magnetism on the angrite parent body and the early differentiation of planetesimals angrites are among the oldest known pristine basaltic meteorites and record the earliest stages of planet formation and differentiation our paleomagnetic analysis of three angrites found that they record a past magnetic field of 10 microteslas on the angrite parent body extending from 4564 to at least 4558 million years ago because the angrite paleomagnetic record extends beyond the expected lifetime of the early circumstellar disk these paleofields were probably generated internally on the angrite parent body possibly by an early dynamo in a rapidly formed metallic core

the crystallization of lava lakes thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above we show that such kinetic undercooling is sufficient to drive convection at high rayleigh numbers even in shallow lava lakes we then employ a theoretical model that includes the effects of convection at high rayleigh number to make predictions of the complete evolution of the lava we show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection however convective motions do shorten the time for complete solidification because when they are driven by kinetic undercooling there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava this important effect which is absent from purely conductive models predicts stratification and zonation of the lava in agreement with field observations we also examine the effect of taking into account the heat transfer to the country rock below the lava lake and determine how the evolution of the lava lake varies with the viscosity of the lava

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

importance of the accretion process in asteroid thermal evolution 6 hebe as an example widespread evidence exists for heating that caused melting thermal metamorphism and aqueous alteration in meteorite parent bodies previous simulations of asteroid heat transfer have assumed that accretion was instantaneous for the first time we present a thermal model that assumes a realistic incremental accretion scenario and takes into account the heat budget produced by decay of 26al during the accretion process by modeling 6 hebe assumed to be the h chondrite parent body we show that in contrast to results from instantaneous accretion models an asteroid may reach its peak temperature during accretion the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface and the volume of high grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core we show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities and thereby highlight the importance of the accretion process in shaping an asteroid s thermal history incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities peak temperatures cooling rates radioisotope closure times in meteorites that were determined by their thermal histories

26al in plagioclase rich chondrules in carbonaceous chondrites evidence for an extended duration of chondrule formation abstract the 26al 26mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase rich chondrules prcs from 13 carbonaceous chondrites ccs one ungrouped acfer 094 six cr five cv and one co reveal large variations in the initial 26al 27al ratio 26al 27al 0 well resolved 26mg excesses 26mg from the in situ decay of the short lived nuclide 26al t1 2 0 72 ma were found in nine chondrules two from acfer 094 five from the cv chondrites allende and efremovka and one each from the paired cr chondrites eet 92147 and eet 92042 with 26al 27al 0 values ranging from 3 10 6 to 1 5 10 5 data for seven additional chondrules from three cv and two cr chondrites show evidence suggestive of the presence of 26al but do not yield well defined values for 26al 27al 0 while the remaining chondrules do not contain excess radiogenic 26mg and yield corresponding upper limits of 11 2 10 6 for 26al 27al 0 the observed range of 26al 27al 0 in prcs from ccs is similar to the range seen in chondrules from unequilibrated ordinary chondrites uocs of low metamorphic grade 3 0 3 4 however unlike the uoc chondrules there is no clear trend between the 26al 27al 0 values in prcs from ccs and the degree of thermal metamorphism experienced by the host meteorites high and low values of 26al 27al 0 are found equally in prcs from both ccs lacking evidence for thermal metamorphism e g crs and ccs where such evidence is abundant e g cvs the lower 26al 27al 0 values in prcs from ccs relative to most cais are consistent with a model in which 26al was distributed uniformly in the nebula when chondrule formation began approximately a million years after the formation of the majority of cais the observed range of 26al 27al 0 values in prcs from ccs is most plausibly explained in terms of an extended duration of 2 3 ma for the formation of cc chondrules this interval is in sharp contrast to most cais from ccs whose formation appears to be restricted to a narrow time interval of less than 105 years the active solar nebula appears to have persisted for a period approaching 4 ma encompassing the formation of both cais and chondrules present in ccs and raising important issues related to the storage assimilation and mixing of chondrules and cais in the early solar system

evolution of double diffusion convection in a felsic magma chamber abstract the onset of double diffusion convection ddc is modeled in a two dimensional case in respect to magma chambers the viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component h 2 o the upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary the case of positive compositional and thermal buoyancy of melt is studied assuming a h 2 o diffusion coefficient small in comparison with thermal diffusivity the ddc has been modeled using a system of equations solved by the finite difference method on a square grid the convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing due to the higher viscosity in the upper zone the upper boundary layer is thicker than the lower one the variation of water concentration in this zone of the convective cell can be significant in nature the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt e g biotite because of a high lewis number 100 temperature variations in the magma chamber decay much faster than the water concentration in this case the intensive convection can continue at a constant temperature due to the non zero water content in the chamber in principle the effect can be applied to the formation of magmatic bodies if the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature water content throughout the body may still remain variable

the role of magma buoyancy on the eruption of lunar basalts it has long been recognized that mare basalts on the moon are preferentially located both on the earth facing hemisphere and within large impact basins a popular model that accounts for this observation assumes that these magmas were denser than the lunar crust that they accumulated at the crust mantle interface and that eruptions occurred only when this magma chamber became overpressurized in this paper we re evaluate this model and argue that it is not consistent with the available data nor with models of dike propagation as an alternative hypothesis we propose that magma buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead we have computed the densities of mare basaltic magmas and find that some are in fact less dense than the moon s upper anorthositic crust based on the widely accepted view that the lunar crust becomes more mafic with depth we also show that all mare basaltic magmas should be less dense than the lower portion of the crust thus if the upper anorthositic crust was regionally removed by an impact event then any mare basaltic magma could have risen to the surface there based on buoyancy considerations alone in support of this model we note that mare basalts are indeed found wherever geophysical crustal thickness models predict the upper crust to be absent furthermore many of the basalts that erupted within the anorthositic highlands are predicted to be less dense than the underlying crust based on remote sensing data the high titanium flows within oceanus procellarum are somewhat problematical to our model in that an anorthositic crust is predicted to be present beneath them using results from recent lunar thermal models we suggest that these magmas may have overcome their negative buoyancy in the crust by possessing superliquidus temperatures if magma buoyancy does indeed control whether or not a basaltic eruption will occur then this implies that the quantity of magma produced beneath the south pole aitken basin was about 10 times less than that of the nearside fl 2001 elsevier science b v all rights reserved

a thermal model for the differentiation of asteroid 4 vesta based on radiogenic heating a finite element code has been developed to model the thermal history of asteroid 4 vesta this is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained the results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from vesta the work solves two major problems with the hypothesis of heating by decay of26al an extinct radionuclide postulated to be a plausible heat source in the early solar system first the model demonstrates that it is possible to keep the mantle of vesta hot for 100 ma thereby explaining the observed difference in ages between cumulate and noncumulate eucrites second the simulation offers a possible explanation of why detectable excesses of26mg the decay product of26al are not observed in noncumulate eucrites the simulation draws a model chronology of vesta and predicts times relative to cai formation for accretion at 2 85 myr core formation at 4 58 myr crust formation at 6 58 myr and geochemical closure at 100 myr for a h chondrite asteroidal bulk composition decay of60fe is found to cause no perceptible difference in the thermal history of vesta even when sequestered into a central core although chondritic xenoliths have not been described in hed igneous lithologies the thermal model suggests the possibility that a veneer of unmelted near surface material should remain

formation of tube pumice structure under pure shear insights from extension tests of solidifying foam abstract tube pumice the bubbles of which are highly elongated in one direction is a common product of explosive silicic eruptions especially caldera forming eruptions to understand the conditions necessary to generate tube like bubbles we performed extensional experiments on polyurethane foam which experiences vesiculation deformation and solidification at room temperature the strain and strain rate of pure shear deformation were controlled by the pull length and pull rate of the extension respectively the shapes of bubbles in the solidified polyurethane foam were analyzed in three dimensions by x ray computed tomography the experimental results demonstrate that strain and capillary number control the degree of bubble deformation highly deformed tube like bubbles were found in the experiments with large strain and large capillary number in each experimental sample there is scatter in the relation between the degrees of bubble deformation and expected capillary numbers which we interpret to be the result of the inhomogeneous shear field around individual bubbles and bubble coalescence despite the large scatter the average deformation degree is predicted well by the dynamics of a single bubble where the viscous stress acting on the bubble surface is taken as the product of bulk strain rate and effective viscosity using this approach the average deformation degree of experimental bubbles in each sample was approximately calculated from the deformation model of a single bubble at least in the range of pure shear strain smaller than 1 5 small bubbles in the samples extended at low viscosities tend to be less elongated than those calculated from the deformation model due to shape relaxation the effect of shape relaxation is quantified by a new dimensionless number i e the quench number which represents the ratio of the timescale of increasing viscosity and shape relaxation our experiment suggests that bubble shapes in highly vesicular pumice can be used to analyze their viscosity and strain rate during the elongation process an analysis of the published data was presented as a test case

