late eocene to early miocene ice sheet dynamics and the global carbon cycle paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial pacific ocean drilling program site 1218 the records include the two largest abrupt 1 myr increases in the cenozoic benthic oxygen isotope record oi 1 in the earliest oligocene 34 ma and mi 1 in the earliest miocene 23 ma the paired mg ca and oxygen isotope records are used to calculate seawater 18o w calculated w suggests that a large antarctic ice sheet formed during oi 1 and subsequently fluctuated throughout the oligocene on both short 0 5 myr and long 2 3 myr timescales between about 50 and 100 of its maximum earliest oligocene size the magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy the transient expansion of the antarctic ice sheet at mi 1 is marked in the benthic 18o record by two positive excursions between 23 7 and 22 9 ma each with a duration of 200 300 kyr bottom water temperatures decreased by 2 c over the 150 kyr immediately prior to both rapid 18o excursions however the onset of each of these phases of ice growth is synchronous within the resolution of the records with the onset of a 2 c warming over 150 kyr we suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as antarctic basement silicate rocks became blanketed by an ice sheet this represents a negative feedback process that might have operated during major abrupt growth phases of the antarctic ice sheet

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

resolving a late oligocene conundrum deep sea warming and antarctic glaciation abstract changes in ice volume and resulting changes in sea level were determined for the late oligocene 26 23 ma astronomical timescale ats by applying 18 o to sea level calibrations to deep sea 18 o records from odp sites 689 690 929 1090 and 1218 our results show that maximum global ice volume occurred during two late oligocene 18 o events oi2c 24 4 ma and mi1 23 0 ma inferred glacioeustatic lowering with volumes up to 25 greater than the present day east antarctic ice sheet eais ice volume during glacial minima was on the order of about 50 of the present day eais this is supported by late oligocene stratigraphic records from antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the antarctic continental shelf in the ross sea and prydz bay in contrast composite deep sea 18 o records show a significant decrease 1 between 26 7 and 23 5 ma which have long been interpreted as bottom water warming combined with deglaciation of antarctica however a close examination of individual 18 o records indicates a clear divergence after 26 8 ma between records from southern ocean locations i e ocean drilling program sites 689 690 744 and those of other ocean basins high 18 o values 2 9 3 3 from these southern ocean 18 o records are consistent with an ice sheet on the east antarctic continent equivalent to present day values and cold bottom water temperatures 2 0 c these differences suggest a reduction in deep water produced near the antarctic continent i e proto antarctic bottom water proto aabw which were quickly entrained and mixed with warmer and presumably more saline bottom water originating from lower latitudes expansion of a warmer deep water mass and the weakening of the proto aabw may explain the large intra basinal isotopic gradients that developed among late oligocene benthic 18 o records these conclusions are also supported by ocean modeling suggesting a reduction of deep water formed in the southern ocean strengthening of deep water from the northern hemisphere and decreasing temperatures in high southern latitudes occurred as the drake passage opened to deep water low 18 o values reported from deep sea locations other than the southern ocean are shown to bias estimates of antarctic ice volume calling for a re evaluation of the notion that antarctic ice volume was significantly reduced during the late oligocene

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

i xe ages of campo del cielo silicates as a record of the complex early history of the iab parent body using in situ laser analyses of a polished thin section from the iab iron meteorite campo del cielo we identified two silicate grains rich in radiogenic 129 xe cr diopside and oligoclase excavated them from the metal and irradiated them with thermal neutrons for i xe dating the release profiles of 129 xe and 128 xe are consistent with these silicates being diopside and oligoclase with activation energies estimated using arrhenius plots of 201 and 171 kcal mole 1 respectively the 4556 4 0 4 ma absolute i xe age of the more refractory diopside is younger than the 4558 0 0 7 ma i xe age of the less refractory oligoclase we suggest that separate impact events at different locations and depths on a porous initial chondritic iab parent body led to the removal of the melt and recrystallization of diopside and oligoclase at the times reflected by their respective i xe ages the diopside and oligoclase grains were later brought into the studied inclusion by a larger scale catastrophic collision that caused breakup and reassembly of the debris but did not reset the i xe ages dating the first events the metal melt most probably was 1250 c when it surrounded studied silicate grains this reassembly could not have occurred earlier than the i xe closure in diopside at 4556 4 0 4 ma

phase equilibrium constraints on the viscosity of silicic magmas 1 volcanic plutonic comparison by using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at 104 5 pa s irrespective of their temperature and level of emplacement magmas crystallizing as granitic plutons are not water poor and thus not more viscous than their extrusive equivalents instead comparison between pre eruption magma viscosities of extrusive silicic intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous given the typical strain rates in silicic magma chambers magma rheological behavior is expected to be dominantly newtonian bubbles having a minor rheological influence at depth although exceptions can exist thus whether a silicic intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors but not on the rheology of the magma itself however preemptive viscosities of extrusive magmas rarely exceed 106 pa s which suggests that crystal melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes the narrow range of viscosities displayed by silicic intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas considerations of the relationships between magma crystallinities bulk sio2 and preemptive melt h2o contents show that the higher the melt h2o content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted an empirical correlation is proposed that enables us to estimate preemptive melt h2o contents of erupted magmas by knowing their crystallinity and bulk sio2

40ar 39ar dating of microgram feldspar grains from the paired feldspathic achondrites gra 06128 and 06129 abstract 40 ar 39 ar ages of single feldspar grains from the paired meteorites graves nunatak 06128 gra8 8 grains and 06129 gra9 26 grains are presented plateau ages 70 of the 39 ar released ranged from 4000 to 4600 ma with an average 1 uncertainty of 90 ma the most precise ages obtained were 4267 17 ma for a grain from gra8 and 4437 19 ma and 4321 18 ma for two grains from gra9 isotope correlation diagrams yield less precise ages ranging from 3800 to 5200 ma with an average 1 uncertainty of 250 ma they indicate a negligible trapped component plateau ages integrated total fusion ages and isochron ages are internally concordant at the 95 confidence level the distribution of the plateau ages for gra9 is bimodal with peaks at 4400 and 4300 ma in contrast the plateau age distribution for gra8 peaks at about 4260 ma with broad wings extending toward younger and older ages to explain the distributions of grain ages we prefer a scenario that includes a major post formation event about 4400 ma ago and a later melt intrusion event that heated gra8 more than some parts of gra9

the bombardment history of the moon as recorded by 40 ar 39 ar chronology new petrography and 40ar 39ar ages have been obtained for 1 3 mm sized rock fragments from apollo 16 station 13 soil 63503 north ray crater ejecta and chips from three rocks collected by apollo 16 and apollo 17 missions selection of these samples was aimed at the old 40ar 39ar ages to understand the early history of the lunar magnetic field and impact flux fifteen samples were studied including crustal material polymict feldspathic fragmental breccias and impact melts the impact ages obtained range between approximately 3 3 and 4 3 billion years ga polymict fragmental breccia 63503 1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4 547 0 027 the plateau age of 4 293 0 044 ga obtained for impact melt rock 63503 13 represents the oldest known age for such a lithology possibly this age represents the minimum age for the south pole aitken spa basin in agreement with literature data these results show that impact ages 3 9 ga are found in lunar rocks especially within soil 63503 impact exhumation of deep seated warm crustal material onto the lunar surface is considered to explain the common 4 2 ga ages obtained for weakly shocked samples from soil 63503 and apollo 17 this would directly imply that one or more basin forming events occurred at that time some rock fragments showing none to limited petrologic features indicate thermal annealing these rocks may have lost ar while resident within the hot ejecta of a large basin concurrent with previous studies these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1 3 ga

