 47 ga however iab silicate ar ar ages should depend much more on the pre and post impact cooling rate and burial depth than on the time of the actual impact this is supported by a compilation of our and literature iab and winonaite ar ar ages ranging smoothly from the time of accretion of the chondritic iab parent body down to the time of its final cooling through the k ar blocking temperature after impact and reassembly instead of showing a peak in ar ar ages at the time of the destructive impact

chemical variations of tephra fall deposit leachates for three eruptions from popocatepetl volcano abstract leachates from ash samples of the popocatepetl eruptions of april 30 1996 may 12 1997 and october 17 1998 settled at different distances from the crater were analyzed for anions so 4 2 cl f and some metals this study is aimed at determining the causes of the compositional variations of the leachates to assist the assessment of water soil and crop contamination due to ash deposits different behavior was observed in the ion concentrations with distance for the three eruptions on april 30 1996 so 4 2 and f concentrations increased with distance and cl remained almost constant on may 12 1997 concentrations of the three anions decreased with distance on october 17 1998 f cl and so 4 2 increased more than three fold with distance tephra size distributions were also different for the three eruptions the observed trends of the leachates anion concentrations may have different causes the type and intensity of the eruptions the distribution of the tephra sizes the degree of interaction of the tephras with volcanic gases humidity static charge the original characteristics of the solid material the transport time from the crater to the site of settling and the relative angle between the wind direction and the sampling line enrichment factors and concentration trends for metals with distance suggest that co ni cu and pb in the leachates resulted mostly from volcanic gas adsorption

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

comment on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty journal of structural geology 27 2005 1599 1609

a comparison of metallographic cooling rate methods used in meteorites abstract the primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites both types of metal occur in some group iab meteorites which are mixtures of massive metal with well developed widmanstatten structures and chondritic inclusions with dispersed metal grains the grains have textures and compositions similar to chondritic metal including negligible p the meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before widmanstatten patterns formed during cooling cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central ni contents modeling entails solving diffusion equations using experimental diffusion coefficients phase relations and bulk or local ni and p contents taking into account geometry undercooling and impingement there is one set of parameters for grains and another quite different set for widmanstatten lamellae including a factor of 30 difference in diffusion coefficients yet cooling rates obtained from widmanstatten structures and metal grains in chondritic inclusions of the same meteorite are consistent uncertainties in the best data are 10 ma equivalent to a factor of 1 0 25 this agreement implies that the data and models are correct or contain fortuitously offsetting errors which is quite unlikely cooling rates range from 40 ma to 70 ma in iab meteorites that contain both grains and widmanstatten structures rates based on grains in ni poor and ni rich meteorites lacking widmanstatten patterns expand the range from 30 ma to perhaps 200 ma cooling rates correlate with ni content ni poor meteorites have slower rates than ni rich ones evidently iab meteorites were radially distributed over 30km in a body with a radius 50km a comparison of the available ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established

assessment of leachable elements in volcanic ashfall a review and evaluation of a standardized protocol for ash hazard characterization abstract volcanic ash presents a widespread and common hazard during and after eruptions complex interactions between solid ash surfaces and volcanic gases lead to the formation of soluble salts that may be mobilized in aqueous environments a variety of stakeholders may be concerned about the effects of ash on human and animal health drinking water supplies crops soils and surface runoff as part of the immediate emergency response rapid dissemination of information regarding potentially hazardous concentrations of soluble species is critical however substantial variability in the methods used to characterize leachable elements makes it challenging to compare datasets and eruption impacts to address these challenges the international volcanic health hazard network www ivhhn org organized a two day workshop to define appropriate methods for hazard assessment the outcome of this workshop was a consensus protocol for analysis of volcanic ash samples for rapid assessment of hazards from leachable elements which was subsequently ratified by leading volcanological organizations the purpose of this protocol is to recommend clear standard and reliable methods applicable to a range of purposes during eruption response such as assessing impacts on drinking water supplies and ingestion hazards to livestock and also applicable to research purposes where possible it is intended that the methods make use of commonly available equipment and require little training to evaluate method transferability an interlaboratory comparison exercise was organized among six laboratories worldwide each laboratory received a split of pristine ash and independently analyzed it according to the protocol for a wide range of elements collated results indicate good repeatability and reproducibility for most elements thus indicating that the development of this protocol is a useful step towards providing standardized and reliable methods for ash hazard characterization in this article we review recent ash leachate studies report the outcomes of the comparison exercise and present a revised and updated protocol based on the experiences and recommendations of the exercise participants the adoption of standardized methods will improve and facilitate the comparability of results among studies and enable the ongoing development of a global database of leachate information relevant for informing volcanic health hazards assessment

volcanic controls on ash iron solubility new insights from high temperature gas ash interaction modeling abstract recent studies strongly suggest that volcanic ash can fertilize the surface ocean by releasing soluble iron however the volcanic and atmospheric processes that solubilize ash iron during its transport from the volcano to the ocean are poorly understood using thermodynamic equilibrium calculations we investigate the influence of gas ash interaction within the hot core t 600 c of the volcanic plume and the consequences of this for ash iron solubility simulations are performed by considering the plume hot core as a box model in which 1000 c magmatic gas ash and 25 c ambient air are mixed together we show that mixing and the resulting cooling of the gas ash air mixture affect the mineralogy and oxidation state of iron in the ash surface rim iron mineralogy in the ash surface layer after high temperature plume processing is primarily governed by the ratio of the h2 and h2s content of the magmatic gas to the amount of entrained o2 into the hot plume xmix the model results indicate that most of the iron in the ash surface layer is oxidized to ferric iron fe iii when log xmix drops below 3 5 in the hot core such conditions may be encountered at convergent plate volcanoes which release h2o rich magmatic gases in contrast high temperature gas ash interaction at divergent plate and hot spot volcanoes which tend to be associated with co2 rich and so2 rich magmatic gases respectively may produce ash surfaces where iron mostly occurs as ferrous fe ii these volcanoes seem to be more favorable for iron fertilization because log xmix does not fall below 3 5 and 80 of the iron in the ash surface remains ferrous fe ii which is more soluble in water than fe iii