thermal diffusivity of rhyolitic glasses and melts effects of temperature crystals and dissolved water thermal diffusivity d was measured using laser flash analysis on pristine and remelted obsidian sam ples from mono craters california these high silica rhyo lites contain between 0 013 and 1 10 wt h2o and 0 to 2 vol crystallites at room temperature dglass varies from 0 63 to 0 68 mm 2 s 1 with more crystalline samples having higher d a st increases dglass decreases approaching a constant value of 0 55 mm 2 s 1 near 700 k the glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity dissolved water contents up to 1 1 wt have no statistically significant effect on the thermal diffusivity of the glass upon crossing the glass transition d decreases rapidly near 1 000 k for the hydrous melts and 1 200 k for anhydrous melts rhy olitic melts have a dmelt of 0 51 mm 2 s 1 thermal conduc tivity k0d cp of rhyolitic glass and melt increases slightly with t because heat capacity cp increases with t more strongly than density and d decrease the thermal conductivity of rhyolitic melts is 1 5 wm 1 k 1 a nd should vary little over the likely range of magmatic temperatures and water contents these values of d and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian and those that crystallize results from the release of latent heat of crystallization latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies the time to solidification is similar in both cases however because solidification requires cooling through the glass transition in the first case and cooling only to the solidus in the second

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

radiative heat transfer in molten and glassy obsidian we have measured optical transmittance spectra in rhyolitic obsidian samples in the wavelength range 380 5500 nm and at temperatures t from 19 1145 c above and below the softening point from the transmittance we calculated the absorption coefficient t and the radiative thermal conductivity kr t kr ranged from 3 10 3 cal cm 1 s 1 k 1 1 2 w m 1 k 1 at 700 c to 12 10 3cal cm 1 s 1 k 1 5 w m 1k 1 at 1145 c the 700 c value is comparable with lattice thermal conductivity klof about 4 10 3 cal cm 1 s 1k 1 1 7 w m 1k 1 removing scattering effects due to bubbles from the transmittance spectra by lowering the absorption baseline increased kr to 20 10 3 cal cm 1 s 1 k 1 8 4 w m 1 k 1 at 1145 c because scattering by bubbles is likely to be small in confined magmas these numbers are probably minimum values for kr and indicate that in active plutons radiative heat transport could be greater than lattice conductivity by more than a factor of 2 at 1000 c thus melting markedly strengthens kr and radiative heat transport is probably the dominant component of the total conductivity k kl kr in silicic magmas these relatively large values of k can be applied to models of the thermal evolution of magma bodies and to cooling of intrusives

thermal properties of vesicular rhyolite abstract thermal diffusivity of rhyolite melt and rhyolite foam 70 80 porosity has been measured using the radial heat transfer method cylindrical samples length 50 55 mm diameter 22 mm of rhyolite melt and foam have been derived by heating samples of little glass mountain obsidian using available data on heat capacity and density of rhyolite melt the thermal conductivity of samples has been determined the difference in thermal conductivity between rhyolite melt and foam at igneous temperatures 1000 c is about one order of magnitude the effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

26al 26mg systematics of ca al rich inclusions amoeboid olivine aggregates and chondrules from the ungrouped carbonaceous chondrite acfer 094 we report in situ magnesium isotope measurements of 7 porphyritic magnesium rich type i chondrules 1 aluminum rich chondrule and 16 refractory inclusions 14 ca al rich inclusions cais and 2 amoeboid olivine aggregates aoas from the ungrouped carbonaceous chondrite acfer 094 using a cameca ims 6f ion microprobe both aoas and 9 cais show radiogenic 26mg excesses corresponding to initial 26al 27al ratios between 5 x 10 5 and 7 x 10 5 suggesting that formation of the acfer 094 cais may have lasted for 300 000 years four cais show no evidence for radiogenic 26mg three of these inclusions a corundum rich a grossite rich and a pyroxene hibonite spherule cai are very refractory objects and show deficits in 26mg suggesting that they probably never contained 26al the fourth object without evidence for radiogenic 26mg is an anorthite rich igneous type c cai that could have experienced late stage melting that reset its al mg systematics significant excesses in 26mg were observed in two chondrules the inferred 26al 27al ratios in these two chondrules are 10 3 7 4 10 6 and 6 0 3 8 10 6 errors are 2 suggesting formation and myr after cais with the canonical 26al 27al ratio of 5 10 5 these age differences are consistent with the inferred age differences between cais and chondrules in primitive ordinary ll3 0 ll3 1 and carbonaceous co3 0 chondrites

cooling dynamics of spatter fed phonolite obsidian flows on tenerife canary islands abstract relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on tenerife to determine their thermal history across the glass transition the phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits the detailed sampling resolution within the two series enabled an accurate quantification of their thermal history cooling rates within the investigated spatter fed flows vary over more than two orders of magnitude the highest cooling rates of 0 39 k min were modeled for the central vesiculated part of one flow the dense basal obsidian layers of both flows were cooled at substantially lower rates of 0 0042 and 0 0028 k min respectively there appears to be an influence of in situ vesiculation processes on the thermal budget of the investigated flows in addition the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with quasi isothermal annealing within this decollement numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows the effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data in addition viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition

multiple parent bodies of ordinary chondrites abstract thermal histories of chondrite parent bodies are calculated from an initial state with material in a powder like form taking into account the effect of consolidation state on thermal conductivity the very low thermal conductivity of the starting materials makes it possible for a small body with a radius of less than 100 km to be heated by several hundred degrees even if long lived radioactive elements in chondritic abundances are the only source of heat the maximum temperature is determined primarily by the temperature at which sintering of the constituent materials occurs the thermal state of the interior of a chondrite parent body after sintering has begun is nearly isothermal near the surface however where the material is unconsolidated and the thermal conductivity is much lower the thermal gradient is quite large this result contradicts the conventional onion shell model of chondrite parent bodies but because the internal temperature is almost constant through the whole body it supports a multiple parent bodies model according to which each petrologic type of chondrite comes from a different parent body

particle water heat transfer during explosive volcanic eruptions 1 thermal interaction between volcanic particles and water during explosive eruptions has been quantified using a numerical heat transfer model for spherical particles the model couples intraparticle conduction with heat transfer from the particle surface by boiling water in order to explore heat loss with time for a range of particle diameters the results are combined with estimates of particle settling times to provide insight into heat removal during eruption from samples of volcanic particles produced by explosive eruption heat removal is restricted by resistance to heat transfer from the volcanic particles with intraparticle thermal conduction important for large particles and surface cooling by boiling dominating for small particles in most cases volcanic particles approach thermal equilibrium with the surrounding fluid during an explosive eruption application of the results to a sample from the gjalp 1996 iceland eruption indicates that relative to 0 c 70 80 of the heat is transferred from the particles to boiling water during the settling time before burial in the stratigraphic succession the implication is that for subglacial explosive eruptions much of the heat content of the magma is coupled into melting ice extremely rapidly if all particles of the gjalp 1996 deposit were cooled to the local boiling point by the end of the eruption then approximately 78 of the initial heat content was removed from the erupting magma during the eruption this is consistent with calorimetric calculations based on volumes of ice melted during and after the eruption

nonisothermal viscous sintering of volcanic ash volcanic ash is often deposited in a hot state volcanic ash containing glass deposited above the glass transition interval has the potential to sinter viscously both to itself particle particle and to exposed surfaces here we constrain the kinetics of this process experimentally under nonisothermal conditions using standard glasses in the absence of external load this process is dominantly driven by surface relaxation in such cases the sintering process is rate limited by the melt viscosity the size of the particles and the melt vapor interfacial tension we propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering we apply our model to viscous sintering scenarios for cooling crystal poor rhyolitic ash using the 2008 eruption of chaiten volcano as a case example we predict that moderate linear cooling rates of 0 1 c min 1 can result in the common observation of incomplete sintering and the preservation of pore networks