39ar 40ar chronology of the enstatite chondrite parent bodies ar ar isochron ages of el chondrites suggest closure of the k ar system at 4 49 0 01 ga for el5 and 6 chondrites and 4 45 0 01 ga for el3 mac 88136 the high temperature release regimes contain a mixture of radiogenic 40ar and trapped primordial argon solar or q type with 40ar 36artr 0 which does not affect the 40ar budget the low temperature extractions show evidence of an excess 40ar component the 40ar 36ar is 180 270 it is defined by intercept values of isochron regression excess 40ar is only detectable in petrologic types 4 5 these lost most of their primordial 36ar from low temperature phases during metamorphism and retrapped excess 40ar the origin of this excess 40ar component is probably related to metamorphic ar mobilization homogenization of primordial and in situ radiogenic ar and trapping of ar by distinct low temperature phases ar ar ages of eh chondrites are more variable and show clear evidence of a major impact induced partial resetting at about 2 2 ga ago or alternatively prolonged metamorphic decomposition of major k carrier phases eh impact melt lap 02225 displayed the highest ar ar isochron age of 4 53 0 01 ga this age sets a limit of about 25 45 ma for the age bias between the k ar and u pb decay systems

granitic melt viscosity and silicic magma dynamics in contrasting tectonic settings models for granitic melt viscosities reveal the influences of h 2 o temperature and tectonics on melt segregation ascent and emplacement low t magmas are wetter than those formed at high t so log 10 melt viscosity pa s is 3 2 6 3 for leucogranite to tonalite only 10 1000 times more viscous than basalts there should be no correlation between critical dyke width to prevent freezing and tectonic setting granitic magma ascent rates in dykes vary by

melt segregation and magma interactions during crustal melting breaking out of the matrix abstract differentiation of the continental crust begins with its partial melting the products of crustal melting are silicic hydrous h2o undersaturated granitic liquids that are generated within matrices of residual crystals crustal differentiation requires that felsic magmas form and escape from these solid residua an important question is whether granitic magmas collect into large batches within or near their sources which then give rise to ascent or alternatively bleed out of the sources in smaller streams or pulses we demonstrate that the physical reality is closer to the second alternative and thus question the validity of the concepts of magma segregation and source fertility as they are sometimes visualised granitic plutons contain chemically distinct sub populations formed by source level entrainment of the peritectic assemblages into the melts rapid evacuation at source levels and high ascent rates protect the magmas from wall rock interactions during their tenure at source depths and during their ascent through cool upper crust the existence of different types of granites and hence of clearly defined chemically different magmas within plutons dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move exposed deep crustal sections are typically lithologically diverse with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions if the melts were required to segregate and accumulate into large volumes within their hot sources especially by slow gravity driven melt percolation it is very likely that they would be substantially modified by reaction with diverse source rocks thus the processes mooted to occur in melting assimilation storage and homogenisation mash or deep crustal hot dch zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma perhaps carrying evidence of extensive magma mixing from various lines of evidence we conclude that mash and dch zones may not exist and even if they do they cannot form the sources of most granitic magmas the intrinsic heterogeneity in crustal source rocks the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source inherited chemical and isotopic heterogeneities rapid semi continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole rock trace element chemistry and accessory mineral chemistry will be decoupled from major element variations this also means that a large source with low nominal fertility due to low hydrous mineral content for example could still produce a substantial granitic pluton by additions of individually small increments of magma

effects of chaotic advection on the timescales of cooling and crystallization of magma bodies at mid crustal levels we numerically define the thermochemical evolution of a subduction related crystal bearing magmatic mass at mid crustal levels 0 7 gpa 20 25 km two main dynamic mechanisms are considered 1 a pure buoyancy driven system where the convective flow is induced by density changes during magma cooling 2 a buoyancy driven convective system governed by chaotic advection the non newtonian rheology of natural magmas is taken into account linking the herschel bulkley formulation with the results of fractional crystallization experiments of magmas with the same composition and at the same conditions of temperature and pressure of the studied system the latent heat of crystallization is also considered in order to address the thermal release in the system induced by the crystallization results indicate that the development of chaotic advection generates a complex thermochemical evolution of the system speeding up the crystallization process and the timing required to reach the jamming condition relative to the pure buoyancy driven convective system nearly 2 times faster our results have important implications for both the rheological history of the magmatic body and the refilling of shallower magmatic systems in particular 1 a time dependent composition ranging from basalt to andesite can be extracted from an initial basaltic magmatic batch 2 at the attainment of the maximum packing fraction i e just before the jamming condition homogeneous andesitic melts can be potentially extracted from the system and 3 the development of chaotic advection within the system allows for the extraction of andesitic melt in shorter times compared to a buoyancy dominated system

phase relations in silicic systems at one atmosphere pressure an important control on magma rheology is the extent to which the magma crystallizes during ascent as a result of the effective undercooling created by volatile exsolution to assess this undercooling we need to know the final anhydrous one atmosphere phase relations of silicic magmas for this reason we have performed one atmosphere controlled fo2 crystallization experiments on dacitic to rhyolitic melt compositions 67 78 wt sio2 and determined equilibrium phase assemblages melt fractions and some phase compositions over a range of temperatures experiments were run at oxygen fugacities between nno 1 and nno 2 and temperatures of 1 000 to 1 250 c constant phase compositions and sample crystallinities in runs longer than 3 5 days suggest that these runs closely approached compositional equilibrium additionally melting experiments with similar compositions yielded results closely resembling those obtained in crystallization experiments all samples have liquidus temperatures between 1 250 and 1 200 c with plagioclase the liquidus phase for the two most mafic samples and quartz for the most silicic sample when associated glass compositions are projected into the qz ab or system they define a revised one atmosphere quartz feldspar cotectic 5 10 less quartz normative than previously estimated glass compositions from each sample plot along this cotectic between 1 100 and 1 000 c consistent with the plagioclase quartz co crystallization textures found in runs at these temperatures this cotectic constrains glass compositions to a maximum silica content of 76 1 wt sio2 reported glass compositions in excess of 77 wt sio2 in volcanic samples suggest non equilibrium crystallization perhaps a consequence of large melt undercoolings

towards a coupled physical and chemical model for tonalite trondhjemite granodiorite magma formation abstract tonalite trondhjemite granodiorite ttg magmatism provides a window into the composition of the earth s earliest crust and on rates of heat transfer during the archaean however the tectonic setting in which these magmas form and the physical mechanisms by which ttg melt segregates from its partially molten source rock are poorly understood here we present some simple models aimed at predicting the compositions of partial melts which segregate from thickened mafic amphibolitic or eclogitic lower crust our results suggest that buoyancy driven compaction with melt flow along grain edges can yield large volumes of segregated ttg melt over geologically realistic timescales 4000 years 10 my petrologic diversity is predicted even from a homogenous protolith the chemical composition of the segregated melt varies both spatially and temporally and is governed not only by the composition and mineralogy of the source rock the depth of melting and the melting reactions but also by the physical processes through which the melt migrates and segregates from its partially molten host this simple model represents the first step towards a properly coupled physical and chemical model of ttg formation which is closely constrained by geochemical field and laboratory data such models will compliment the existing geochemical work and help resolve some of the outstanding issues concerning ttg petrogenesis

comment on 40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology by paul r renne

reply to comment on 40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology by mario trieloff elmar k jessberger and christine fieni

characteristics of internal contacts in the tuolumne batholith central sierra nevada california usa implications for episodic emplacement and physical processes in a continental arc magma chamber internal contacts along the eastern margin of the tuolumne batholith central sierra nevada california and structures preserved along these contacts are highly variable the contacts range from relatively sharp to gradational boundaries to sheeted zones to very complex boundaries formed by multiple processes fractional crystallization kilometer scale mixing within broad transition zones voluminous magmatic stoping along sharp contacts and downward return flow and or margin collapse of older magma units were important large scale processes along these contacts during chamber construction in contrast sheeting extensional cracking and diking represent only second order small scale complexities formation of the most complex zones along internal contacts resulted from the interaction of the sequential emplacement of different units with the irregular geometry of these contacts which often resulted from removal of earlier phases by stoping our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved we also argue that fairly large magma chambers existed in this batholith and thus that large accumulations of magma may exist in upper crustal chambers for significant periods of time