a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body abstract we have studied iab iron meteorites and their silicate bearing inclusions to elucidate the origin of their parent body we have divided iab irons into five categories which best describe the inclusions and other properties of the irons these are 1 sulfide rich irons such as mundrabilla that contain a significant volume fraction of troilite with or without associated silicates 2 irons with non chondritic silicate inclusions such as basaltic inclusions in caddo county 3 irons with rounded troilite and or graphite inclusions sometimes containing embedded angular silicate inclusions 4 irons with angular silicate inclusions similar to those of the winonaites where silicates may comprise up to 40 vol of the rock and 5 irons with phosphate bearing inclusions such as the brianite bearing iab san cristobal similarities in mineralogy mineral and oxygen isotopic compositions suggest that iab irons and winonaites are almost certainly from a common parent body iiicd irons may also have originated on this parent body although evidence supporting this suggestion is less convincing previous models for the origin of iab irons propose that they are either products of selective impact melting in the megaregoliths of a chondritic asteroid or products of partial melting and fractional crystallization during formation of a sulfur rich core the former model suffers from the fact that impact is incapable of causing preferential melting of metal and sulfide on a large scale and pooling of these selective melts into sizable bodies the latter model does not readily explain the occurrence of often fragmentary angular silicate clasts in a metallic fe ni host we propose a new model for the origin of iab irons that incorporates aspects of both previous models we suggest that a chondritic possibly heterogeneous parent body was heated by a non collisional heat source which caused partial but not complete melting of the body after peak temperatures were reached and cooling and crystallization had begun a major impact catastrophically fragmented this body and the debris gravitationally reassembled scrambling the parent body lithologies we suggest that the impact caused extensive mixing of molten metallic fe ni and solid silicate material resulting in the formation of iab irons with silicate inclusions whereas winonaites represent metamorphosed rocks and partial melt residues of the precursor silicate lithologies high temperatures of some of the reassembled debris and deep burial resulted in continued fractionation and formation of the 2

a microstructural indicator of shear sense in volcanic rocks and its relationship to porphyroblast rotation in metamorphic rocks an asymmetrical arrangement of microfolds in flow layering adjacent to phenocrysts indicates the sense of shear and is a potential indicator of the local flow direction in flow layered volcanic rocks this microstructure is analogous to a similar arrangement of microfolds around rotated porphyroclasts in some metamorphic rocks it is also relevant to the incorporation of inclusion trails in porphyroblasts that grow while undergoing rotation relative to the matrix foliation suggesting that asymmetrical microfolds may form outside porphyroblasts during their growth and relative rotation asymmetrical microfolds adjacent to garnet porphyroblasts have been observed in some metamorphic rocks

noble gas retention chronologies for the st severin meteorite abstract 40 ar 39 ar and 129 xe 128 xe analyses were performed on two lithologies light and dark of the st severin ll6 chondrite for the light and dark fractions respectively we obtained 40 ar retention ages of 4 38 and 4 42 ae and 129 xe retention ages of 8 4 and 15 2 myr after bjurbole the two methods give age differences of opposite sense and by both methods the differences are significant both the 40 ar and the 129 xe ages are interpreted as dating relaxation of metamorphic conditions these two chronometers are decoupled however and do not date the same events 40 ar 39 ar reflect chondritic metamorphisrn on a 10 8 yr time scale the 129 xe 128 xe ages reflect isotopic closure at higher temperatures and earlier times

reply to the comment by marques et al on numerical models of flow patterns around a rigid inclusion in a viscous matrix undergoing simple shear implications of model parameters and boundary conditions by n mandal s k samanta and c chakraborty

hf w chronometry of primitive achondrites metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies the timing of these events can be constrained using the short lived 182hf 182w radionuclide system here we present new 182hf 182w data for major types of primitive achondrites including acapulcoites winonaites and one lodranite these meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites for acapulcoites we derived a 182hf 182w age of tcai 4 1 1 2 1 1 ma a model age for winonaite separates calculated from the intercept of the isochron defines an age of tcai 4 8 3 1 2 6 ma assuming a bulk hf w ratio of 1 2 both ages most likely define primary magmatic events on the respective parent bodies such as melting of metal although metal stayed in place and did not segregate to form a core a later thermal event is responsible for resetting of the winonaite isochron yielding an age of tcai 14 3 2 7 2 2 ma significantly younger than the model age assuming a co genetic relationship between winonaites and silicates present in iab iron meteorites based on oxygen isotope composition and including data by schulz et al 2009 a common parent body chronology can be established magmatic activity occurred between 1 5 and 5 ma after cais more than 5 ma later intensive thermal metamorphism has redistributed hf w average cooling rates calculated for the winonaite iab parent asteroid range between 35 and 4 k ma most likely reflecting different burial depths cooling rates obtained for acapulcoites were 40 k ma to 720 k and then 3 k ma to 550 k accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies e g angrites ureilites and eucrites in this case supporting the concept of an inverse correlation between accretion age of asteroids and intensity of heating in their interiors as expected from heating by 26al and 60fe decay however the early accretion of the parent asteroid of primitive iab silicates 1 0 ma after cais schulz et al 2009 and the possibly impact induced melting history of winonaites show that this concept is too simplistic parent body size impact driven melting as well as heat insulating regolith cover also need to be considered in the early history of asteroid differentiation

interpreting the i xe system in individual silicate grains from toluca iab detailed isotopic and mineralogical studies of silicate inclusions separated from a troilite nodule of the toluca iab iron meteorite reveal the presence of radiogenic 129 xe in chlorapatite plagioclase perryite and pyroxene grains subsequent i xe studies of 32 neutron irradiated pyroxene grains indicate that high mg and low mg pyroxenes have distinctive i xe signatures the i xe system in high mg pyroxenes closed at 4560 5 2 4 ma probably reflecting exsolution of silicates from the melt while the low mg pyroxenes closed at 4552 0 3 7 ma 8 5 ma later providing a means for determining the cooling rate at the time of exsolution if the host toluca graphite troilite rich inclusion formed after the breakup and reassembly of the iab parent body as has been suggested the i xe ages of the high mg pyroxenes separated from this inclusions indicate that this catastrophic impact occurred not later than 4560 5 ma 6 7 ma after formation of cais the cooling rate at the time of silicates exsolution in toluca is 14 5 10 0 c ma