an early i xe age for cb chondrite chondrule formation and a re evaluation of the closure age of shallowater enstatite the iodine xenon system has been analyzed in samples of 7 chondrules from the cb chondrites gujba and hammadah al hamra hah 237 one sample from gujba defined a high temperature iodine xenon isochron corresponding to closure 1 87 0 4 ma before closure of shallowater enstatite motivated by this result we employ outlier rejection to re evaluate the shallowater age leading to a modified value of 4562 3 0 4 ma 1 in this process the datum obtained by combining our i xe age for gujba with the literature pb pb age is rejected as an outlier indicating that in this sample the i xe system closed earlier than the accepted pb pb age of chondrules from cb chondrites the need for a formation environment distinct from that of chondrules from other meteorites is thus reduced

discriminating secondary from magmatic water in rhyolitic matrix glass of volcanic pyroclasts using thermogravimetric analysis abstract pyroclasts from explosive eruptions such as the 1060 ce explosive glass mountain eruption of medicine lake volcano california contain large amounts of water this may be the consequence of diffusive rehydration of the volcanic glass by meteoric secondary water after the eruption discriminating between magmatic and secondary water in the matrix glass of pyroclasts is important because the degassing of magmatic water affects the intensity of volcanic eruptions such discrimination has remained a challenging problem especially because some aspects of water diffusion in silicate glasses at low temperatures and atmospheric pressure remain poorly constrained we used thermogravimetry to analyze the loss of water from natural volcanic glasses and glasses that were hydrated in the laboratory at magmatic temperatures and pressures numerical modeling of diffusive water loss during thermogravimetric analyses accounted for the interconversion of molecular water h 2 o m and hydroxyls groups oh and indicates that glass mountain pumices contain 0 2 0 5 wt primary water but gained 1 2 wt of meteoric water by diffusive rehydration during the past 950 years these results confirm that the majority of magmatic water is lost from the magma during explosive eruptions furthermore the integration of thermogravimetric analysis and numerical modeling facilitates discrimination between the magmatic and secondary water content of volcanic glasses

numerical simulations of the differentiation of accreting planetesimals with 26al and 60fe as the heat sources numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26al and 60fe as the heat sources planetesimal accretion was started at chosen times up to 3 ma after ca al rich inclusions cais were formed and was continued for periods of 0 001 1 ma the planetesimals were initially porous unconsolidated bodies at 250 k but became sintered at around 700 k ending up as compact bodies whose final radii were 20 50 100 or 270 km with further heating the planetesimals underwent melting and igneous differentiation two approaches to core segregation were tried in the first labelled a the core grew gradually before silicate began to melt and in the second labelled b the core segregated once the silicate had become 40 molten in a when the silicate had become 20 molten the basaltic melt fraction began migrating upward to the surface carrying 26al with it the 60fe partitioned between core and mantle the results show that the rate and timing of core and crust formation depend mainly on the time after cais when planetesimal accretion started they imply significant melting where accretion was complete before 2 ma and a little melting in the deep interiors of planetesimals that accreted as late as 3 ma the latest melting would have occurred at 10 ma the effect on core and crust formation of the planetesimals final size the duration of accretion and the choice of 60fe 56fe initial were also found to be important particularly where accretion was late the results are consistent with the isotopic ages of differentiated meteorites and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 ma after cais

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

two competing effects of volatiles on heat transfer in crystal rich magmas thermal insulation vs defrosting the build up and eruption of crystal rich ignimbrites commonly referred to as monotonous intermediates has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal rich magma bodies mushes in this study we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush intrusion system we solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension our calculations span a range of pressures from 1 to 4 kbar mush composition andesite to rhyolite and initial water contents from 1 5 to 6 wt the mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy and as a consequence by the amount of melting and the injection rate of volatiles from the intrusion to the mush we find that volatiles affect the melting of the mush or defrosting in two opposite ways depending on pressure and the initial water content of the intrusion when the intrusion volatile content is high and pressure relatively low 44 wt h2o at 2 kbar the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush for lower initial volatile contents 54 wt h2o in the intrusion and or higher pressure 3 4 kbar volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion we propose that the dual role played by volatiles during the thermal evolution of crystal rich magmas reheated by an underplating intrusion can explain the presence of crystal rich ignimbrites in arcs and their absence in drier hotspots or extensional regions

conditions for pore water convection within carbonaceous chondrite parent bodies implications for planetesimal size and heat production abstract the critical rayleigh number for convective flow of pore water through a permeable spherical body is derived the result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production if the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water then the present calculations suggest that the parent bodies were less than 80 km in diameter the maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water rock ratios in altered carbonaceous chondrites pore water would have flowed through parent bodies larger than 120 km in diameter with typical rock like permeabilities convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other

on the mechanics of caldera resurgence the mechanisms suggested to cause caldera resurgence can be categorized as 1 viscous rebound of magma 2 regional detumescence and 3 pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma the critical observation that resurgence takes of the order of 103 105 years insists that the causative mechanism be regulated by a highly viscous e g 1022 pa s structural member of the crust the relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale the most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation detumescence compresses the magma chamber squeezing magma upward against and through the caldera block vesiculation also compresses the chamber and tends to dome the caldera block the times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber but the rate of cooling of the chamber is proportional to its size because solidification spreads viscous relaxation over a wide region of the crust and curtails resurgence there is likely to be a minimum volume of magma that can undergo resurgent doming it is difficult to be exact about this critical size until more precise models have been developed

the use of a shear thinning polymer as a bubbly magma analogue for scaled laboratory experiments abstract analogue materials are commonly used in volcanology to perform scaled laboratory experiments analogue experiments inform on fundamental fluid dynamic structural and mechanical processes that are typically very difficult to observe and quantify directly in the natural volcanic system here we investigate the suitability of an aqueous solution of hydroxyethyl cellulose polymer hec for use as a lava magma analogue with a particular focus on its rheological behaviour we characterize a range of physical properties as functions of the concentration and temperature of the solution density specific heat capacity thermal diffusivity thermal conductivity surface tension as well as rheology hec has a non newtonian shear thinning rheology that depends on the concentration and temperature of the solution we show that the rheology is well described by the cross model which was originally developed for polymer solutions but has also been applied to bubbly magmas using this similarity an approach for scaling analogue experiments that use shear thinning polymers like hec to bubbly magma is presented a detailed workflow and a spreadsheet are provided to allow experimentalists to investigate the effects of non newtonian behaviour in their existing laboratory set ups this contribution will allow for the more complex but often more realistic case of bubble bearing magmas to be rigorously studied in experimental volcanology

progressive alteration in cv3 chondrites more evidence for asteroidal alteration abstract the oxidized cv3 chondrites can be divided into two major subgroups or lithologies bali like cv3oxb and allende like cv3oxa in which chondrules calcium aluminum rich inclusions cais and matrices show characteristic alteration features weisberg et al 1997 krot et al 1997d kimura and ikeda 1997 the cv3oxb lithology is present in bali kaba parts of the mokoia breccia and possibly in grosnaja and allan hills alh 85006 it is characterized by the presence of the secondary low ca phyllosilicates saponite and sodium phlogopite magnetite ni rich sulfides fayalite fa 90 ca fe rich pyroxenes fs10 50wo45 50 and andradite phyllosilicates replace primary ca rich minerals in chondrules and cais which suggests mobilization of ca during aqueous alteration magnetite nodules are replaced to various degrees by fayalite ca fe rich pyroxenes and minor andradite fayalite veins crosscut fine grained rims around chondrules and extend into the matrix thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures 300 c in the presence of aqueous solutions oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large 18ofayalite magnetite fractionation 20 we infer that phyllosilicates magnetite fayalite ca fe rich pyroxenes and andradite formed at relatively low temperatures 300 c by fluid rock interaction in an asteroidal environment secondary fayalite and phyllosilicates are virtually absent in chondrules and cais in the cv3oxa lithology which is present in allende and its dark inclusions axtell alha81258 alh 84028 lewis cliff lew 86006 and parts of the mokoia and vigarano breccias instead secondary nepheline sodalite and fayalitic olivine are common fayalitic olivine in chondrules replaces low ca pyroxenes and rims and veins forsterite grains it also forms coarse lath shaped grains in matrix secondary ca fe rich pyroxenes are abundant we infer that the cv3oxa lithology experienced alteration at higher temperatures than the cv3oxb lithology the presence of the reduced and cv3oxa lithologies in the vigarano breccia and cv3oxa and cv3oxb lithologies in the mokoia breccia indicates that all cv3 chondrites came from one heterogeneously altered asteroid the metamorphosed clasts in mokoia krot and hutcheon 1997 may be rare samples of the hotter interior of the cv asteroid we conclude that the alteration features observed in the oxidized cv3 chondrites resulted from the fluid rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced cv3 chondrites