maximum and minimum water contents of granitic melts implications for chemical and physical properties of ascending magmas new results obtained by the investigation of liquidus and solidus phase relations in the system qz ab or and by the determination of h2o solubility in haplogranitic melts are used to discuss the evolution of granitic quartzo feldspathic magmas during their ascent in the crust particular attention is given to the role of h2o and its effect on physical properties of ascending magmas the evolution of melt fraction h2o content of the melt and viscosity of melt and magma during ascent are discussed for several examples adiabatic ascent ascent with cooling ascent with crystal fractionation it is shown that decompression melting processes may be very important significant increase of the melt proportion especially at low pressure in magmas ascending along adiabats or with slow cooling rates the comparison of the discussed examples shows that melts formed at low t are significantly less viscous than at high t although the degree of melting may remain very low in the case of dehydration melting at low temperature the low viscosity of the generated melts may enhance the extraction of these melts from the source area to produce leucogranites

quantifying magma segregation in dykes abstract the dynamics of magma flow is highly affected by the presence of a crystalline load during magma ascent it has been demonstrated that crystal melt segregation constitutes a viable mechanism for magmatic differentiation moreover crystal melt segregation during magma transport has important implications not only in terms of magma rheology but also in terms of differentiation of the continental crust however the influences of the crystal volume percentage of their geometry their size and their density on crystal melt segregation are still not well constrained to address these issues we performed a parametric study using 2d direct numerical simulations which model the ascension of a crystal bearing magma in a vertical dyke using these models we have characterised the amount of segregation as a function of different physical properties including the density contrast between crystals and the melt phase the size of the crystals ac and their aspect ratio r results show that small values of r do not affect the segregation in this case the amount of segregation depends upon four parameters segregation is highest when and ac are large and lowest for large pressure gradient pd and or large values of dyke width wd these four parameters can be combined into a single one the snumber which can be used to quantify the amount of segregation occurring during magma ascent based on systematic numerical modelling and dimensional analysis we provide a first order scaling law which allows quantification of the segregation for an arbitrary snumber and encompassing a wide range of typical parameters encountered in terrestrial magmatic systems although developed in a simplified system this study has strong implications regarding our understanding of crystal segregation processes during magma transport our first order scaling law allows to immediately determine the amount of crystal melt segregation occurring in any given magmatic dyke system

development of complex sub vertical layering in the cortaderas gabbro intrusion central chile abstract the cortaderas intrusion is a small gabbroic unit spatially associated with the lower cretaceous felsic illapel plutonic suite of central chile it is unmetamorphosed largely unaltered and not significantly tilted most of the intrusion has well developed vertical to steep layering dominantly defined by variations in abundance in plagioclase and amphibole in some places the layers are regular and rhythmic with a spacing of about 0 5 2 cm but elsewhere layering is more complex with solitary plagioclase rich bands 1 cm wide in some areas layers are convoluted sometimes with channel like structures apparent cross cutting relationships and balloon like structures pegmatoids occur as patches and layers in some places with miarolitic cavities plagioclase and amphibole dominate the mineralogy with minor clinopyroxene as relicts in amphibole and minor olivine chemical and isotopic data from different samples cannot be linked by any single simple process and several different magma sources are necessary we propose that the intrusion is a fossil conduit originally linking magma sources possibly associated with the base of the illapel plutonic suite to basaltic andesite to andesitic volcanism the first magmas flowed along a major fault heated the host rocks and solidified as unlayered gabbros later magmas flowed up the same structure but cooled slowly in the preheated host rocks producing the layered parts of the intrusion layering was produced by non dynamic processes perhaps similar to that which produces liesegang banding plagioclase and pyroxene crystallised first to produce a loose framework close to the walls of the conduit latent heat from growth of amphibole may have facilitated equilibration by development of layering and coarsening the channel like and cross cutting structures may have resulted from interactions of chemical diffusion waves with discontinuities in permeability of the crystal framework during the development of the crystal framework the interstitial silicate liquid became enriched in water partly dissolving earlier structures and crystallising as pegmatoids some with miarolitic cavities

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

layered granodiorites at chebucto head south mountain batholith nova scotia abstract a sequence of more than 40 distinct rhythmically banded layers typically 10 30 cm thick occurs in the contact zone granodiorite of the south mountain batholith nova scotia the layered sequence shows well developed modal mineralogical variations grain size variations layer bifurcations scour and fill structures cross bedding slump structures and feeder dykes various processes granitization of sediments closed system fractionation double diffusion shear flow are incapable of explaining the field observations moreover field relations suggest that the layers formed stratigraphically toward the roof of the batholith and petrographic evidence suggests that they formed episodically from a magma of eutectoid composition a physical model for individual layers invokes stokes s law to describe microphenocryst settling to form the fine grained mafic layer bottoms and jackson s growth rate equation to describe formation of the coarser grained felsic layer tops model input variables include the initial grain sizes and their distribution in the layers chemical diffusion rates magma viscosities and newtonian or bingham rheologies the model predicts settling times of approximately 10 years for the microphenocrysts to form the fine grained biotite rich layer bottoms and mechanical solidification times of approximately 10 25 years for the coarser grained felsic upper parts of the layers thus we believe that a coupled and repeating sequence of congealed roof block subsidence to create space and magma injection into that space can account for the rhythmic layering in the granodiorite at chebucto head

thermal and dynamical regimes of single and two phase magmatic flow in dikes finite element calculations of magma flow in dikelike channels with length to width ratios of 1000 1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature dependent viscosity and dissipation steady state solutions with realistic thermal and dynamical parameter values have been obtained the models show that the onset of solidification on the boundaries of a basaltic or andesitic dike as predicted by idealized laminar flow models can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles boundary roughness or turbulence this effect will reduce the critical initial widths of dikes estimated by bruce and huppert 1989 in the absence of transverse flow the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike with converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected for viscous heating to offset significant rates of heat loss 2 kw m2 in a plane parallel 1 m wide basaltic dike an average flow velocity of 2 7 m s driven by a pressure gradient of 1 7 mpa km is required a 7 void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient the ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long furthermore a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer however the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two component magma mixture in which the components have different viscosities rises within a dike within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part effectively lubricating the passage of the more viscous component compositional variations in the obsidian dome volcano support the occurrence of this pressure reduction or self lubrication mechanism such a self lubrication process often may be necessary to permit very viscous magmas to reach earth s surface if so chemical and lithologic zoning would be anticipated as a common feature of near surface intrusions or lava flows that are characterized by a high silica and or high crystal content such as at deadman dome in long valley california

the role of viscous particle segregation in forming chromite layers from slumped crystal slurries insights from analogue experiments the question of how the mineral layering in layered intrusions forms has been extensively debated for many decades there are many types of layering and it is of course possible that a number of mechanisms are involved of particular interest is how chromite layers form because these may contain valuable metals such as platinum group elements in addition to cr one model for the formation of these layers is that they formed through slumping of semi consolidated cumulates from the margins of the intrusion into the magma chamber during this slumping the grains are sorted by density and or size differences this study examines the viability of this process using analogue modelling starting materials beads and glycerine were scaled to match the density and size of the minerals chromite orthopyroxene and plagioclase present in layered mafic ultramafic layered intrusions and to match the density and viscosity of the silicate magma a perspex flume tank divided by a removable partition at one end was fully filled with glycerine a homogenized mixture of the beads was placed in the smaller partition of the tank representing the margins of the magma chamber the tank was then inclined between 16 and 45 the partition was removed and the beads flowed into the main part of the box the experiments were recorded by video camera allowing us to follow the dynamics of the flow during each run segregation of the beads was observed in the final deposits the larger less dense beads representing plagioclase concentrated at the top of the flow with the intermediate sized and medium density beads representing orthopyroxene in the middle and the smaller denser beads representing chromite at the bottom thus mimicking natural examples in experiments where the angle of inclination was low long thin layers formed such as those found in the bushveld complex in experiments where the angle of inclination was high thick but short layers formed a dimensionless analysis allows better understanding of the dynamics of the flow at the macroscopic scale the flow regime is strongly influenced by the viscosity of the fluid and is considered macro viscous where the role of the interstitial liquid is non negligible