spatial analysis of mount st helens tephra leachate compositions implications for future sampling strategies tephra particles in physically and chemically evolving volcanic plumes and clouds carry soluble sulphate and halide salts to the earth s surface ultimately depositing volcanogenic compounds into terrestrial or aquatic environments upon leaching of tephra in water these salts dissolve rapidly previous studies have investigated the spatial and temporal variability of tephra leachate compositions during an eruption in order to gain insight into the mechanisms of gas tephra interaction which emplace those salts however the leachate datasets analysed are typically small and may poorly represent the natural variability and complexity of tephra deposits here we have conducted a retrospective analysis of published leachate analyses from the 18 may 1980 eruption of mount st helens washington analysing the spatial structure of the concentrations and relative abundances of soluble ca cl na and s across the deposits we have identified two spatial features 1 concentrated tephra leachate compositions in blast deposits to the north of the volcano and 2 low s cl and na cl ratios around the washington idaho border by reference to the bulk chemistry and granulometry of the deposit and to current knowledge of gas tephra interactions we suggest that the proximal enrichments are the product of pre eruptive gas uptake during cryptodome emplacement we speculate that the low s cl and na cl ratios reflect a combination of compositional dependences on high temperature so2 uptake and preferential hcl uptake by hydrometeor tephra aggregates manifested in terrestrial deposits by tephra sedimentation and fallout patterns however despite our interrogation of the most exhaustive tephra leachate dataset available it has become clear in this effort that more detailed insights into gas tephra interaction mechanisms are prevented by the prevalent poor temporal and spatial representativeness of the collated data and the limited characterisation of the tephra deposits future leachate studies should aim to extensively sample across tephra deposit limits whilst simultaneously characterising deposit stratigraphy and tephra chemistry mineralogy and granulometry taking steps to ensure the quality and comparability of collected leachate datasets

a petrologic chemical and isotopic study of monument draw and comparison with other acapulcoites evidence for formation by incipient partial melting abstract we have conducted petrologic chemical and isotopic studies of acapulcoites acapulco monument draw yamato 74063 alh a77081 alh a81261 alh a81315 alh 78230 alh a81187 and alh 84190 in an attempt to constrain their genesis acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in iab and iiicd irons winonaites and ureilites and thus formed on a different parent body oxygen isotopic compositions which are slightly heterogeneous within the group overlap with lodranites indicating a likely origin on a common parent body these groups can be distinguished on the basis of mafic silicate grain size all acapulcoites have mafic silicate compositions intermediate between e and h chondrites roughly chondritic mineralogies achondritic equigranular textures micrometer to centimeter sized veins of fe ni fes which cross cut silicate phases rapid metallographic cooling rates at 600 400 c 103 105 c myr and trapped noble gas abundances comparable to type 3 4 ordinary chondrites they exhibit variable mafic silicate zoning abundance of fe ni fes veins ree abundances and patterns and possibly cosmic ray exposure ages 5 7 ma momument draw and yamato 74063 retain rare relict chondrules phosphates are associated with fe ni fes veins or form separate veins in monument draw and acapulco heating and cooling of acapulcoites occurred very early in the history of the solar system as evidenced by the 39ar 40ar ages of 4 51 ga these ages appear distinctly younger than the likely formation time for acapulco of 4 557 ga but are older than analogous 39ar 40ar ages for most chondrites acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions heating to 950 1000 c resulted in melting at the fe ni fes cotectic but silicates did not melt silicate textures resulted from extensive solid state recrystallization heating was by noncollisional sources e g 26al electromagnetic induction despite uncertainties owing to a lack of data acapulcoites may have experienced a three stage thermal history of slow cooling at high temperature rapid cooling at intermediate temperatures and slow cooling at low temperatures possibly resulting from breakup and gravitational reassembly of the parent body the complex thermal history is also reflected in disequilibrium ree abundances one or at most two impact events 7 ma and possibly 5 ma ago are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist

the cooling history of the acapulco meteorite as recorded by the 244pu and 40ar 39ar chronometers 244pu fission track densities recorded in phosphates merrillite and apatite and in orthopyroxenes adjacent to phosphates along with 40ar 39ar dating were used to retrace the cooling evolution in the low temperature regime of the acapulco meteorite over a time interval of 110 ma high resolution 40ar 39ar dating yielded a plateau age of 4514 16 ma and a precise 40ar closure temperature of 560 20 k in plagioclase this ar closure temperature is essentially indistinguishable from the 50 retention temperature of fission tracks registered in orthopyroxene 550 25 k this allows us to anchor the floating relative 244pu timescale to the absolute timescale defined by the 40ar 39ar chronometer plutonium contents of bulk phosphates inferred from fission xenon are 2x higher than those inferred from tracks in the same phosphates reflecting the low track retention temperatures of phosphates however the pu contents from tracks in orthopyroxenes adjacent to phosphates agree with those from fission xenon suggesting similar retention temperatures and times our results together with an estimated peak metamorphic temperature 1300 k and the u pbpb datum of acapulco phosphates gopel et al 1992 outline an early thermochronological history of acapulco that is more detailed than obtained by earlier attempts and spans a time of 160 ma fast cooling in the high temperature regime 1300 720 k of 100 40 k ma was followed by a drastic change of the cooling rate between 720 and 560 k 3 7 k ma down to a very slow cooling 1 7 0 5 k ma from 550 to 360 k the fast cooling at high temperatures shared also by other acapulcoite lodranites a l suggests that the parent body of acapulco was distinctly smaller than the h chondrite asteroid in contrast the very slow cooling in the low temperature regime should imply that acapulco like material became effectively insulated during a later stage of its history 6 ma after the asteroid formation if a formation time of 4565 ma is assumed for the a l body extrapolations back in time of the pu contents of phosphates give initial 244pu 238u ratios of 0 4 10 2 and 11 10 2 for apatite and merrillite respectively for the bulk phosphates an initial pu u value of 0 8 0 16 10 2 is obtained for acapulco which is in full agreement with the preferred chondritic ratio of 0 7 10 2 proposed by hagee et al 1990 trapped ar contents are surprisingly high both primordial 36ar at levels of type 4 ordinary chondrites and excess radiogenic 40ar were detected in our sample as in some acapulcoites by other authors this partial closed system behaviour could be the result of the partial melt and sintering effects that the high peak metamorphic temperatures have induced in a l meteorites in addition the spread in the early peak temperatures 1300 1450 k as reflected by the diverse degrees of partial melting observed in a l meteorites suggests heterogeneously distributed short lived heating sources 26al60fe and disfavours the homogeneous heating expected from an electromagnetic induction arising from an active pre main sequence sun the possibility that some specific collisional event s with or without disruption of the a l planetesimal could satisfactorily explain the thermal histories and diverse characteristics of these meteorites appears difficult to envision in the absence of evident shock effects in most if not all a l meteorites