effects of thermal quenching on mechanical properties of pyroclasts contact with water can promote magma fragmentation obsidian chips and glass spheres typically crack when quenched vesicular pyroclasts are made of glass so thermal quenching by water may damage them if water enters eruption columns or if pyroclastic density currents interact with water hot pumice can be quenched we performed a set of experiments on air fall pumice from medicine lake california we made quenched samples by heating natural clasts to 600 c quenching them in water at 21 c drying them at 105 c and then cooling them to room temperature we compare these samples with untreated air fall pumice from the same deposit hereafter referred to as regular pumice we tested whether quenched pumice would 1 shatter more easily in collisions and 2 abrade faster we also tested whether individual clasts lose mass upon quenching and whether they increase in effective wet density two measurements which may help characterize the magnitude of clast damage during quenching we also compare pre quenching and post quenching textures using x ray microtomography xrt images results from collision experiments show no clear difference between quenched pumice and regular pumice quenched pumice abraded faster than regular pumice on average 0 3 of mass may have been lost during quenching effective wet density increased 1 5 on average as measured after 5 minutes of immersion in water overall we find modest differences between quenched pumice and regular pumice in experiments and measurements the experimental results imply that quenching may damage small parts of a clast but tends not to cause cracks that propagate easily through the clast post quenching xrt imaging shows no obvious change in clast texture this is in stark contrast to non vesicular glass that develops large cracks on quenching we present four factors that explain why pumice is resistant to damage from thermal quenching thin glass films experience lower transient thermal stresses many internal surfaces are initially vapor cooled vesicles can arrest cracks and cracks from thermal quenching may not occur in the locations most susceptible to fracture in later collisions

a novel apparatus for the simulation of eruptive gas rock interactions the chemical interactions between hot volcanic gases and co erupted magmatic and lithic particles within eruption plumes and pyroclastic flows are increasingly investigated for their relevance to the impacts of ash emission on natural and human environments laboratory experiments are critical to our understanding of high temperature gas ash interactions but previous studies are yet to replicate the chemical composition of the high temperature volcanic gases involved here we present a unique apparatus the advanced gas ash reactor capable of generating an atmosphere of h2o co2 so2 and hcl at temperatures ranging from 200 to 900 c under variable heating and cooling rates experiments utilising the reactor can inform investigations of a range of topics from subsurface gas rock interaction and in plume gas adsorption processes to the effect of ash surface chemistry on marine nutrient loadings and atmospheric chemistry our results demonstrate the differences in high temperature gas uptake by volcanic glass powders under both hydrous and anhydrous atmospheres and accordingly demonstrate the utility of the new reactor

vesiculation and quenching during surtseyan eruptions at hunga tonga hunga ha apai volcano tonga surtseyan eruptions are shallow to emergent subaqueous explosive eruptions that owe much of their characteristic behavior to the interaction of magma with water the difference in thermal properties between water and air affects the cooling and postfragmentation vesiculation processes in magma erupted into the water column here we study the vesiculation and cooling processes during the 2009 and 2014 2015 surtseyan eruptions of hunga tonga hunga ha apai volcano by combining 2 d and 3 d vesicle scale analyses of lapilli and bombs and numerical thermal modeling most of the lapilli and bombs show gradual textural variations from rim to core the vesicle connectivity in the lapilli and bombs increases with vesicularity from fully isolated to completely connected and also increases from rim to core in transitional clasts we interpret the gradual textural variations and the connectivity vesicularity relationships as the result of postfragmentation bubble growth and coalescence interrupted at different stages by quenching in water the measured vesicle size distributions are bimodal with a population of small and large vesicles we interpret this bimodality as the result of two nucleation events one prefragmentation with the nucleation and growth of large bubbles and one postfragmentation with nucleation of small vesicles we link the thermal model with the textural variations in the clasts showing a dependence on particle size leidenfrost effect and initial melt temperature in particular the cooling profiles in the bombs are consistent with the gradual textural variations from rim to core in the clasts likely caused by variations in time available for vesiculation before quenching

eruption of magmatic foams on the moon formation in the waning stages of dike emplacement events as an explanation of irregular mare patches abstract volcanic eruptions on the moon take place in conditions of low gravity and negligible atmospheric pressure very different from those on earth these differences lead to characteristic lunar versions of hawaiian and strombolian explosive activity and to the production of unusual eruption products neither predicted nor observed on earth in the terminal stages of eruptions these include the unusual mounds and rough hummocky blocky floors of some small shield summit pit crater floors elongate depressions and mare flows similar to those named irregular mare patches imps by braden et al 2014 we examine the ascent and eruption of magma in the waning stages of the eruptive process in small shield summit pit crater floors and show that many imp characteristics can be plausibly explained by basaltic magma behavior as the rise rate of the ascending magma slows to zero volatiles exsolve in the dike and lava lake to form a very vesicular foam and the dike begins to close stresses in the very vesicular and porous lava lake crust produce fractures through which the foam extrudes at a rate determined by its non newtonian rheology waning stage extrusion of viscous magmatic foams to the surface produces convex mounds whose physical properties inhibit typical impact crater formation and regolith development creating an artificially young crater retention age this mechanism for the production and extrusion of very vesicular magmatic foams is also applicable to waning stage dike closure associated with pit craters atop dikes and fissure eruptions in the lunar maria providing an explanation for many irregular mare patches this mechanism implies that imps and associated mare structures small shields pit craters and fissure flows formed synchronously billions of years ago in contrast to very young ages less than 100 million years proposed for imps by some workers

initial 26al 27al in carbonaceous chondrite chondrules too little 26al to melt asteroids we report 26mg excesses correlated with al mg ratios in five chondrules from the primitive co3 0 chondrite yamato 81020 that yield a mean initial 26al 27al ratio of only 3 8 0 7 10 6 about half that of ordinary chondrite oc chondrules even if asteroids formed immediately after chondrule formation this ratio and the mean al content of co chondrites is only capable of raising the temperature of a well insulated co asteroid to 940 k which is more than 560 k too low to produce differentiation the same ratio combined with the higher al content of cv chondrites results in a cv asteroid temperature of 1100 k we calculate that the mean initial 26al 27al ratio of about 7 4 10 6 found in ll chondrules is only able to produce small amounts of melting too little to produce differentiation these results cast serious doubt on the viability of 26al as the heat source responsible for asteroid differentiation inclusion of 60fe raises temperatures about 160 k but this increment is not enough to cause differentiation even of an ll chondrite asteroid

thermodynamic and rheological properties of rhyolite and andesite melts the heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative enthalpy measurements made between 400 and 1800 k for the glass phases the experimental data agree with empirical models of calculation of the heat capacity for the liquid phases the agreement is less good owing to strong interactions between alkali metals and aluminum which are not currently accounted for by empirical heat capacity models the viscosity of both liquids has been measured from the glass transition to 1800 k the temperature dependence of the viscosity is quantitatively related to the configurational heat capacity determined calorimetrically through the configurational entropy theory of relaxation processes for both rhyolite and andesite melts the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions

transport properties of low sanidine single crystals glasses and melts at high temperature thermal diffusivity d was measured using laser flash analysis from oriented single crystal low sani dine k0 92na0 08al0 99fe 3 si2 95o8 and three glasses near kalsi3o8 viscosity measurements of the three super cooled liquids in the range 10 6 8 to 10 12 3 pa s confirm near arrhenian behavior varying subtly with composition for crystal and glass d decreases with t approaching a constant near 1 000 k dsat 0 65 0 3 mm 2 s 1 for bulk crystal and 0 53 0 03 mm 2 s 1 for the glass a rapid decrease near 1 400 k is consistent with crossing the glass transition melt behavior is approximated by d 0 475 0 01 mm 2 s 1 thermal conductivity klat of glass calculated using previous heat capacity cp and new density data increases with t because cp strongly increases with t for melt klat reaches a plateau near 1 45 w m 1 k 1 and is always below klat of the crystal melting of potassium feldspars impedes heat transport providing positive thermal feedback that may promote further melting in continental crust