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

melts mush and more evidence for the state of intermediate to silicic arc magmatic systems understanding the physical state of intermediate to silicic arc magmatic systems is necessary for our petrologic models of these systems researchers have generated a plethora of data geophysical geochronological petrological theoretical over the past few decades these data have changed how we view arc magmatic systems leading to a model of crystal mush that is rejuvenated repeatedly over the lifespan of the magmatic system however much data are either circumstantial or incomplete paterson et al 2016 october issue use a combined set of textural geochemical and temporal data to demonstrate the changing physical state of the tuolumne intrusive complex in california over its 10 million year history they offer evidence for magmatic erosion and recycling along with the potential for a surge growth of the batholith that allows for significant volumes of magma to exist ephemerally under arc volcanoes most intermediate to silicic arc magmas henceforth arc magma are stored 3 km depth so direct observation is impossible instead this question has been approached in four main ways 1 geophysical inspections of active arcs 2 mineral geochronology 3 field observations of plutons and 4 thermal and physical modeling of magmatic systems each of these four avenues has their own strengths and shortcomings many times the interpretation of these data appears to be at odds some overarching observations can be made about what we know of the state of active intermediate to silicic arc magmatic systems geophysical observations such as seismic tomography have yielded little evidence for large bodies of fully molten magma persistently underneath arc volcanoes if anything only small lenses of partial melt are observed at

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

mafic schlieren crystal accumulation and differentiation in granitic magmas an integrated case study observations in the s type granites s l of the wilson s promontory batholith demonstrate that one type of schlieren in granitic rocks represent accumulations of mainly mafic magmatic minerals with internal layering formed through pulsed magma flow loss of interstitial magmatic liquid played at most a minor role in shaping the preserved compositions of the schlieren filter pressing was not involved and simple gravity settling of crystals was also insignificant through dissolution reprecipitation and reactions with residual magmatic liquids in the wilsons promontory schlieren the original accumulated crystals of mafic minerals were largely supplanted by later generations or completely new phases in the present case the original accumulating minerals were garnet and orthopyroxene with minor biotite and accessory minerals the schlieren retain some of the early euhedral compositionally distinct accumulated biotite but most biotite formed through reaction of accumulated orthopyroxene and garnet with residual liquid some early accumulated garnet remains but this is not the peritectic garnet that was originally entrained into the magmas at source depths rather these are magmatic crystals formed at mid crustal depths through dissolution reprecipitation of the original peritectic garnet at emplacement level another episode of garnet dissolution reprecipitation occurred close to the solidus extensively reorganising the grain scale igneous textures although schlieren preserve structural chemical and some textural features that can be used to infer their origins their present microtextures do not fully reflect their initial formation mechanisms also the physical mechanisms of schlieren formation most commonly have little similarity with the processes that were responsible for the main chemical variations in the batholith nevertheless the presence of schlieren indicates that the granitic magmas were flowing in sheet like laminae and they provide information on the high temperature igneous minerals that were suspended in their parent magmas prior to emplacement

origin of megacrysts in granitoids by textural coarsening a crystal size distribution csd study of microcline in the cathedral peak granodiorite sierra nevada california microcline megacrysts in the cathedral peak granodiorite and other parts of the tuolumne intrusive suite were formed by textural coarsening ostwald ripening of earlier formed crystals the early formed crystals nucleated and grew in an environment of increasing undercooling probably during the ascent of the magma emplacement of the magma into warm host rocks promoted textural coarsening crystals smaller than a certain size the critical size dissolved in the interstitial melt whilst large crystals grew microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts megacryst growth was halted when cooling resumed at the end of the intrusive cycle k feldspar nucleation was then renewed and k feldspar crystals grew to form part of the groundmass it was the particular thermal history of this pluton that promoted textural coarsening chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture

new insights into the origin of ladder dikes implications for punctuated growth and crystal accumulation in the cathedral peak granodiorite abstract ladder dikes are steep tabular bodies typically a meter or less thick composed of moderately dipping concave upward alternating dark i e schlieren and light bands oriented roughly perpendicular to the ladder dike margins these structures occur widely but sparsely in granitic rocks and are found prominently in the cathedral peak granodiorite cpg of the tuolumne intrusive suite previous studies have interpreted that ladder dikes form as a result of processes including the downward flow of crystal mush in cracks within strong crystal mush or by upward flow in steep tubes that migrate within a strong crystal mush our new observations indicate that ladder dikes formed by downward flow of crystal mush in troughs or valleys in a manner potentially comparable to trough bands in mafic layered intrusions extensions of the schlieren outward and upward away from the ladder dike margins into the host granite demonstrate that the host granite was deposited as mounds on both sides at the same time as the ladder dikes ladder dikes therefore record lateral flows of crystal mush on a magma chamber floor vertical exposures suggest these flows are on the order of ten meters thick some steep exposures on granite domes indicate multiple ladder dikes and flows over a stratigraphic height of 80 100 m later stratigraphically higher flows commonly deform and erode the top of an earlier flow and granitic material rich in k feldspar megacrysts has locally engulfed large blocks of ladder dikes demonstrating that the megacrysts were also transported in flows flows in the cpg are directed away from the center of the pluton toward the western and eastern margins and apparently spread along a strong crystal mush floor and into a rheologically complex cpg magma whereas established dynamical models for spreading single phase gravity currents with simple and complex rheologies explain the elongate geometry spacing and orientation of the tabular bodies the origin and character of the downward flows required to explain the trough band schlieren structures is challenging however an intermittent and progressive deposition of trough bands consistent with field observations is potentially explained if the two phase crystals and melt dynamics governing the response of the cpg magma to a new injection are considered

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

silicic magma differentiation in ascent conduits experimental constraints abstract crystallization of water bearing silicic magmas in a dynamic thermal boundary layer is reproduced experimentally by using the intrinsic thermal gradient of piston cylinder assemblies the standard agv2 andesite under water undersaturated conditions is set to crystallize in a dynamic thermal gradient of about 35 c mm in 10 mm length capsules in the hotter area of the capsule the temperature is initially set at 1200 c and decreases by programmed cooling at two distinct rates of 0 6 and 9 6 c h experiments are conducted in horizontally arranged assemblies in a piston cylinder apparatus to avoid any effect of gravity settling and compaction of crystals in long duration runs the results are conclusive about the effect of water rich fluids that are expelled out the crystal rich zone mush where water saturation is reached by second boiling in the interstitial liquid expelled fluids migrate to the magma ahead of the solidification front contributing to a progressive enrichment in the fluxed components sio 2 k 2 o and h 2 o the composition of water rich fluids is modelled by mass balance using the chemical composition of glasses quenched melt the results are the basis for a model of granite magma differentiation in thermally zoned conduits with application of in situ crystallization equations the intriguing textural and compositional features of the typical autoliths accompanying granodiorite tonalite batholiths can be explained following the results of this study by critical phenomena leading to splitting of an initially homogeneous magma into two magma systems with sharp boundaries magma splitting in thermal boundary layers formed at the margins of ascent conduits may operate for several km distances during magma transport from deep sources at the lower crust or upper mantle accordingly conduits may work as chromatographic columns contributing to increase the silica content of ascending magmas and at the same time leave behind residual mushes that eventually are dragged as enclaves or autoliths