mt st helens evidence of increased magmatic gas component abstract this paper presents measurements of so 2 flux and ash leachate chemistry from mt st helens volcano during the period may 18 to july 22 which are in contrast to similar data from before may 18 comparison of post 18 may so 2 data with similar data from other volcanoes leads to the conclusion that between 15 and 25 may the so 2 flux reached typical magmatic levels ash leachate chemistry from the eruption of 18 may 22 june and 22 july also indicate a large magmatic gas component concentrations of cl and so 2 4 on may 18 ash increase with distance from the volcano and decrease with time leachate chemistry is used to estimate that a minimum of 2 4 10 5 tons of s were released on may 18 low f concentrations on the ash though increasing present to health hazards

porphyry copper deposit formation by sub volcanic sulphur dioxide flux and chemisorption the processes that create economic grade accumulations of metals above magma chambers are unclear high temperature laboratory experiments show that rapid reactions between magmatic gases and earth s crust can trigger efficient metal deposition

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

compositional heterogeneity of tephras from the 1980 eruptions of mount st helens samples collected at hourly intervals on may 18 19 1980 at three sites 200 km downwind from mount st helens have made possible a detailed reconstruction of the conditions that contribute to the compositional heterogeneity of mineral and glass components observed in distal tephra layers the air fall tephra deposited at the sites during the first 7 hours of the may 18 eruption is mostly coarse grained microlite rich nonjuvenile glass and feldspar grain size maxima in this initial tephra can be related to the cataclysmic blast at 0832 and a subsequent pulse of the eruption at 1200 juvenile microlite free glass increases in relative abundance at the sampling sites beginning at about 1900 such a change between nonjuvenile and juvenile tephra can be related to a 5 km increase in column height associated with the last major pulse of the eruption which occurred at 1700 at the volcano electron microprobe study of both microlite rich and microlite free pumice in the time series samples reveals significant compositional differences interstitial glass in nonjuvenile pumice deposited during the first few hours at the sampling sites is enriched in sio2 and k2o and depleted in tio2 feo and mgo relative to juvenile glass by comparison major element composition of the least evolved juvenile glass sampled during the last several hours of the eruption displays a slight trend toward less evolved composition least squares calculations suggest that the more evolved character of the nonjuvenile glass can be explained by greater fractional crystallization brought about by enhanced cooling in a cryptodome prior to eruption whereas the temporal changes observed in juvenile glass composition during the last several hours of the eruption suggest the presence of a small slightly zoned magma chamber at depth electron microprobe study of glass coated ilmenites magnetites and plagioclases provides the following estimates of the physical conditions in this reservoir 865 50 c ph2o 2 2 kbar and log o2 11 7 analyses of bulk pumice glass and selected mineral phases from may 25 june 12 july 22 and october 16 18 pumices erupted from mount st helens indicate that the bulk pumice magma compositions have become slightly more andesitic with time while mineral and co existing glass compositions have changed significantly in post may 18 eruptions with both being more highly evolved than those associated with the may 18 eruption an application of the magnetite ilmenite geothermometer to june 12 and july 22 samples indicates temperatures of 919 30 c and 930 50 c respectively least squares calculations suggest that such evolved post may 18 glass and mineral phases can be derived by fractional crystallization of a magma composition like bulk may 18 pumice into approximately 50 crystals and 50 residual liquid such partitioning between crystals and residual liquid appears to have occurred on the scale of centimeters and is interpreted as a consequence of accelerated crystallization under reduced water pressure

experimental investigations of trace element fractionation in iron meteorites ii the influence of sulfur abstract we have investigated the partitioning of ir ge ga w cr au p and ni between solid metal and metallic liquid as a function of temperature and s concentration of the metallic liquid partition coefficients for siderophile elements such as ir w ga and ge increase by factors of 10 100 as the sconcentration of the metallic liquid increases from 0 30 wt partition coefficients for other siderophile elements such as ni au and p increase by only factors of 2 3 in contrast partition coefficients for the more chalcophile element cr decrease these experimentally determined partition coefficients have been used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of ni p au and ir during the magmatic evolution of groups iiab iiiab iva and ivb iron meteorites the mean s concentration for each group increases in the order ivb iva iiiab iiab in accord with cosmochemical prediction however we are unable to reproduce the geochemical behavior of ge ga w and cr in an internally consistent way we conclude that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed

siderophile element systematics of iab complex iron meteorites new insights into the formation of an enigmatic group siderophile trace element abundances and the 187re 187os isotopic systematics of the metal phases of 58 iab complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related close adherence of 187re 187os isotopic data of most iab iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental disturbance soon after formation minor presumably late stage open system behavior however is observed in some members of the slm slh shl and shh subgroups the new siderophile element abundance data are consistent with the findings of prior studies suggesting that the iab subgroups cannot be related to one another by any known crystallization process equilibrium crystallization coupled with crystal segregation solid liquid mixing and subsequent fractional crystallization can account for the siderophile element variations among meteorites within the iab main group mg the data for the slm subgroup are consistent with equilibrium crystallization combined with crystal segregation and mixing by contrast the limited fractionation of siderophile elements within the sll subgroup is consistent with metal extraction from a chondritic source with little subsequent processing the limited data for the other subgroups were insufficient to draw robust conclusions about crystallization processes involved in their formation collectively multiple formational processes are represented in the iab complex and modeling results suggest that fractional crystallization within the mg may have been a more significant process than has been previously recognized

leaching characteristics of ash from the may 18 1980 eruption of mount st helens volcano washington leaching of freshly erupted air fall ash unaffected by rain from the may 18 1980 eruption of mount st helens volcano washington shows that ca2 na mg2 so 4 2 and cl are the predominant chemical species released on first exposure of the ash to water extremely high correlation of ca with so4 and na with cl in water leachates suggests the presence of caso4 and nacl salts on the ash the amount of water soluble material on ash increases with distance from source and with the weight fraction of small less than 63 micrometers ash particles of high surface area this suggests that surface reactions such as adsorption are responsible for concentrating the soluble material caso4 nacl and other salts are probably formed as microscopic crystals in the high temperature core of the eruption column and are then adsorbed by silicate ash particles

volcanic ash leachates a review and recommendations for sampling methods tephra in plumes can scavenge and thereby rapidly deposit volatiles including sulphur halogen and metal species these may then be leached e g by rainfall potentially releasing heavy loadings to soils and water bodies several eruptions have resulted in contamination of pasture sometimes with serious impacts on livestock water quality has also been an issue in some areas affected by tephra fall this work synthesises the literature on volcanic ash leachates and considers the controls on volatile adsorption general trends emerge for basaltic intermediate and silicic tephra as well as for variable particle size and transport distance the applications of ash leachate data to plume gas geochemistry calculation of volatile budgets and environmental impact assessment are evaluated comparisons for different eruptions are hampered by disparities in leachate analysis techniques a standardised methodology is therefore proposed to facilitate future health impact assessment and volcanological interpretation of results from different sites