the fate of pyroclasts produced in explosive eruptions on the asteroid 4 vesta we evaluate the consequences of explosive activity having taken place during volcanic eruptions on the differentiated asteroid 4 vesta which is the likely parent body of the howardite eucrite diogenite hed meteorites for a wide range of magma volatile contents we calculate the eruption speeds and subsequent trajectories of the pyroclastic magma droplets produced by considering the size distribution and eruption speeds of the droplets and the mass fluxes at which they are ejected we show that under all realistic circumstances the droplets will have formed lava fountains that were extremely optically dense as a result virtually all of the droplets will have reached the surface having suffered a negligible amount of radiative cooling and will have coalesced into lava ponds feeding lava flows typically 1 of the pyroclasts will have undergone enough cooling to allow them to accumulate into a recognizable fall deposit consisting of unwelded or partly welded volcanic glass beads this result is consistent with the apparent absence of identifiable pyroclastic material in the hed and other types of differentiated meteorites

biot number and thermos bottle effect implications for magma chamber convection thermal boundary conditions model the coupling between a convecting magmatic body and its host such conditions need to be considered in models of igneous systems that involve thermal histories crystallization and fractionation of melt formation of aureoles by contact metamorphism and any other processes in which transport of heat plays a role usually investigations of magmatic systems have tended to emphasize modeling the interior convective regime relative to treatment of the thermal coupling yet it is found that the thermal nature of an intrusion is likely to be influenced more by coupling to its host than by the details of internal convective flows evaluation of a parameter having the form of a biot number bi provides a basis for estimating which boundary conditions are most appropriate it is found that bi 0 1 constant heat flux limit for models of several caldera systems for such values of the biot number the host regime behaves somewhat like a thermos bottle by limiting the flow of heat through the magma host system so that convective stirring of magma has little effect on the cooling rate of the intrusion because of this insulating effect boundary temperatures assumed in convection models should approach magmatic values even if an active hydrothermal system is present however high boundary temperatures do not imply that melting and assimilation of host rock by magma must occur despite the thermos bottle effect magmatic convection can still be quite vigorous

pb isotopic age of the allende chondrules we have studied pb isotope systematics of chondrules from the oxidized cv3 carbonaceous chondrite allende the chondrules contain variably radiogenic pb with a 206pb 204pb ratio between 19 5 268 pb pb isochron regression for eight most radiogenic analyses yielded the date of 4566 2 2 5 ma internal residue leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566 6 1 0 ma our best estimate for an average age of allende chondrule formation this pb pb age is consistent with the range of model 26al 26mg ages of bulk allende chondrules reported by bizzarro et al 2004 and is indistinguishable from pb pb ages of ca al rich inclusions cais from cv chondrites 4567 2 0 6 ma amelin et al 2002 and the oldest basaltic meteorites we infer that chondrule formation started contemporaneously with or shortly after formation of cv cais and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids the entire period of chondrule formation lasted from 4566 6 1 0 ma allende to 4564 7 0 6 ma cr chondrite acfer 059 to 4562 7 0 5 ma cb chondrite gujba and was either continuous or consisted of at least three discrete episodes since chondrules in cb chondrites appear to have formed from a vapor melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated krot et al 2005 there were possibly a variety of processes in the early solar system occurring over at least 4 5 myr that we now combine under the umbrella name of chondrule formation

hf w evidence for rapid differentiation of iron meteorite parent bodies abstract new high precision w isotope data on iron meteorites are presented that provide important constraints on the timing of silicate metal segregation in planetesimals magmatic iron meteorites all have e 182 w within error or less radiogenic than initial e 182 w estimated by studies of chondritic meteorites at face value this implies that iron meteorites are as old and older than refractory calcium aluminium rich inclusions cai which are widely thought to be the oldest solar system objects moreover different meteorites from the same magmatic groups believed to be derived from the same planetissimal core display a range of e 182 w we suggest that the paradoxical e 182 w values more negative than initial solar system initial ssi are most readily explained as a result of secondary spallation reactions with cosmic rays during transit between parent body and the earth this is supported by the most negative e 182 w being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions on the other hand it is also striking that none of the magmatic iron group meteorites have e 182 w analyses outside error more radiogenic than the estimated solar system initial ratio this suggests that core formation in parent bodies of magmatic iron meteorites occurred 1 5 myr after the formation age of cai y amelin a n krot i d hutcheon and a a ulyanov lead isotopic ages of chondrules and calcium aluminum inclusions science 297 1678 1683 2002 this extremely early metal silicate differentiation is coeval with the first chondrules m bizzarro j a baker and h haack mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions nature 431 275 278 2004 a n krot y amelin p cassen and a meibom young chondrules in cb chondrites from a giant impact in the early solar system nature 436 989 992 2005 formation of later chondrules and hence the parent bodies of some chondritic meteorites must therefore have occurred in the presence of planetesimals large enough to possess iron cores we conclude that early planetary accretion and differentiation was sufficiently fast for 26 al decay to be an important heat source non magmatic iron meteorites however display more radiogenic and varied w isotope signatures this is in keeping with them being generated later by impact melting during which the metal partially re equilibrated with the then more radiogenic silicate fraction

conduit margin heating and deformation during the ad 1886 basaltic plinian eruption at tarawera volcano new zealand during explosive eruptions a suspension of gas and pyroclasts rises rapidly within a conduit here we have analysed textures preserved in the walls of a pyroclastic feeder dyke of the ad 1886 tarawera basaltic plinian fissure eruption the samples examined consist of basaltic ash and scoria plastered onto a conduit wall of a coherent rhyolite dome and a welded rhyolitic dome breccia we examine the textural evidence for the response of the wall material built of 75 vol glass and 25 vol crystals pore free equivalent to mass movement in the adjacent conduit in the rhyolitic wall material we quantify the orientation and aspect ratio of biotite crystals as strain markers of simple shear deformation and interpret juxtaposed regions of vesiculation and vesicle collapse as evidence of conduit wall heating systematic changes occur close to the margin 1 porosity is highly variable with areas locally vesiculated or densified 2 biotite crystals are oriented with their long axis parallel to the margin 3 the biotites have greater aspect ratios close to the margin and 4 the biotite crystals are fractured we interpret the biotite phenocryst deformation to result from crystal fracture rotation and cleavage parallel bookcase translation these textural observations are inferred to indicate mechanical coupling between the hot gas ash jet and the conduit wall and reheating of wall rock rhyolite we couple these observations with a simple 1d conductive heating model to show what minimum temperature the conduit wall needs to reach in order to achieve a temperature above the glass transition throughout the texturally defined deformed zone we propose that conduit wall heating and resulting deformation influences conduit margin outgassing and may enhance the intensity of such large basaltic eruptions

deformation of foamed rhyolites under internal and external stresses an experimental investigation the style of magma eruption depends strongly on the character of melt degassing and foaming depending on the kinetics of these processes the result can be either explosive or effusive volcanism in this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments 2 min 24 h followed by quenching the samples the volumetric gas melt ratio has been estimated through the density measurements of foamed samples

the porosity of pyroclasts as an indicator of volcanic explosivity abstract the porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption here we analyse the porosity distribution of deposits of 17 different volcanic eruptions covering a broad range of inferred magma properties and observed eruption styles histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean variance modality and skewness we present characteristic porosity distributions for five different eruption style categories dome forming explosive basaltic cryptodome forming subplinian plinian ultraplinian and phreatomagmatic we find that the mean porosity values of the deposits correlate positively with the volcanic explosivity index vei of newhall and self 1982 regardless of the degree of complexity of the individual distributions we then propose causes of the deviations from a general trend in terms of magma viscosity degassing efficiency and cap rock confinement finally a generalised correlation between clast porosity and explosivity of an eruption is developed that should help together with conventional field techniques facilitate the interpretation of pyroclastic sequences even in cases where no direct observation of the eruption exists