eocene circulation of the southern ocean was antarctica kept warm by subtropical waters 1 near the eocene s close 34 million years ago the climate system underwent one of the largest shifts in earth s history antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed previous studies conjectured that poleward penetration of warm subtropical currents the east australian current eac in particular caused eocene antarctic warmth late eocene opening of an ocean gateway between australia and antarctica was conjectured to have disrupted the eac cooled antarctica and allowed ice sheets to develop here we reconstruct eocene paleoceanographic circulation in the tasmanian region using 1 biogeographical distributions of phytoplankton including data from recently drilled ocean drilling program leg 189 sites and 2 fully coupled climate model simulations we find that the eac did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern our results do not support changes in thermal isolation as the primary driver of the eocene oligocene climatic transition

hf w chronometry of the iab iron meteorite parent body abstract the iab iron meteorites are unusual in having abundant silicate inclusions but the relationship of metal and silicates is not well understood in order to study the sequence and the timing of events that led to the observed structure of iab iron meteorites the hf w isotope system was applied to metal and silicate separates from seven iab iron meteorites a major metal segregation event on the iab parent body is dated by the average 182 w 184 w ratio of the iab metal phases 3 1 0 2 e 182 w units corresponding to a time of 1 8 2 3 2 0 ma after the formation of ca al rich inclusions cais the 182 hf 182 w data for three bulk inclusions from landes copiapo and el taco define a linear trend interpreted as an isochron corresponding to an age of 2 5 2 3 2 0 ma after cai formation this age most likely dates silicate differentiation on the iab parent body and is within analytical uncertainty indistinguishable from the major metal segregation event several silicate inclusions are displaced from the iab silicate isochron towards less radiogenic w compositions most likely reflecting thermal events on the parent body metal segregation and silicate differentiation occurred early enough so that these processes could have at least partially been triggered by an internal heat source e g 26 al basaltic inclusions of the caddo county specimen define an age of 4 6 0 7 0 6 ma most likely dating a partial melting event on the iab parent body three silicate separates from another el taco inclusion yield an age that postdates cai formation by 10 8 2 4 2 0 ma and the iab silicate isochron by 8 3 ma this prolonged interval of local thermal events on the iab parent body are best explained by collisional heat sources i e impacts the hf w ages for thermal events on the iab parent body are within the range of previously reported 129 i 129 xe ages but are significantly older than published 87 rb 87 sr 147 sm 143 nd and most 40 k 40 ar ages most likely these chronometers were reset during later thermal events that had only a minor effect on the 182 hf 182 w and 129 i 129 xe systems

thermodynamic modeling for an incrementally fractionated granite magma system implications for the origin of igneous charnockite abstract understanding fractionation of silicic magma is crucial to advance our knowledge of differentiation of continental crust enrichment of elements of economic interest and plutonic volcanic connection microstructural records afford critical appraisals for silicic magma fractionation yet are rarely reported in granite plutons here we combine detailed microstructural observations and thermodynamic modeling to quantify the components and the conditions of silicic magma fractionation using the peraluminous jiuzhou pluton south china as an example the pluton shows compositional gradients from primitive orthopyroxene bearing granite charnockite at stratigraphically low levels to relatively evolved orthopyroxene free granite at stratigraphically high levels deviation of whole rock compositions from metasediment sourced experimental melts and step zoned plagioclase and alkali feldspar crystals in the exposed rocks suggest open system fractionation by melt extraction partial dissolution and subsequent crystallization from trapped minimum melt crystal cluster and chain fabrics and viscous deformation are more abundant in the charnockites than in the overlying orthopyroxene free granites suggesting that gravitational compaction driven fractionation increased towards the bottom of the pluton field observations thermodynamic modeling and petrographic studies further demonstrate that gravitational compaction reduces the trapped melt fraction of a crystal mush with thickness of 100 m from 30 wt at the upper level to 10 wt at the lower level of the pluton significant melt extraction restricted back reaction with high temperature phases during progressive crystallization which preserved orthopyroxene during the solidification of granitic magma the compositional mineralogical and textural zoning of the jiuzhou pluton suggests that incremental fractionation of granite systems may be an important process that produces compositionally zoned cumulate incremental fractionation may occur in many zoned granite plutons worldwide causing their whole rock composition to deviate from their primary melt composition detailed microstructural examination for these granite plutons may provide insights into the mechanism s for melt extraction and crystal accumulation of silicic magma providing key insights towards quantifying fractionation of magma systems

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

new implications for the origin of the iab main group iron meteorites and the isotopic evolution of the noncarbonaceous nc reservoir abstract the origin of the iab main group mg iron meteorites is explored through consideration of 182w isotopic compositions thermal modeling of 26al decay and mass independent nucleosynthetic mo isotopic compositions of planetesimals formed in the noncarbonaceous nc protosolar isotopic reservoir a refined 182w model age for the meteorites campo del cielo canyon diablo and nantan suggests that the iab mg parent body underwent some form of metal silicate segregation as early as 5 3 0 4 myr after calcium aluminum rich inclusion cai formation or as late as 13 8 1 4 myr after cai formation if melting of the iab mg occurred prior to 7 myr after cai formation it was likely driven by 26al decay for a parent body radius 40 km otherwise additional heat from impact is required for melting metal this late in solar system history if melting was partially or wholly the result of internal heating a thermal model of 26al decay heat production constrains the accretion age of the iab mg parent body to 1 7 0 4 myr after cai formation if melting was instead dominantly caused by impact heating thermal modeling suggests the parent body accreted more than 2 myr after cai formation comparison of mo mass independent isotopic compositions of the iab mg to other nc bodies with constrained accretion ages suggests that the mo isotopic composition of the nc reservoir changed with time and that the iab mg parent body accreted between 2 to 3 myr after cai formation thus requiring an origin by impact the relationship between nucleosynthetic mo isotopic compositions and accretion ages of planetesimals from the nc reservoir suggests that isotopic heterogeneity developed from either addition of s process material to or removal of coupled r p process material from the nc reservoir

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

is stoping a volumetrically significant pluton emplacement process magmatic stoping is widely called upon to explain discordant pluton contacts but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons first xenoliths rarely make up more than a few percent of most plutons this sparseness even on well exposed pluton floors argues that few xenoliths make it into plutons second piles of rock fragments have sufficiently high porosities 50 that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much if not all of the available magma producing pluton scale magmatic breccias that are not observed third although settling calculations show that large meter scale or larger xenoliths could sink rapidly in magma natural fragmentation processes are likely to produce a fractal distribution of particle sizes with small particles far more abundant than large ones the absence of small xenoliths at pluton contacts where magma cools most rapidly thus argues against stoping fourth although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks finally there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally stoping cannot be volumetrically important in incrementally emplaced plutons because blocks can only fall through the volume that is molten at any given time where xenoliths are abundant in plutons they may commonly represent isolation of wall rock bodies between successive intrusive increments rather than wall rock fragments that were engulfed by magma however even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber although stoping undoubtedly occurs during the emplacement of some plutons it is unlikely to be a volumetrically significant process