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

a user friendly one dimensional model for wet volcanic plumes 1 this paper presents a user friendly graphically based numerical model of one dimensional steady state homogeneous volcanic plumes that calculates and plots profiles of upward velocity plume density radius temperature and other parameters as a function of height the model considers effects of water condensation and ice formation on plume dynamics as well as the effect of water added to the plume at the vent atmospheric conditions may be specified through input parameters of constant lapse rates and relative humidity or by loading profiles of actual atmospheric soundings to illustrate the utility of the model we compare calculations with field based estimates of plume height 9 km and eruption rate 4 105 kg s during a brief tephra eruption at mount st helens on 8 march 2005 results show that the atmospheric conditions on that day boosted plume height by 1 3 km over that in a standard dry atmosphere although the eruption temperature was unknown model calculations most closely match the observations for a temperature that is below magmatic but above 100 c

comparative petrology of silicates in the udei station iab and miles iie iron meteorites implications for the origin of silicate bearing irons abstract the textures and mineral chemistries of silicate inclusions in the udei station iab and miles fractionated iie iron meteorites were studied using optical and electron microscopy sem empa and la icp ms techniques to better understand the origin of silicate bearing irons inclusions in udei station include near chondritic basaltic gabbroic feldspathic orthopyroxenitic and harzburgitic lithologies in miles most inclusions can be described as feldspathic pyroxenite or pyroxene enriched basalt gabbro the trace element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from udei station whereas in different inclusions from miles the compositions of orthopyroxene grains are similar while those of clinopyroxene plagioclase and especially cl apatite are variable orthopyroxene in miles tends to be enriched in ree compared to that in udei station but the reverse is true for plagioclase and clinopyroxene the data can be explained by models involving partial melting of chondritic protoliths silicate melt migration and redox reactions between silicate and metal components to form phosphate the extent of heating melt migration and phosphate formation were all greater in miles silicates in miles were formed from liquids produced by 30 partial melting of a chondritic precursor brought to a peak temperature of 1250 c this silicate melt crystallized in two stages during stage 1 crystallizing minerals orthopyroxene clinopyroxene chromite and olivine were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling with a residence time of 20 ka at 1150 c during stage 2 following probable re melting of feldspathic materials and after the silicate mush was mixed with molten metal plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus in udei station despite a lower peak temperature we suggest that a model involving major collisional disruption and mixing of partly molten endogenically heated planetesimals can best explain the data for iab and fractionated iie silicate bearing irons the extent of endogenic heating was different less for the iabs and the amount of parent body disruption was different scrambling with collisional unroofing for the iab iiicd winonaite body more complete destruction for the fractionated iie body but both bodies were partly molten and incompletely differentiated at the time of impact we suggest that the post impact secondary body for iab iiicd winonaite meteorites was mineralogically zoned with ni poor metal in the center and that the secondary body for fractionated iie meteorites was a relatively small melt rich body that had separated from olivine during collisional break up

simple shear of deformable square objects abstract finite element models of square objects in a contrasting matrix in simple shear show that the objects deform to a variety of shapes for a range of viscosity contrasts we catalogue the changing shapes and orientations of objects in progressive simple shear at moderate simple shear 1 5 the shapes are virtually indistinguishable from those in equivalent pure shear models with the same bulk strain r s 4 examined in a previous study in theory differences would be expected especially for very stiff objects or at very large strain in all our simple shear models relatively competent square objects become asymmetric barrel shapes with concave shortened edges similar to some types of boudin incompetent objects develop shapes surprisingly similar to mica fish described in mylonites

an experimental study of so 2 reactions with silicate glasses and supercooled melts in the system anorthite diopside albite at high temperature sulfur dioxide so2 g is the most abundant sulfur bearing volcanic gas species on earth from its magmatic origin at depth to expulsion at the surface via either persistent degassing or large explosive volcanic eruptions so2 g interacts with silicate materials at elevated temperatures similar high temperature reactions also occur in the volcanic systems and the atmospheres of venus the galilean moon io and in mars past as well in industrial flue gas processing we present an experimental investigation of the reaction between so2 g glasses and supercooled melts in the system anorthite diopside albite caal2si2o8 camgsi2o6 naalsi3o8 the samples were exposed to so2 g at 600 800 c for experimental durations of 10 min to 24 h the reactions resulted in the formation of sulfate coatings and modified the near surface composition of the silicate samples the predominant sulfate reaction product is caso4 with hydrated mgso4 or na2so4 also observed in some experiments in the anorthite diopside system the reaction extent strongly depends on the temperature relative to the glass transition temperature tg above tg in reactions with supercooled melts the reaction forms up to 20 times more sulfate the overall rate of sulfate formation is controlled by the diffusive flux of ca mg and na from the increasingly depleted silicate to the surface where the reaction with so2 g occurs the sulfate forming reaction results in a volume increase relative to the unreacted silicate when this reaction occurs in the subvolcanic environment it causes an increased molar volume that may close veins reducing the permeability and decrease the so2 g flux at the surface an increase in the so2 g flux would then result in the opening of new veins which may be accompanied by seismic activity additionally the change in molar volume may itself trigger seismicity the strong preferential uptake of ca into the sulfate reaction product results in a si and al enriched silicate in the sulfate the ca component may be mobilized by secondary processes such as through the interaction with meteoric fluids we recommend that the products of such gas solid reactions should be the object of remote and robotic investigations of planetary environments with volcanic histories such as on mars io venus and mercury