the distribution of short lived radioisotopes in the early solar system and the chronology of asteroid accretion differentiation and secondary mineralization abstract we evaluate initial 26 al 27 al i 53 mn 55 mn i and 182 hf 180 hf i ratios together with 207 pb 206 pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common closed isotopic system i e the early solar nebula the relative initial isotopic abundances of 26 al 53 mn and 182 hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values here denoted by i al ss i mn ss and i hf ss respectively i mn ss and i hf ss 9 1 1 7 10 6 and 1 07 0 08 10 4 respectively correspond to canonical i al ss 5 1 10 5 i hf ss so determined is consistent with i hf ss 9 72 0 44 10 5 directly determined from an internal hf w isochron for cai minerals i mn ss is within error of the lowest value directly measured for cais we suggest that erratically higher values measured for cais in carbonaceous chondrites may reflect proton irradiation of unaccreted cais by the early sun after other asteroids destined for melting by 26 al decay had already accreted the 53 mn incorporated within such asteroids would have been shielded from further local spallogenic contributions from within the solar system the relative initial isotopic abundances of the short lived nuclides are less consistent with the 207 pb 206 pb ages of the corresponding materials than with one another the best consistency of short and long lived chronometers is obtained for 182 hf 180 hf i and the 207 pb 206 pb ages of angrites 182 hf 180 hf i decreases with decreasing 207 pb 206 pb ages at the rate expected from the 8 90 0 09 ma half life of 182 hf the model solar system age thus determined is t ss hf w 4568 3 0 7 ma 26 al 27 al i and 53 mn 55 mn i are less consistent with 207 pb 206 pb ages of the corresponding meteorites but yield t ss mn cr 4568 2 0 5 ma relative to i al ss 5 1 10 5 and a 207 pb 206 pb age of 4558 55 0 15 ma for the lew86010 angrite the mn cr method with i mn ss 9 1 1 7 10 6 is useful for dating accretion if identified with chondrule formation primary igneous events and secondary mineralization on asteroid parent bodies all of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies for i mn ss 9 1 1 7 10 6 parent body differentiation is found to extend at least to 5 ma post t ss i e until differentiation of the angrite parent body 4563 5 ma ago or 4564 5 ma ago using the directly measured 207 pb 206 pb ages of the d orbigny clan angrites the 1 ma difference is characteristic of a remaining inconsistency for the d orbigny clan between the al mg and mn cr chronometers on one hand and the 207 pb 206 pb chronometer on the other differentiation of the iiiab iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body and at nearly the same time as one another as shown by the mn cr ages of iiiab irons and ureilites respectively the latest recorded episodes of secondary mineralization are for carbonates on the ci carbonaceous chondrite parent body and fayalites on the cv carbonaceous chondrite parent body both extending to 10 ma post t ss

size limits for rounding of volcanic ash particles heated by lightning volcanic ash particles can be remelted by the high temperatures induced in volcanic lightning discharges the molten particles can round under surface tension then quench to produce glass spheres melting and rounding timescales for volcanic materials are strongly dependent on heating duration and peak temperature and are shorter for small particles than for large particles therefore the size distribution of glass spheres recovered from ash deposits potentially record the short duration high temperature conditions of volcanic lightning discharges which are hard to measure directly we use a 1 d numerical solution to the heat equation to determine the timescales of heating and cooling of volcanic particles during and after rapid heating and compare these with the capillary timescale for rounding an angular particle we define dimensionless parameters capillary fourier stark biot and peclet numbers to characterize the competition between heat transfer within the particle heat transfer at the particle rim and capillary motion for particles of different sizes we apply this framework to the lightning case and constrain a maximum size for ash particles susceptible to surface tension driven rounding as a function of lightning temperature and duration and ash properties the size limit agrees well with maximum sizes of glass spheres found in volcanic ash that has been subjected to lightning or experimental discharges demonstrating that the approach that we develop can be used to obtain a first order estimate of lightning conditions in volcanic plumes

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

original structures and fragmentation and reassembly histories of asteroids evidence from meteorites abstract if chondritic meteorites were internally heated after accretion had ended then the hottest material would have been buried the deepest and should have cooled the slowest if this is correct there ought to be a correlation between cooling rate and petrographic type a measure of the extent to which chondrites were metamorphosed i e heated published and new cooling rates derived from the compositions of metallic iron nickel grains do not display this correlation implying either that chondrite parent asteroids never had onion shell structures or that bodies with onion shell structures were broken up and reassembled prior to cooling to below 500 c the temperature at which cooling rate information is recorded in metallic iron nickel chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids metallic iron nickel grains in h and l chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to 1000 k myr based on thermal calculations these cooling rates suggest that the materials spread out on the surfaces of h and l chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them therefore it appears that at least some asteroids namely the parent bodies of h and l chondrites were disrupted after cooling to below 300 c and then reassembled to create surfaces containing rocks that originated at a wide range of depths these results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

from rock to magma and back again the evolution of temperature and deformation mechanism in conduit margin zones abstract explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing the zone at the margin of a volcanic conduit thought to play an important role in the outgassing of magma and therefore pore pressure changes and explosivity is the boundary through which heat is exchanged from the hot magma to the colder country rock using a simple heat transfer model we first show that the isotherm for the glass transition temperature whereat the glass within the groundmass transitions from a glass to an undercooled liquid moves into the country rock when the magma within the conduit can stay hot or into the conduit when the magma is quasi stagnant and cools on the centimetric scale over days to months we then explore the influence of a migrating viscous boundary on compactive deformation micromechanisms in the conduit margin zone using high pressure effective pressure of 40 mpa high temperature up to 800 c triaxial deformation experiments on porous andesite our experiments show that the micromechanism facilitating compaction in andesite is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass t g i e rock in this regime porosity is only reduced within the bands of crushed pores the porosity outside the bands remains unchanged further the strength of andesite is a positive function of temperature below the threshold t g due to thermal expansion driven microcrack closure the micromechanism driving compaction above t g i e magma is the distributed viscous flow of the melt phase in this regime porosity loss is distributed and is accommodated by the widespread flattening and closure of pores we find that viscous flow is much more efficient at reducing porosity than cataclastic pore collapse and that it requires stresses much lower than those required to form bands of crushed pores our study therefore highlights that temperature excursions can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the material within the conduit margin zone with possible implications for pore pressure augmentation and explosive behaviour

airborne gravity measurements over mountainous areas by using a lacoste romberg air sea gravity meter this paper introduces a new approach to airborne gravity data reduction well suited for surveys flown at high altitude with respect to gravity sources mountainous areas classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western french alps by using a lacoste romberg air sea gravity meter the part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft this finding is theoretically argued by solving the equation of the gravimetric system gravity meter and stabilized platform the transfer function of the system is derived and a new formulation of airborne gravity data reduction which takes care of the sensitive response of spring tension to observable gravity field wavelengths is given the resulting gravity signal exhibits a residual noise caused by electronic devices and short wavelength eotvos effects the use of dedicated exponential filters gives us a way to eliminate these high frequency effects examples of the resulting free air anomaly at 5100 m altitude along one particular profile are given and compared with free air anomaly deduced from the classical method for processing airborne gravity data and with upward continued ground gravity data the well known trade off between accuracy and resolution is discussed in the context of a mountainous area

organic thermometry for chondritic parent bodies abstract a unique spectroscopic feature has been identified in a study of twenty five different samples of meteoritic insoluble organic matter iom spanning multiple chemical classes groups and petrologic types using carbon x ray absorption near edge structure xanes spectroscopy the intensity of this feature a 1s exciton appears to provide a precise measure of parent body metamorphism the intensity of this exciton is also shown to correlate well with a large negative paramagnetic shift observed through solid state 13c nmr experiments reveal that upon heating primitive iom is transformed into material that is indistinguishable from that in thermally processed chondrites including the development of the 1s exciton a thermo kinetic expression is derived from the experimental data that allows the intensity of the 1s exciton to be used to estimated the effective temperature integrated over time a good correlation is observed between the intensity of the 1s exciton and previously published microraman spectral data these data provide a self consistent organic derived temperature scale for the purpose of calibrating raman based thermometric expressions