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

late oligocene to early miocene geochronology and paleoceanography from the subantarctic south atlantic mi2 as well as several other distinctive isotope events our data suggest that the d 18 o maximum commonly associated with the oligocene miocene o m boundary falls within c6cn 2r 23 86 ma the d 13 c maximum coincides within the temporal resolution of our record with c6cn 2n r boundary and hence to the o m boundary comparison of the stable isotope record from site 1090 to the orbitally tuned stable isotope record from odp site 929 across the o m boundary shows that variability in the two records is very similar and can be correlated at and below the o m boundary site 1090 stable isotope records also provide the first deep southern ocean end member for reconstructions of circulation patterns and late oligocene to early miocene climate change comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow oxygen isotope gradients between sites suggest that the deep southern ocean was cold in comparison to the north atlantic indian and the pacific oceans dominance of cold southern component deep water at site 1090 at least until 17 ma suggests that relatively cold circumpolar climatic conditions prevailed during the late oligocene and early miocene we believe that a relatively cold southern ocean reflects unrestricted circumpolar flow through the drake passage in agreement with bathymetric reconstructions index terms 4267 oceanography general paleoceanography 1035 geochemistry geochronology 4870 oceanography biological and chemical stable isotopes keywords paleoceanography 1035 geochronology stable isotopes

origin of rapidly solidified metal troilite grains in chondrites and iron meteorites inclusions of troilite and metallic fe ni 0 2 4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites cooling rates in the interval 1400 950 c calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10 7 to 104 c sec in 8 of these chondrites which are breccias containing some normal slow cooled metal grains the inclusions solidified before they were incorporated into the breccias their cooling rates of 1 300 c sec indicate cooling by radiation or by conduction in contact with cold silicate or hot silicate volumes only 6 40 mm in size this is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body by impact and were not formed at depth from an internally derived melt in ramsdorf rose city and shaw which show extensive reheating to 1000 c fe fes textures in melted areas are coarser and indicate cooling rates of 10 1 to 10 4 c sec during solidification this metal may have solidified inside hot silicate volumes that were 10 300 cm in size as shaw and rose city are breccias of unmelted and melted material their melted metal did not necessarily cool through 1000 c within a few m of the surface shock melted fine grained irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of 105 c sec their kamacite and taenite compositions may result from annealing at 250 c of metallic glass or exceedingly fine grained quench products

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

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

petrogenesis of late early cretaceous high silica granites from thebangong nujiang suture zone central tibet abstract high silica granites are common in collisional orogenic belts however the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated in this study we present zircon u pb ages and hf isotopes and whole rock geochemical and sr nd isotopic compositions for biotite granites from the jiang co area in the bangong nujiang suture zone central tibet zircon u pb dating reveals that the jiang co granites were emplaced during the late early cretaceous ca 114 ma these granites are characterized by high and variable sio2 69 7 78 2 wt and k2o 4 7 8 4 wt and low mgo 0 09 1 25 wt and p2o5

repeated multiscale magmatic erosion and recycling in an upper crustal pluton implications for magma chamber dynamics and magma volume estimates the tuolumne intrusive complex an upper crustal 7 11 km emplacement depths incrementally constructed 95 85 ma growth history plutonic complex 1100 km 2 preserves evidence from several data sets indicating the repeated multiscale magmatic erosion of older units occurred and that some eroded material was recycled into younger magma batches these include 1 map patterns of internal contacts hundreds of kilometers that show local hybrid units truncations and evidence of removal of older units by younger 2 the presence of widespread xenolith and cognate inclusions thousands including composite inclusions 3 the presence of widespread enclaves millions including composite enclaves plus local enclave swarms that include xenoliths and cognate inclusions 4 the presence of widespread schlieren bound magmatic structures 9000 showing evidence of local meter scale truncations and erosion 5 antecrystic zircons billions and other antecrystic minerals from older units now residing in younger units 6 whole rock geochemistry including major element ree and isotopic data and 7 single mineral petrographic and geochemical studies indicating mixing of distinct populations of the same mineral synthesis of the above suggests that some erosion and mixing occurred at greater crustal depths but that thousands of erosion events at the emplacement site resulted in removal of 35 55 of the original plutonic material from the presently exposed surface with some 25 being recycled into younger magmas and the remainder was either erupted or displaced downward the driving mechanisms for mixing recycling are varied but likely include buoyancy driven intrusion of younger batches into older crystal mushes collapse and avalanching along growing and over steepened solidification fronts within active magma chambers 1 to 500 km 2 in size and local convection in magma chambers driven by internal gradients e g buoyancy temperature and composition

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

contrasting magmatic structures between small plutons and batholiths emplaced at shallow crustal level sierras de cordoba argentina abstract processes like injection magma flow and differentiation and influence of the regional strain field are here described and contrasted to shed light on their role in the formation of small plutons and large batholiths their magmatic structures the final geometric and compositional arrangement of magma bodies are a complex record of their construction and internal flow history magma injection flow and differentiation as well as regional stresses all control the internal nature of magma bodies large magma bodies emplaced at shallow crustal levels result from the intrusion of multiple magma batches that interact in a variety of ways depending on internal and external dynamics and where the early magmatic growth related structures are commonly overprinted by subsequent history in contrast small plutons emplaced in the brittle ductile transition more likely preserve growth related structures having a relatively simple cooling history and limited internal magma flow outcrop scale magmatic structures in both cases record a rich set of complementary information that can help elucidate their evolution large and small granitic bodies of the sierra pampeanas preserve excellent exposures of magmatic structures that formed as magmas stepped through different rheological states during pluton growth and solidification these structures reveal not only the flow pattern inside magma chambers but also the rheological evolution of magmas in response to temperature evolution

diffusion reaction in a thermal gradient implications for the genesis of anorthitic plagioclase high alumina basalt and igneous mineral layering abstract piston cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt rock reaction processes occurring during magma differentiation in the crust in two experiments juxtaposing basaltic andesite and gabbro at 0 5 gpa pressure for durations of either 13 or 26 days diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase specifically the basaltic andesite gains al2o3 mgo and cao from the gabbro and loses na2o k2o sio2 and feo to it with a plagioclase rich layer developing at the interface between the two materials in a process termed diffusion reaction the percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores up to an90 that abruptly shift in composition to thin rims that are in na ca exchange equilibrium with the co existing melt both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion reaction isotopic tracers 45ca 6li 84sr and 136ba initially deposited at the basaltic andesite gabbro interface in the 13 day experiment were detected in the cores of the anorthitic plagioclase after the experiment demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion reaction experiment the formation of anorthitic plagioclase during diffusion reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra calcic melts textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 c for instance apatite fenis olivine and almost pure albite occur at distinct specific horizons in the gabbro within the 26 day experiment because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro equilibration between minerals and fluids melts must be rapid the overall effect of the diffusion reaction process is to make an ascending magma more primitive in composition and in this case produce anorthitic plagioclase while making surrounding crustal wall rocks more evolved several observations within igneous rocks support the occurrence of this process suggesting that the genesis of porphyritic high alumina basalt ubiquitously observed at convergent margins could reflect a diffusion reaction process in the crust

granular flow and viscous fluctuations in low bagnold number granitic magmas recent thermal and fluid dynamical models have shown that density driven ascent of isoviscous granitic melt through the crust in narrow dykes is geologically instantaneous provided the role of suspended crystals is ignored however it is well known that solids in suspension can avect significantly the rheology and hence flow properties of magmas leading to the subjective criticism that granite ascent and emplacement rates based on simple dyke transport models may be unrealistic by estimating the magnitude of shear stresses developed during continuous flow in a 6 m wide granitic dyke we show yield strengths in excess of 10 4 pascals are required to stop flow entirely if the pressure gradient driving the flow is high using granular flow theory we show how fluctuations in particle velocity in the shear field can be used to predict the rheological behaviour of magmas with crystal contents solidosity in the range 0 1 specimen calculations for the solidosity and fluctuation on particle velocity for ascending granitic magmas show that the energy associated with these irregular movements is much greater than that expected on purely brownian grounds we find no clear correlation between viscosity i and the yield strength and while it is possible that magma has curious unexpected bonding properties magmatic suspensions where the grain size is on the scale of centimetres e g some coarsely porphyritic magmas low estimated bagnold numbers c 10 8 render bingham flow models inappropriate self organization of the flow into a crystal poor margin and crystal rich core flow differentiation by diffusion of particles towards the region of minimal shear results in the familiar plug type flow profiles traditionally associated with an inherent yield strength shear enhanced diffusion is an inevitable consequence of granular flow that leads to flow differentiation providing a simple mechanistic explanation for high concentrations of pheoncrysts in the cores of porphyritic dykes bagnold effect deviation from simple newtonian flow is unlikely to be significant until the magma has stopped ascending and emplacement begins

magmatic life at low reynolds number the extremely high viscosities of high silica liquids mean that intermediate and high silica magmas exist deep within the laminar range of fluid behavior many common interpretations of layering in plutonic rocks rely on analogies with sedimentation in water but for such high viscosity liquids including interstitial liquid in partially crystallized intermediate magmas these analogies cannot be correct particle reynolds numbers for high silica liquids are exceedingly small on the order of 10 6 or less in such a regime inertia is negligible and everyday experience is inapplicable layering and other pseudo sedimentary structures in granodiorites and granites likely arise from processes such as chemical diffusion self organization and crystal ripening