megaregolith insulation and the duration of cooling to isotopic closure within differentiated asteroids and the moon ages determined for extraterrestrial samples by the sm nd and rb sr techniques are commonly assumed to record igneous crystallization events because in solid silicates nd and sr diffuse at exceedingly slow rates however we find that for coarse grained igneous cumulate rocks from the moon or from a large thoroughly brecciated asteroid this assumption may not be reliable the moon and at least one asteroid the parent body of the eucrite diogenite and howardite meteorites appear to have been largely molten at or about the time they formed we have modeled global cooling of the moon and large r 40 250 km asteroids starting at or near the solidus a crucial factor in determining the prevailing interval ic of cooling between igenous crystallization and isotopic closure for any given depth in the crust is the extent to which the body is insulated by a regolith megaregolith layer of porous fragmental impact debris given plausible assumptions regarding the thicknesses of such layers on the moon and the eucrite parent asteroid and regarding the radius of the eucrite asteroid our results indicate that deep crustal regions tend to remain above the nd and sr isotopic closure temperature for intervals that are long in comparison to the precision of modern nd and sr based age measurements and in comparison to suggested chronologic scenarios of global differentiation ic intervals of as long as 100 m y may be common among available samples of primordial deep crustal cumulates from both bodies chronologies for the gross solidification of the moon and the eucrite asteroid should allow for the possibility that any single age for a coarse grained plutonic or cumulate textured rock might be many tens of millions of years younger than the igneous crystallization age

surface area and volume measurements of volcanic ash particles using micro computed tomography micro ct a comparison with scanning electron microscope sem stereoscopic imaging and geometric considerations volcanic ash particles are important components of explosive eruptions and their surface textures are the subject of intense research characterization of ash surfaces is crucial for understanding the physics of volcanic plumes remote sensing measurements of ash and aerosols interfacial processes modelling transportation and deposition of tephra and characterizing eruptive styles a number of different methods have been used over the years to arrive at surface area estimates the more common methods include estimates based on geometric considerations geometric surface area and physisorption of gas molecules on the surface of interest physical surface area in this study micro computed tomography micro ct which is a non destructive method providing three dimensional data enabled the measurement of surface area and volume of individual ash particles results were compared with the values obtained from sem stereoscopic imaging and geometric considerations surface area estimates of micro ct and sem stereoscopic imaging are similar with surface area volume ratios sa v of 0 0368 and 0 0467 respectively ash particle surface textures show a large deviation from that of simple geometric forms and an approximation both to spheres and ellipsoids do not seem adequate for the representation of ash surface sem stereoscopic and or micro ct imaging are here suggested as good candidate techniques for the characterization of textures on macro pore regions of ash particles

thermoluminescence sensitivity and thermal history of type 3 ordinary chondrites eleven new type 3 0 3 1 chondrites and possible explanations for differences among h l and ll chondrites we review induced thermoluminescence tl data for 102 unequilibrated ordinary chondrites uocs many data just published in abstracts in order to identify particularly primitive uocs and further explore tl systematics that may have implications for the history of the chondrites and their parent body we have identified 11 uocs of petrologic types 3 0 3 1 adrar 003 elephant moraine eet 90066 eet 90161 grosvenor mountains gro 95502 lewis cliff lew 88477 meteorite hills met 96503 yamato y 790787 y 791324 y 791558 y 793565 and y 793596 these samples represent an important new resource for researchers interested in the nature of primitive solar system materials previously reported trends in which tl sensitivity increases with tl peak temperature and tl peak width which we interpret in terms of crystallization of feldspar in the ordered or disordered forms during metamorphism are confirmed by the new data importantly the present data strengthen the trend described earlier in which the mean level of metamorphism experienced by uocs increases along the series ll l and h this suggests either different burial depths for the uocs from each class or formation at similar depths in regoliths of different thickness

smnd systematics of a silicate inclusion in the caddo iab iron meteorite we report the first sm nd data from an iron meteorite silicate inclusion mineral separates from an inclusion in the caddo iab iron define a 147 sm 143 nd age of 4 53 0 02 ga and an initial 146 sm 144 sm of 0 0086 0 0021 a plagioclase separate from the inclusion yields the lowest 142 nd 144 nd ratio measured to date which provides an absolute maximum initial solar system 142 of 2 1 0 4 when the 146 sm 142 nd data are combined with the 147 sm 143 nd data we obtain our preferred initial solar system 142 value of 3 2 u which corresponds to an initial 146 sm 144 sm of 0 009 rare earth element data from the caddo silicate inclusion suggest that it was derived by partial melting of a chondritic source prior to incorporation into the metal therefore differentiation of the silicate portion of the parent body must have started approximately within the first 30 ma of solar system history the data reported here clearly show that iab meteorites preserve a record of very early planetary differentiation processes modeling of element diffusion indicates that if the silicate material was incorporated into its feni host at a time much later than 4 53 ga then the mixing must have taken place at shallow levels i e 50 m in the parent body in order to preserve the sm nd systematics it is more likely that incorporation of the silicate took place at depths greater than 2 km and that it occurred within the first 30 ma of solar system history this is consistent with estimates of cooling rates determined for iab irons

fate and agricultural consequences of leachable elements added to the environment from the 2011 cordon caulle tephra fall abstract the june 2011 eruption of cordon caulle volcano chile dispersed tephra over 350 000 km 2 including productive agricultural land this resulted in the death of nearly one million livestock two distinct environments were affected a proximal temperate andean setting and the semi arid argentine steppe farther from the volcano the purpose of this study was to better understand the fate and agricultural consequences of leachable elements added to the environment by this large silicic tephra fall tephra soil and surface water samples across the depositional area were collected both immediately after the eruption tephra and water and nine months afterwards tephra soil and water tephra samples were analysed following a new hazard assessment protocol developed by the international volcanic health hazard network ivhhn water extractable element concentrations in freshly collected tephra were very low to low compared to other eruptions and showed no trends with distance from the volcano surface water analyses suggested short term changes to water composition due to the release of elements from tephra no effect on the fertility of soils underlying tephra was apparent after nine months water extractable fluorine f in freshly collected tephra ranged from 12 to 167 mg kg with a median value of 67 mg kg based on parallels with the 11 12 october 1995 eruption of ruapehu volcano new zealand we conclude that f toxicity was a possible contributing factor to the large scale livestock deaths as well as to chronic fluorosis widely reported in wild deer populations across the cordon caulle tephra depositional area finally we recommend that effective response to widespread tephra fall over agricultural areas should include 1 rapid statistically representative field sampling of tephra soils surface water supplies and forage crops 2 analysis using appropriate and reliable laboratory methods 3 modelling both short and long term impacts on the ecosystem especially for elements that may generate chronic hazard 4 timely dissemination of results to agricultural agencies 5 longitudinal sampling and monitoring to adapt impact models and 6 developing reliable animal fatality diagnoses through autopsies and chemical analysis

ar ar and i xe ages and the thermal history of iab meteorites studies of several samples of the large caddo county iab iron meteorite reveal andesitic material enriched in si na al and ca which is essentially unique among meteorites this material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening such an origin implies extended metamorphism of the iab parent body new 39ar 40ar ages for silicate from three different caddo samples are consistent with a common age of 4 50 4 51 gyr less well defined ar ar degassing ages for inclusions from two other iabs eet elephant moraine 83333 and udei station are 4 32 gyr whereas the age for campo del cielo varies considerably over about 3 23 4 56 gyr new 129i 129xe ages for caddo county and eet 83333 are 4557 9 0 1 myr and 4557 4560 myr respectively relative to an age of 4562 3 myr for shallowater considering all reported ar ar degassing ages for iabs and related winonaites the range is 4 32 4 53 gyr but several iabs give similar ar ages of 4 50 4 52 gyr we interpret these older ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40ar diffusion loss the older ar ar ages for iabs are similar to sm nd and rb sr isochron ages reported in the literature for caddo county considering the possibility that iab parent body formation was followed by impact disruption reassembly and metamorphism e g benedix et al 2000 the ar ar ages and iab cooling rates deduced from ni concentration profiles in iab metal herpfer et al 1994 are consistent if the time of the postassembly metamorphism was as late as about 4 53 gyr ago however i xe ages reported for some iabs define much older ages of about 4558 4566 myr which cannot easily be reconciled with the much younger ar ar and sm nd ages an explanation for the difference in radiometric ages of iabs may reside in combinations of the following a i xe ages have very high closure temperatures and were not reset during metamorphism about 4 53 gyr ago b a bias exists in the 40k decay constants which makes these ar ar ages approximately 30 myr too young c the reported sm nd and rb sr ages for caddo are in error by amounts equal to or exceeding their reported 2 sigma uncertainties and d about 30 myr after the initial heating that produced differentiation of caddo silicate and mixing of silicate and metal a mild metamorphism of the iab parent body reset the ar ar ages

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

palladium silver chronology of iab iron meteorites abstract the extinct 107pd 107ag decay system half life 6 5 ma is a useful chronometer to constrain the thermal evolution of the iab parent body to this end pd ag concentrations and the ag isotope compositions of metals separated from 6 different iab iron meteorites were determined the samples show e107ag variations between 0 1 and 15 8 with 108pd 109ag ratios between 38 and 200 the data can be divided into two groups based on their petrology each defining an isochron a graphite and troilite rich inclusion bearing group a with the iab meteorites toluca odessa and canyon diablo and a more silicate rich group b which includes campo del cielo caddo county and goose lake using the initial abundance of 107pd derived from carbonaceous chondrites the corresponding age for the group a is 18 7 3 6 5 0 ma after the start of the solar system and 14 9 2 5 4 9 ma for the group b this suggests that the last thermal event to reach high enough temperatures to melt metal on the iab parent body occurred within the first 15 ma of our solar system

how does the hot core of a volcanic plume control the sulfur speciation in volcanic emission 1 volcanic eruptions inject sulfur into the atmosphere mainly in form of so2 and h2s the ratio of these species h2s so2 which is usually used as the mirror of the oxidation state of the source magma varies significantly according to the type of activity tectonic setting etc this study aims to investigate the role of the hot core of plinian and sub plinian volcanic plumes t 600 c in sulfur speciation based on the thermodynamic equilibrium in this temperature range we consider the hot core as a box model in which 1000 c magmatic gas and 25 c ambient air are mixed and show that it functions as a hot oxidizing reactor for s species processes inside the hot core usually decrease the h2s content of the system but can either increase or decrease so2 depending on initial oxidation state thus the so2 injected into the atmosphere is not essentially generated directly from the magma but it can be produced in the hot core as the result of h2s oxidation besides volcanic cloud compositions do not mirror the source conditions considering three types of tectonic settings convergent plate divergent plate and hot spot we propose that h2s emission is more likely under reduced conditions in divergent plate and hot spot volcanic settings

metal phases of l chondrites their formation and evolution in the nebula and in the parent body abstract metal phases of six three equilibrated and three unequilibrated l chondrites were studied by inaa sem and mossbauer spectroscopy characteristics retained in the bulk metals of unequilibrated chondrites abundant carbon high contents of cr v mn and low contents of w mo and ga compared to metals of equilibrated chondrites less enrichment of w than mo and fractionation of co from ni demonstrate that chondrite metals are not nebular condensates all those characteristics can be well explained by melting chondrite metals are not melting remnants of previously condensed metals rather they were produced by reduction of ci or cm like material during the melting process the complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the low temperature diffusion products and must have been developed in the chondrite parent body the difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body thus while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity the crystallization of eoc silicates resulted from an external heating the internal metamorphism was mild 400 600 c and long whereas the external heating was intense with a maximum temperature in range of 800 950 c and short tetrataenite is present not only in uocs but also in eocs suggesting that the external heating occurred during the internal metamorphism i e within 100 myr of chondrite formation

trace element inventory of meteoritic ca phosphates most extraterrestrial samples feature the two accessory ca phosphates apatite group minerals and merrillite which are important carrier phases of the rare earth elements ree the trace element concentrations ree sc ti v cr mn co as rb sr y zr nb ba hf ta pb th and u of selected grains were analyzed by la icp ms and or sims ree only this systematic investigation includes 99 apatite and 149 merrillite analyses from meteorites deriving from various asteroidal bodies including 1 carbonaceous chondrite 8 ordinary chondrites 3 acapulcoites 1 winonaite 2 eucrites 5 shergottites 1 ureilitic trachyandesite 2 mesosiderites and 1 silicate bearing iab iron meteorite although ca phosphates predominantly form in metamorphic and or metasomatic reactions some are of igneous origin as late stage phases that often incorporate the vast majority of their host s bulk ree budget the investigated ca phosphates have ree enrichments of up to two orders of magnitude compared to the host rock s bulk concentrations within a single sample each phosphate species displays a uniform ree pattern and variations are mainly restricted to their enrichment therefore indicating similar formation conditions exceptions are brecciated samples i e the adzhi bogdo ll3 6 ordinary chondrite despite this uniformity within single samples distinct meteorite groups do not necessarily have unique ree patterns four basic shapes dominate the ree patterns of meteoritic ca phosphates 1 flat patterns smoothly decreasing from la lu with prominent negative eu anomalies acapulcoites eucrites apatite from the winonaite and the ureilitic trachyandesite merrillite from ordinary chondrites 2 unfractionated patterns with only minor or no anomalies mesosiderites enriched shergottites iab iron meteorite 3 lree enriched patterns with either positive or slightly negative eu anomalies chondritic apatite and 4 strongly lree depleted patterns with negative eu anomalies depleted shergottites the patterns do not correlate with the grade of metamorphism petrologic type specific adjacent mineral assemblages or with ca phosphate grain size neither the proportions of different ree nor particular ree patterns themselves are universally correlated to a specific formation mechanism yet eu i e magnitude of the eu anomaly is a sensitive indicator to evaluate the timing of plagioclase and phosphate crystallization based on our data u and th abundances in apatite increase almost linearly with the grade of metamorphism as well as with the differentiation of their host rock

a petrologic and isotopic study of winonaites evidence for early partial melting brecciation and metamorphism we have conducted detailed petrologic chemical and isotopic studies of winonaites to ascertain the genesis of this group of meteorites winonaites have reduced mineral compositions and mineralogy and oxygen isotopic compositions distinct from primitive achondrite groups other than silicate inclusions in iab and iiicd irons however winonaites differ from iab and iiicd irons in that they lack the metallic matrices of the latter and consist mostly of silicates on the basis of these criteria winona mount morris wisconsin tierra blanca pontlyfni y 74025 y 75300 y 8005 and que 94535 are winonaites and y 75305 and y 75261 may be winonaites winonaites are fine to medium grained mostly equigranular rocks pontlyfni and mount morris wisconsin contain what appear to be relict chondrules several winonaites contain mm sized areas that differ substantially in grain size and or silicate mineralogy from the surrounding matrix fe ni fes veins are common in many winonaites mineral compositions are intermediate between e and h chondrites and reduced sulfides are observed in low feo winonaites bulk major element compositions are roughly chondritic although ree elements are fractionated the 39ar 40ar ages of three winonaites range from 4 40 ga mount morris wisconsin to 4 54 ga pontlyfni cosmic ray exposure ages are 20 80 ma trapped noble gases in these winonaites resemble those in enstatite chondrites we suggest that the winonaites formed from a chondritic precursor material unlike that of known chondrites in mineral and oxygen isotopic compositions and this material may have been heterogeneous in composition extensive heating caused metamorphism and partial melting of both fe ni fes and silicate material impact brecciation during cooling mixed lithologies with different thermal histories and subsequent metamorphism produced recrystallization grain growth and reduction of mafic silicates the 39ar 40ar ages indicate that cooling may have been more rapid than observed in iab irons although later resetting may have occurred

sulfide rich metallic impact melts from chondritic parent bodies sacramento wash 005 saw 005 meteorite hills 00428 met 00428 and mount howe 88403 how 88403 are s rich fe ni rich metal meteorites with fine metal structures and homogeneous troilite we compare them with the h metal meteorite lewis cliff 88432 phase diagram analyses suggest that saw 005 met 00428 and how 88403 were liquids at temperatures above 1350 c tridymite in how 88403 constrains formation to a high temperature and low pressure environment the morphology of their metal troilite structures may suggest that met 00428 cooled the slowest saw 005 cooled faster and how 88403 cooled the quickest saw 005 and met 00428 contain h chondrite like silicates and saw 005 contains a chondrule bearing inclusion that is texturally and compositionally similar to h4 chondrites the compositional and morphological similarities of saw 005 and met 00428 suggest that they are likely the result of impact processing on the h chondrite parent body saw 005 and met 00428 are the first recognized iron and sulfide rich meteorites which formed by impact on the h chondrite parent body which are distinct from the iie iron meteorite group the morphological and chemical differences of how 88403 suggest that it is not from the h chondrite body although it likely formed during an impact on a chondritic parent body

sedimentation from strong volcanic plumes 1 we develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes these are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy we describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind free environment new parameterizations are used to describe the wind field below the spreading current particle aggregation and particle density variations we conducted a broad study to investigate the effects of these processes and made comparisons with field observations the greatest variations resulted from wind advection below the spreading current which shifts the plume corner mass accumulation and the position of transitions in fallout regimes downwind particle aggregation strongly depends on the initial grain size distribution and significantly affects deposit thinning depending on the aggregate size and density and on the relative amount of aggregating particles small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field variation of particle density also affects the resulting thinning trend the model provides acceptable reproduction of observations of the propagation of the spreading current tested with the plinian phase of the 18 may 1980 eruption of mount st helens and of observed thinning of tephra fallout deposits tested with deposits from mount st helens 1980 askja d 1875 quizapu 1932 minoan eruption 3 6 ka b p novarupta a 1912 and hudson 1991

boudinage and shearband boudins a meso to micro scale tool in structural analysis abstract a classification of boudinage and boudins is made presenting a simplified parametric description of boudin block geometry end members the most important shearband boudins parameters ratios microstructural domains and internal evolutions are described and interpreted shearband boudins are related to a large number of geological meso and microstructures e g pinch and swell drawn or torn boudins mica fish porphyroclasts foliation boudinage tectonic lozenges and c s c several factors can control the asymmetric boudinage process such as the obliquity of a competent layer the orientation of an initial fracture boudin axial ratio the viscosity ratio between matrix and layer flow type and shear strain this review highlights the correlation of shearband boudins with other asymmetrically deformed structures and shows their geological relevance finally this study presents a phenomenological framework for shearband boudins

mathematical model for folding of layering near rigid objects in shear deformation abstract the asymmetric arrangement of deformed layering around rigid objects in shear zones is among the most reliable criteria for inferring shear sense in rock a mathematical model which simulates shear deformation of a ductile medium containing a rigid inclusion gives insight into the genesis of this type of shear sense indicator the model which is based on the velocity field solution for viscous flow around a sphere demonstrates that many common strtuctures can arise in a mechanically passive matrix as the model simulates increasing shear strain structures in the matrix pass through distinct evolutionary stages similar results have been obtained in experimental models suggesting that it may be possible to read shear strain magnitude from the geometry of deformed layering near clasts and porphyroclasts in sheared rocks