pseudopillow fracture systems in lavas insights into cooling mechanisms and environments from lava flow fractures abstract detailed field observations of structures within the flow front of a holocene trachyandesite lava from snaefellsnes iceland are presented the lava provides exceptional three dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water the flow front interior is characterised by structures consisting of a large metre scale curviplanar master fracture with many smaller centimetre scale subsidiary fractures perpendicular to the master fracture such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures we propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together all documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant i e liquid water ice or snow we use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures master fracture surface textures used to identify fracture mechanisms include chisel marks striae cavitation dimples river lines and rough smooth fracture surface textures these indicate both brittle and ductile fracture happening on different types of master fracture chisel marks on subsidiary fractures indicate comparative cooling rates cooling directions and isotherm orientations at the time of fracture we propose a model for pseudopillow fracture system formation taking into account all the various fracture types textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants

expansion and quenching of vesicular magma fragments in plinian eruptions the conditions of pumice generation in plinian eruptions are studied a physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column the effects of pressure release and cooling are calculated for a range of eruption conditions the magma fragment expands in the conduit and stops expanding soon after leaving the vent when a thin viscous rind forms against the cold mixture of magmatic gas and air this rind prevents further volume changes pumice vesicularity is a function of the decompression rate in the conduit which depends on ascent velocity and fragmentation depth it is also sensitive to the cooling rate in the atmospheric column which depends on vent radius and mass discharge rate different fragments follow different trajectories in the column and are subjected to different cooling rates this generates a range of vesicularities which reflects the eruptive conditions all else being equal pumice vesicularity increases as magma viscosity decreases these predictions are consistent with observations pumices provide quantitative constraints on conduit flow conditions and mass discharge rate these concepts are applied to two plinian eruptions vesicularity values for the bishop tuff long valley caldera require a mass discharge rate between 108 and 109 kg s 1 vesicularity variations during plinian phase 1 of the minoan eruption santorini are explained by the conduit radius increasing from about 30 m to 120 m both cases require large decompression rates in the eruption conduit suggesting that flow pressures were close to lithostatic values

constraints on cooling and degassing of pumice during plinian volcanic eruptions based on model calculations during explosive volcanic eruptions pumice clasts are transported into the atmosphere and their thermal and degassing histories determine how much volatiles are lost syneruptively this process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice in the model we investigate the impact of various parameters e g mass eruption rate biot number eruption temperature and geometry on the cooling and degassing of pumice and find that the biot number and the eruption temperature are the most influential during typical risetimes of pumices inside eruption columns 200 300 s those smaller than 0 5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume in the case of larger pumices the ratio of the cooling timescale r p 2 where rp is the pumice radius and is the thermal diffusivity to the degassing timescale w 2 d 0 where w is the vesicle wall thickness and d 0 is the species diffusivity controls degassing if this ratio is larger than 50 degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice for a ratio of less than 0 1 no volatiles escape the pumice typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing cl and h 2 o the transition from 0 1 to 50 will be in the pumice size range of 1 10 cm here volatile concentration is a strong function of position inside the pumice suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass a number which is frequently used to estimate the total volatile input of volcanic eruptions this effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward d 80

magma mixing induced by particle settling a time series of experiments at high temperature have been performed to investigate the influence of particle settling on magma mixing a natural rhyolite glass was held above a natural basalt glass in a platinum crucible after melting of the glasses at superliquidus temperatures a platinum sphere was placed on the upper surface of the rhyolitic melt and sank into the experimental column rhyolitic melt above basaltic melt upon falling through the rhyolitic basaltic melt interface the pt sphere entrained a filament of rhyolitic melt in its further fall the quenched products of the experiments were imaged using x ray microct methods the images of our time series of experiments document the formation of a rhyolite filament as it is entrained into the underlying basalt by the falling platinum sphere when the pt particle reached the bottom of the crucible the entrained rhyolitic filament started to ascend buoyantly up to the initial rhyolitic basaltic interface this generated a significant thickness increase of a comingled melange layer at the interface due to liquid rope coiling and piling up of the filament as a consequence the basalt rhyolite interface was greatly enlarged and diffusive hybridisation greatly accelerated further bubbles originating at the interface are observed to have risen into the overlying rhyolite dragging basalt filaments with them upon crossing the basalt rhyolite interface the bubbles have non spherical shapes as they adapt to the differing surface tensions of basaltic and rhyolitic melts major element profiles measured across the rhyolite filaments exhibit asymmetrical shapes from the rhyolite into the basalt na and ti reveal uphill diffusion from the rhyolite towards the interface in the filament cross sections these results reveal the potential qualitative complexity of the mingling process between rhyolitic and basaltic magmas in the presence of sinking crystals they imply that crystal rich magma mingling may be expected to be accelerated with respect to crystal poor systems we urge the further fluid dynamic analysis of these phenomena obtainable for the first time using detailed tomographic imaging

pb pb dating of chondrules from cv chondrites by progressive dissolution abstract to refine absolute pb pb ages of chondrules we have experimented with progressive dissolution procedures of chondrules in the cv chondrite allende in an attempt to effectively remove terrestrial contamination and generate pb fractions of sufficient size and spread in 207 pb 206 pb space to adequately define meaningful isochrons samples are extricated from the matrix ultrasonicated abraded and repeatedly pre cleaned by ultrasonicating in distilled ethanol water and acetone subsequent steps of weak hcl and hbr acids combined with ultrasonic agitation and modest heating effectively removes terrestrial contamination in most samples we analyzed subsequent exposure to warm 4 m hno 3 typically isolates the least radiogenic component of the stepwise dissolution series continuing the dissolution steps with hcl of increasing molarity with longer and higher temperature heating steps returns more radiogenic pb fractions the most radiogenic pb fraction typically corresponds to the first warm 1 m hf step with subsequent steps in stronger hf yielding progressively less radiogenic signatures importantly the pb components extracted in the dissolution of the final residues with 28 m hf 14 m hno 3 typically lie slightly below the isochron defined by previous steps corresponding to a slightly younger 207 pb 206 pb model age than the inferred real age the wide range of total pb contents 20 2000 pg in different fractions returned by this method requires analyses by tims using a high efficiency pb emitter 202 pb 205 pb double spike and diligent monitoring of laboratory pb blank as expected u and pb are highly and variably fractionated during the procedure such that no u pb age information is recovered a multi chondrule fraction from the cv chondrite allende yields an age of 4565 32 0 81 ma which is statistically younger than calcium aluminum inclusions from the cv chondrite efremovka

53mn 53cr dating of fayalite formation in the cv3 chondrite mokoia evidence for asteroidal alteration fayalite grains in chondrules in the oxidized aqueously altered cv3 chondrite mokoia have large excesses of radiogenic chromium 53 these excesses indicate the in situ decay of short lived manganese 53 half life 3 7 million years and define an initial53mn 55mn ratio of 2 32 0 18 10 6 this ratio is comparable to values for carbonates in ci and cm chondrites and for several classes of differentiated meteorites mokoia fayalites formed 7 to 16 million years after allende calcium aluminum rich inclusions during hydrothermal activity on a geologically active asteroid after chondritic components had ceased forming in the solar nebula

stony meteorite porosities and densities a review of the data through 2001 in this review we summarize the data published up to december 2001 on the porosity and density of stony meteorites these data were taken from 925 samples of 454 different meteorites by a variety of techniques most meteorites have densities on the order of 3 to 4 g cm 3 with lower densities only for some volatile rich carbonaceous meteorites and higher densities for stony irons for the vast majority of stones porosity data alone cannot distinguish between different meteorite compositions average porosities for most meteorite classes are around 10 though individual samples can range as high as 30 porosity unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present the measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment a theoretical grain density based on composition can model pre weathered porosities the average model porosity for h and ll chondrites is 10 while l chondrite model porosities average only 6 a statistically significant difference

u pb ages of angrites abstract precise u pb ages determined with double spike 202 pb 205 pb thermal ionization m1ass spectrometry are reported for angrites angra dos reis ador lewis cliff 86010 lew and d orbigny nineteen of 23 acid washed pyroxene fractions from these meteorites and whole rock fractions from d orbigny contain between 0 5 and 1 3 pg of total common pb indistinguishable from analytical blank measured 206 pb 204 pb ratios in these fractions are between 6300 and 14 100 for ador 1160 4500 for lew and 608 8500 for d orbigny blank corrected 206 pb 204 pb ratios for all three meteorites vary from 2160 to over 100 000 these fractions yielded precise and reproducible 207 pb 206 pb dates with the average values of 4557 65 0 13 ma for ador 4558 55 0 15 ma for lew and 4564 42 0 12 ma for d orbigny pb pb isochrons including data with slightly elevated common pb and u pb upper concordia intercepts yield similar dates the implications of these new pb isotopic ages of angrites are threefold first they demonstrate that ador and lew are not coeval and the group of slowly cooled angrites is therefore genetically diverse second the new age of lew suggests an upward revision of 53 mn 53 cr absolute ages by 0 7 ma third a precise age of d orbigny allows consistent linking of the 53 mn 53 cr and 26 al 26 mg extinct nuclide chronometers to the absolute lime scale

the relative formation ages of ferromagnesian chondrules inferred from their initial aluminum 26 aluminum 27 ratios we performed a systematic high precision sims 26al 26mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite bishunpur ll3 1 the chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and or glass the chemical compositions of the chondrules vary from feo poor to feo rich eight chondrules show resolvable 26mg excesses with a maximum 26mg of 1 in two chondrules the initial 26al 27al ratios inferred for these chondrules range between 2 28 0 73 x 10 5 to 0 45 0 21 x 10 5 assuming a homogeneous distribution of al isotopes in the early solar system this range corresponds to ages relative to cais between 0 7 0 2 my and 2 4 0 4 0 7 my the inferred total span of the chondrule formation ages is at least 1 my which is too long to form chondrules by the x wind the initial 26al 27al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine rich chondrules formed earlier than pyroxene rich chondrules though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of si from earlier generations of chondrules followed by addition of si to the precursors of later generation chondrules

pyrrhotite oxidation as a proxy for thermal structure of eruption clouds a comparative study of plinian and vulcanian eruptions of asama and sakurajima volcanoes japan abstract thermal expansion of a volcanic eruption cloud by entrainment of air is a primary control of its behavior oxidation of the pyroclasts in the eruption cloud can be an indicator of the degree of air entrainment and thermal disequilibrium during its development here the oxidation texture of sulfide minerals pyrrhotite in four andesitic pyroclastic eruptions of asama and sakurajima volcanoes was investigated in order to highlight the contrast between the air entrainment and thermal disequilibrium processes of the plinian and vulcanian eruption columns though oxidized pyrrhotite crystals were commonly found in the pumice fall deposit of these four plinian and vulcanian eruptions significant differences in the component proportion and degree of oxidation of the pyrrhotite crystals were found between plinian and vulcanian products greater proportions of oxidized pyrrhotite crystals in the asama 1783 and sakurajima 1914 plinian pumice deposits than those in the asama 2004 and sakurajima 2010 vulcanian eruptions indicate that heat retainment of pumice clasts and the continuous ejection of eruption clouds in the plinian eruptions kept pumice in the eruption cloud at high temperature during the entrainment of air and expansion of the cloud in contrast the rapid thermal equilibrium of pumice clasts and the short lived episodic ejection of vulcanian eruptions reduced the temperature of pumice in the eruption cloud quickly this suggests that pyrrhotite crystals have the potential to be indicators of the development of temperature inside an eruption cloud reflecting the process of thermal disequilibrium of pyroclasts and of air entrainment

on early solar system chronology implications of an heterogeneous spatial distribution of 26al and 53mn abstract early solar system chronology is usually built with the assumption that the distribution of short lived radionuclides was homogeneous through the solar accretion disk at present there is no unambiguous evidence for a homogeneous distribution of short lived radionuclides in the solar accretion disk while some data point to a heterogeneous distribution of short lived radionuclides in this paper we explore a possible chronology based on a heterogeneous distribution of 26 al and 53 mn in the accretion disk our basic assumption is that the different abundances of extinct short lived radionuclides in calcium aluminium rich inclusions cais and chondrules are due to spatial rather than temporal differences we develop a simple model where cais and chondrules form contemporaneously in different spatial locations and are characterised by distinct initial 26 al and 53 mn abundances in this model all evolved bodies are supposed to be originally chondritic i e to be made of a mixture of cais chondrules and matrix this mixture determines the initial content in 26 al and 53 mn of a chondritic parent body as a function of its cai and chondrule abundance fraction this approach enables us to calculate coherent 26 al and 53 mn ages from the agglomeration of the parent body precursors cais and chondrules until the isotopic closure of 26 al and 53 mn thereafter called 26 al 53 mn age we calculate such 26 al 53 mn ages for a diversity of evolved objects with the constraint that they should be found for realistic chondritic parent body precursors i e objects having similar or identical petrograpy to the existing chondrite groups the so defined age of the d orbigny angrite is 4 3 1 1 myr for the asuka 881394 eucrite 2 8 1 0 myr for the h4 chondrite sainte marguerite 3 myr and for h4 forest vale 5 myr the calculated 26 al 53 mn ages give timescales for the evolution of the respective parent bodies meteorites that can be investigated in the light of further petrographic studies we anchor the calculated relative chronology to an absolute chronology using absolute pb pb ages and relative hf w ages of the objects under scrutiny the precursors of sainte marguerite and forest vale agglomerated at the same time 4565 8 1 2 ma ago the precursors of eucrites asuka 881394 agglomerated 4564 8 1 2 ma ago the precursors of angrites agglomerated late 4561 5 1 8 ma ago our model provides a fully compatible al mg mn cr pb pb chronology and is shown to be robust to reasonable changes in the input parameters the calculated initial 26 al 27 al ratios are high enough to have 26 al as a possible heat source for differentiation

26al 26mg systematics of chondrules in a primitive co chondrite abstract we have conducted systematic investigations of formation age chemical compositions and mineralogical characteristics of ferromagnesian chondrules in yamato 81020 co3 05 one of the most primitive carbonaceous chondrites to get better understanding of the origin of chemical groups of chondrites the 26 al 26 mg isotopic system were measured in fourteen feo poor type i six feo rich type ii and two aluminum rich al rich chondrules using a secondary ion mass spectrometer excesses of 26 mg in plagioclase 1 0 13 5 are resolved with sufficient precision mostly 0 4 6 6 at 2 level in all the chondrules studied except one chemical zoning of mg and na in plagioclase were investigated in detail in order to evaluate the applicability of 26 al 26 mg chronometer we conclude that the al mg isotope system of the chondrules in y 81020 have not been disturbed by parent body metamorphism and can be used as chronometer assuming homogeneous distribution of 26 al assuming an initial 26 al 27 al ratio of 5 10 5 in the early solar system 26 al 26 mg ages were found to be 1 7 2 5 ma after cai formation for type i 2 0 3 0 ma for type ii and 1 9 and 2 6 ma for al rich chondrules the formation ages of ferromagnesian chondrules in y 81020 are in good agreement with those of l and ll type 3 0 3 1 chondrites in the literature which indicates that common chondrules in the co chondrite were formed contemporaneously with those in l and ll chondrites the concurrent formation of chondrules of co and l ll chondrites suggests that the chemical differences between co and l ll chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk

chondrites as samples of differentiated planetesimals abstract chondritic meteorites are unmelted and variably metamorphosed aggregates of the earliest solids of the solar system the variety of metamorphic textures in chondrites motivated the onion shell model in which chondrites originated at varying depths within a parent body heated primarily by the short lived radioisotope 26 al with the highest metamorphic grade originating nearest the center allende and a few other chondrites possess a unidirectional magnetization that can be best explained by a core dynamo on their parent body indicating internal melting and differentiation here we show that a parent body that accreted to 200 km in radius by 1 5 ma after the formation of calcium aluminum rich inclusions cais would have a differentiated interior and ongoing accretion would add a solid undifferentiated crust overlying a differentiated interior consistent with formational and evolutionary constraints inferred for the cv parent body this body could have produced a magnetic field lasting more than 10 ma this hypothesis represents a new model for the origin of some chondrites presenting them as the unprocessed crusts of internally differentiated early planetesimals such bodies may exist in the asteroid belt today the shapes and masses of the two largest asteroids 1 ceres and 2 pallas can be consistent with differentiated interiors conceivably with small iron cores with hydrated silicate or ice silicate mantles covered with undifferentiated crusts