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

40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology acapulcoites consistently yield some of the oldest 40ar 39ar ages reported for any meteorites all indistinguishable from the age of 4510 22 ma previously determined for whole rock samples of acapulco these ages are significantly younger at face value than the 4557 2 ma pb pb age reported for phosphates which dates cooling through 550 c metallographic evidence for rapid cooling between 650 and 350 c retention of 129xe in apatite and the absence of shock features all suggest that the 40ar 39ar ages might be expected to be more consistent with the pb pb age new 40ar 39ar data from plagioclase determined to be the sole k rich phase in acapulco yield a high precision isochron age of 4507 18 ma 2 based on currently accepted decay constants and standards but neglecting uncertainties in these quantities these data allow unambiguous association of the 40ar 39ar age with a closure temperature near 300 c and preclude the possibility of artifacts due to 39ar recoil low temperature alteration products or inherited pre solar grains the 40ar 39ar plagioclase age recalculated with decay constants believed to be more accurate than those conventionally used is 4554 ma weakening the basis of a previously inferred dramatic decrease in cooling rate shortly after closure of the pb pb system in phosphate systematic errors in 40ar 39ar dating associated chiefly with 40k decay constants and standards must be redressed before the method can be applied to high resolution inference of early solar system evolution similar limitations affect ages based on other long lived radionuclides such as 147sm

uturuncu volcano bolivia volcanic unrest due to mid crustal magma intrusion uturuncu volcano sw bolivia is a dormant stratovolcano 85 km3 dominated by dacitic lava domes and flows 39ar 40ar ages show that the volcano was active between 890 ka and 271 ka with the lavas becoming younger and less extensive at higher elevations there are current signs of unrest between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm yr deformation indicates volume changes of 400 x 108 m3 over 14 years an average of 1 m3 s 10 2 km3 yr the deformation is attributed to magma intrusion into the altiplano puna regional crustal magma body deformation models indicate a source at depths of 17 to 30 km beneath current local relief in a reconnaissance survey persistent seismic activity mean of 2 6 earthquakes per hour with a maximum of 14 per hour was recorded at about 4 km depth below the center of the uplift 4 km sw of the volcano s summit the seismic events have a normal b value 1 04 and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion the porphyritic dacite lavas 64 68 sio2 have a plagioclase orthopyroxene biotite magnetite ilmenite assemblage and commonly contain juvenile silicic andesite inclusions cognate norite nodules and crustal xenoliths temperature estimates are in the range 805 to 872 c for the dacites and about 980 c for the silicic andesites the dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite reversely zoned orthopyroxene phenocrysts in the andesitic end member are explained by changing oxidation states during crystallization fe3 fe2 ratios from orthopyroxene crystals and fe3 in plagioclase provide evidence for a relatively reduced melt that subsequently ascended degassed and became more oxidized as a consequence of degassing the geophysical and petrological observations suggest that dacite magma is being intruded into the altiplano puna regional crustal magma body at 17 km or more depth consistent with deformation models in the late pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together the current unrest together with geophysical anomalies and 270 ka of dormancy indicate that the magmatic system is in a prolonged period of intrusion such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation

porosity and permeability variations in fractured and liesegang banded breathitt sandstones middle pennsylvanian eastern kentucky diagenetic controls and implications for modeling dual porosity systems abstract middle pennsylvanian fluvial sandstones in eastern kentucky breathitt group manifest visible evidence of alteration related to fluid flow localized through near vertical joints fracture related alterations involve both physical and chemical modifications that together create dramatic permeabilisty variations at the outcrop scale on the fracture surface infiltered detritus combined with mineral and organic coatings have reduced pore sizes and hence permeabilities 0 03 0 44 md by an order of magnitude over values characteristic of the adjacent sandstone 0 32 1 53 md prominent zones of orange brown discoloration contain evidence of oxidation reactions and form an envelope of variable thickness around the fractures authigenic iron oxides are not uniformly distributed within these zones but rather are concentrated as local bands of pervasive mineralization commonly known as liesegang bands petrographic evidence suggests that most of the iron that now resides in oxidized authigenic phases was derived from solutes mobilized through dissolution of older iron bearing authigenic minerals permeability and pore sizes within the oxidation zone are bimodal and vary from high values similar to those in adjacent unoxidized sandstone to low values associated with the zones of pervasive mineralization that approach those values observed for the fracture skin the large magnitude of permeability variation around fracture systems in these sandstones documents the presence of a dual porosity system and suggests that fluid and contaminant transport cannot be realistically modeled using average rock properties

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

formation of igneous layering in granodiorite by gravity flow a field microstructure and geochemical study of the tuolumne intrusive suite at sawmill canyon california layered granites and granodiorites occur in many plutons they occur together with non layered granodiorites at the margin of the tuolumne intrusive suite one of the largest late cretaceous intrusions in the sierra nevada results from field observations quantitative microstructural analysis and whole rock compositions from the non layered kuna crest half dome and cathedral peak granodiorites and layered granodiorites from the sawmill canyon area are compared to determine which processes contributed to the formation of the layering the non layered granodiorites have foliated but homogeneous microstructures which are attributed to the orientation of crystals with high aspect ratios when the magma flowed before solidification their compositions are consistent with magma mixing and fractional crystallization the microstructure in the layered granodiorite is heterogeneous and shows normal gradation of the dense primocryst minerals magnetite hornblende and titanite at the base and inverse size gradation with large crystals of the less dense primocryst phases plagioclase and biotite at the top and a systematic change in the preferred orientation of minerals from base to top macroscopic structures in the layers indicate erosion and the action of traction currents when the layers formed and the sinking of the dense minerals and rise of interstitial melt immediately afterwards the layered rocks have similar grain size and mineral compositions to the non layered granodiorites and thus could be derived from the same magmas as the kuna crest half dome and cathedral peak non layered granodiorites the major and trace element variations in the layered rocks are very different from those in the non layered rocks and indicate a strong segregation of crystals during formation of the layers however the separation process did not fractionate the main rare earth element bearing phases from one another the macrostructure and microstructural features in the layers resemble those from hyper concentrated sediment gravity flows the layering is interpreted to have formed from gravity flows of melt and crystals initiated by syn magmatic tectonic deformation and seismic shaking two types of layering are identified one called normal layering represents material derived from a single granodiorite magma and deposited from a relatively low energy gravity flow whereas the other called melanocratic layering incorporated mafic minerals eroded from the substrate had the felsic tops of layers eroded off by subsequent flows incorporated crystals from more than one magma source and formed from gravity flows that had higher energy

fractionation process of high silica magmas through the lens of zircon crystallization a case study from the tengchong block sw china abstract the generation of high silica magma is generally considered to occur as a result of liquid crystal segregation from a crystal mush yet whether high silica melts could undergo significant fractionation process after the segregation remains unclear this study contributes to this topic through the lens of zircon geochemistry of the 130 ma high silica granite in the tengchong block sw china the high silica granite with sio2 76 displays low zr hf and eun eu n and high rb sr ratios the zircons crystallized from the granite can be divided into two groups the group 1 zircons are characterized by lower hf 12 200 ppm and display elevated u values 936 2429 ppm averaging 1557 ppm and yb values 755 1556 ppm averaging 1134 ppm as well as lower eun eu n 0 01 0 1 the increasing hf content from group 1 to group 2 zircons presence of fluorite inclusions in high hf zircon and similarity in hf isotope between the two groups of zircon consistently support the high silica granite has witnessed a highly fractionated crystallization process the presence of these two groups of zircons is considered to reflect two stage in situ fractionation processes which display distinct ree rare earth elements compositions in the magma system in response to differing precipitated mineral assemblages in light of the change of zircon ree within each group the percentage of minerals co precipitating with zircons in each stage was estimated the results showed that 35 24 quartz 18 60 feldspar 0 25 apatite 0 14 zircon 0 12 allanite and 0 05 titanite fractionated in the early stage and 67 50 quartz 23 41 feldspar 0 15 zircon 0 10 allanite 0 07 apatite and 0 06 titanite fractionated in the second stage the discovery of different zircons and corresponding estimation in this study provide a new line of evidence for the fractionation of high silica melt

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

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

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

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

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

magmatic processes under quizapu volcano chile identified from geochemical and textural studies quizapu is part of a linear system of active volcanos in central chile the volcanic petrology and geology have been used to infer the plumbing system beneath the volcano the 1846 1847 eruption 5 km3 started with small flows of dacite then changed to a range of andesite dacite compositions and finally terminated with large flows of dacite andesitic enclaves 10 occur in some of these flows activity restarted explosively in 1932 4 km3 dre with an initial andesite dacite ash followed by uniform dacite ash and then a terminal andesite ash all samples including the enclaves have chemical compositions that lie on an almost perfect mixing line with a few exceptions the abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold cathode cathodoluminescence images and zonation in visible light all populations of macrocrysts have csds characteristic of coarsening although they differ in detail two classes can be ascribed to growth in andesite and dacite magmas but the three other classes are associated with particular magma batches a model is developed which started with ponding of andesite magma in the crust this differentiated to produce a dacite magma most of which probably solidified to make a granodiorite batholith activation of a n s fault enabled volcanism andesite magma traversed the dacite filled chamber heating and raising it up into storage areas hosted by the fault where it mixed to form a homogeneous magma a short time before the 1846 1847 eruption more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas the first magma to be erupted from quizapu was a dacite but soon other storage areas along the fault started to feed the system first mixed magmas then back to dacites the eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped an undegassed dacite chamber and resulted in a strong explosive eruption the whole story is one of continual andesite magmatism modulated by storage degassing and mixing

formation of k feldspar megacrysts in granodioritic plutons by thermal cycling and late stage textural coarsening k feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases grown to large sizes when the magma was mostly liquid owing to their large size euhedral form and features that suggest deposition by magmatic sedimentation however phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that k feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50 crystallization and is thus largely incapable of flow and sedimentation megacryst size distributions compositions and textural relationships from the cretaceous tuolumne intrusive suite california reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones whereas linear growth rates during cooling are similar thus during thermal cycling material is transferred from small crystals to larger ones megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of k feldspar explains the presence of megacrysts in the inner parts of thetuolumne intrusive suite and elsewhere and may be a common process in formation of megacrystic granitic rocks

the silicate inclusions of the ocotillo iab iron meteorite the ocotillo iab iron meteorite contains small silicate inclusions consisting of olivine low ca pyroxene chromian diopside plagioclase magnesiochromite apatite troilite and metal the ferromagnesian silicates have a small range of fe fe mg ratios that are not due to zoning these phases appear to be not well equilibrated the feo content of magnesiochromite is lower than values normally seen in silicate assemblages in iab iron meteorites the minerals in ocotillo are generally like silicate assemblages in other iab meteorites covering similar composition ranges and exhibiting a metamorphic granoblastic texture an estimate was made of the bulk composition of ocotillo silicate inclusions the bulk composition is close to that of ordinary chondrites with the exception of a deficiency in cao that might be due to a sampling problem associated with the method used to estimate the bulk composition

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

the iab iron meteorite complex a group five subgroups numerous grouplets closely related mainly formed by crystal segregation in rapidly cooling melts abstract we present new data for iron meteorites that are members of group iab or are closely related to this large group and we have also reevaluated some of our earlier data for these irons in the past it was not possible to distinguish iab and iiicd irons on the basis of their positions on element ni diagrams but we now show that plotting the new and revised data yields six sets of compact fields on element au diagrams each set corresponding to a compositional group the largest set includes the majority 70 of irons previously designated ia we christened this set the iab main group the remaining five sets we designate subgroups within the iab complex three of these subgroups have au contents similar to the main group and form parallel trends on most element ni diagrams the groups originally designated iiic and iiid are two of these subgroups they are now well resolved from each other and from the main group the other low au subgroup has ni contents just above the main group two other iab subgroups have appreciably higher au contents than the main group and show weaker compositional links to it we have named these five subgroups on the basis of their au and ni contents the three subgroups having au contents similar to the main group are the low au l subgroups the two others the high au h subgroups the ni contents are designated high h medium m or low l thus the old group iiid is now the slh subgroup the old group iiic is the slm subgroup in addition eight irons assigned to two grouplets plot between sll and slm on most element au diagrams a large number 27 of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the iab complex many of these irons contain coarse silicates having similar properties most are roughly chondritic in composition the mafic silicates show evidence of reduction during metamorphism in each case the silicate o isotopic composition is within the carbonaceous chondrite range 17o 0 3 in all but four cases these are within the so called iab range 0 30 17o 0 68 fine silicates appear to be ubiquitous in the main group and low au subgroups this requires that viscosities in the parental melt reached high values before buoyancy could separate these the well defined main group trends on element au diagrams provide constraints for evaluating possible models we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium the main arguments against the main group having formed by fractional crystallization are a the small range in ir and b the evidence for rapid crystallization and a high cooling rate through the iron stability field the evidence for the latter are the small sizes of the iron crystals parental to the widmanstatten pattern and the limited thermal effects recorded in the silicates including retention of albitic plagioclase and abundant primordial rare gases in contrast crystal segregation in a cooling metallic melt and related processes such as incomplete melting and melt migration can produce the observed trends in the main group we infer that this melt was formed by impact heating on a porous chondritic body and that the melt was initially hotter than the combined mix of silicates and metal in the local region the melt cooled rapidly by heat conduction into the cooler surroundings mainly silicates we suggest that the close compositional relationships between the main group and the low au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids

the space and time complexity of chaotic mixing of silicate melts implications for igneous petrology abstract we present new experimental results on the study of the space and time modulation of compositional fields during chaotic mixing between mafic and felsic silicate melts the experimental strategy was planned using numerical simulations performed using the experimental geometry these mixing experiments were performed using a recently developed experimental apparatus which is capable of mixing high viscosity silicate melts at high temperatures and under precisely controlled conditions of fluid dynamics and strain the compositional variability produced by the mixing process was investigated both along linear analytical transects and on high resolution 2d x ray maps covering the mixing patterns our results indicate that chaotic flow fields represent very powerful dynamics to blend silicate melts even under laminar fluid dynamic conditions reynolds number ca 10 7 and for dissimilar melts with high viscosity ratios on the order of 103 the repetition of stretching and folding processes between the two melts induced a strong increase of contact interfaces thus favoring efficient chemical exchanges as a result the initial mafic composition is no longer detectable in the mixing system after ca 2 h i e the duration of the experiment a further important result is the observation of highly non linear patterns in inter elemental plots produced by the onset of diffusive fractionation processes this is contrary to common thinking that magma mixing should always produce linear trends between pairs of chemical elements a new measure the concentration variance is proposed to quantify chemical element mobility during the mixing process this measure is statistically robust and can be quantitatively used to measure chemical element mobility independently of the geometry in which the compositional variation i e transects areas etc is embedded or the local strain history of the mingling our results highlight concentration variance as a robust probe of the as yet poorly understood processes involved in the common petrological process of magma mixing

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers