ordinary chondrite related giant 800 μm cosmic spherules from the transantarctic mountains antarctica abstract in order to identify the parent bodies of cosmic spherules melted micrometeorites with porphyritic olivine po and cryptocrystalline cc textures we measured the oxygen isotopic composition of 15 giant 800 m cosmic spherules recovered in the transantarctic mountains antarctica with ir laser fluorination mass spectrometry and we conducted a characterization of their petrographic and magnetic properties samples include 6 8 and 1 spherules of po cc and barred olivine bo textural types respectively eleven spherules 70 of the total 4 6 po and 6 8 cc and the bo spherule are related to ordinary chondrites based on oxygen isotopic compositions olivines in ordinary chondrite related spherules have compositions fa 8 5 11 8 they are ni poor to ni rich 0 04 1 12 wt and tend to be richer in cao than other spherules 0 10 0 17 wt ordinary chondrite related spherules also have high magnetite contents 2 12 wt one po and one cc spherules are related to previously identified 17 o enriched cosmic spherules for which the parent body is unknown one cc spherule has an oxygen isotopic signature relating it to cm cr carbonaceous chondrites the majority of po cc cosmic spherules derive from ordinary chondrites this result exemplifies how the texture of cosmic spherules is not only controlled by atmospheric entry heating conditions but also depends on the parent body whether be it through orbital parameters entry angle and velocity or chemistry mineralogy or grain size of the precursor

the classification of micrometeorites due to their small size the mineralogical and chemical properties of micrometeorites mms are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites identifying which groups of mm are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical textural and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies albeit with minimal bias towards preconceived genetic models specifically the interpretation of mms requires a rigorous and meaningful classification scheme at present the classification of mms is however at best ambiguous a unified petrological chemical classification scheme is proposed in the current study and is based on observations of several thousand mms collected from antarctic ice

the weathering of micrometeorites from the transantarctic mountains abstract micrometeorites are cosmic dust particles recovered from the earth s surface that dominate the influx of extraterrestrial material accreting to our planet this paper provides the first in depth study of the weathering of micrometeorites within the antarctic environment that will allow primary and secondary features to be distinguished it is based on the analysis of 366 particles from larkman nunatak and 25 from the transantarctic mountain collection several important morphological categories of weathering effects were identified 1 irregular and faceted cavities 2 surface etch pits 3 infilled cavities 4 replaced silicate phases and 5 hydrated and replaced metal these features indicate that congruent dissolution of silicate phases in particular olivine is important in generating new pore space within particles comparison of the preservation of glass and olivine also indicates preferential dissolution of olivine by acidic solutions during low temperature aqueous alteration precipitation of new hydrous phases within cavities in particular ferrihydrite and jarosite results in pseudomorph textures within heavily altered particles glass in contrast is altered to palagonite gels and shows a sequential replacement indicative of varying water to rock ratios metal is variably replaced by fe oxyhydroxides and results in decreases in ni fe ratio in contrast sulphides within metal are largely preserved magnetite an essential component of micrometeorites formed during atmospheric entry is least altered by interaction with the terrestrial environment the extent of weathering in the studied micrometeorites is sensitive to differences in their primary mineralogy and varies significantly with particle type despite these differences we propose a weathering scale for micrometeorites based on both their degree of terrestrial alteration and the level of encrustation by secondary phases the compositions and textures of weathering products however suggest open system behaviour and variable water to rock ratios that imply climatic variation over the lifetime of the micrometeorite deposits

fayalitic olivine in cv3 chondrite matrix and dark inclusions a nebular origin fayalitic olivine fa32 is the major component of the matrices and dark inclusions of cv3 and other unequilibrated chondrites it occurs most commonly as rims veins and halos in and around chondrule silicates in the allende type cv3oxa chondrites and to a much lesser extent in the reduced cv3r and bali type cv3oxb chondrites the olivines have distinctive platy tabular and lath or irregular shaped crystals with the ratio of the two types varying widely in cv3oxb chondrites matrix fayalitic olivines range up to fag99 9 whereas in the other cv3 chondrites the range is much smaller the platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor and the nature of the occurrences suggests that cv3 matrices are unequilibrated mixtures of nebular materials we argue that the parent body hydration dehydration model has numerous inconsistencies that make this hypothesis highly unlikely these include 1 there is no direct evidence linking fayalitic olivine to precursor phyllosilicates 2 dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines 3 fayalitic olivine clearly predates the formation of the hydrous phases in cv3 chondrites and is one of the phases that breaks down to form phyllosilicates keller et al 1994 4 the unequilibrated nature of the matrix including fine scale zoning in 10 m sized fayalitic olivine crystals would not survive the parent body metamorphism required in the dehydration model 5 a dark inclusion in the ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material zolensky et al 1997 which probably formed by radiation damage this indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting 6 some allende chondrules contain unaltered primary anhydrous glassy mesostasis in contact with the host matrix e g ikeda and kimura 1995 chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario 7 single platy and barrel shaped crystals of fayalitic olivine are present in accretionary rims in calcium aluminum rich inclusions cais macpherson and davis 1997 which developed in the nebula 8 matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine scott et al 1984 which indicates a nebular origin 9 oxygen isotopic compositions of allende matrix and dark inclusions strongly indicate little or no hydration for allende and its components clayton 1997 we favor a nebular vaporization recondensation model in which vaporization of chondritic dust produced a fayalite rich vapor followed by formation of the fayalitic olivine by direct recondensation from the vapor epitactic growth on surfaces of existing forsterite and enstatite in chondrules and replacement of existing forsterite and enstatite by gas solid exchange

micrometeorite flux on earth during the last 50 000 years the flux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2 50 m 2 from the indian ocean the spherules are recovered from sediment samples in close spaced locations from the indian ocean after sieving 293 kg of sediment the terrestrial age of the spherules has a range of 0 50 000 years the spherules have a size range of 57 750 m average size 265 92 m the diameter of the spherules increases from scoriaceous barred cryptocrystalline glassy types the time averaged flux of the spherules is 160 tonnes yr a sizeable mass 60 resides in the 300 m fraction the slope of distribution is similar to that of deepsea spherules dss but significantly different from other collections which have lower average diameters it is observed that a significant population of cosmic dust resides in the higher sizes which can be recovered by sampling large areas in time and space the proportions of textural types observed are similar to that of unbiased collections this collection comprises particles that are larger than other unbiased collections and the proportion of textural types does not vary with size further this collection samples the flux over a longer time span indicating that the textural types of cosmic dust that have been raining on the earth during the last 50 ky have been constant regardless of size major element chemistry of a majority of the spherules shows elemental ratios that are close to a cm or ci chondritic parent body a single spherule 0 2 of the population suggests an achondritic parent body large unbiased collections spanning large areas temporally and spatially expand the inventory of the earth crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion

identification of the parent bodies of micrometeorites with high precision oxygen isotope ratios oxygen isotopic compositions allow identification of potential parent bodies of extraterrestrial materials we measured oxygen isotope ratios of 33 large diameter 500 m silicate melted micrometeorites cosmic spherules from antarctica using ir laser fluorination coupled with mass spectrometry it is the first time that this high precision method is used on individual micrometeorites the selected micrometeorites are representative of the influx of extraterrestrial materials to the earth our results show that most micrometeorites are related to carbonaceous chondrites which is consistent with previous studies however 20 50 of them seem to be related to co cv carbonaceous chondrites whereas cm cr carbonaceous chondrites were thought to be the main source for micrometeorites furthermore 30 of measured samples have oxygen isotope ratios lying above the terrestrial fractionation line which relates them to ordinary chondrites or other as yet unsampled parent bodies

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

chondrules in antarctic micrometeorites previous studies of unmelted micrometeorites 50 m recovered from antarctic ice have concluded that chondrules which are a major component of chondritic meteorites are extremely rare among micrometeorites we report the discovery of eight micrometeorites containing chondritic igneous objects which strongly suggests that at least a portion of coarse grained crystalline micrometeorites represent chondrule fragments six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine grained matrix these particles suggest that at least some coarse grained micrometeorites cgmms may be derived from the same parent bodies as fine grained micrometeorites the new evidence indicates that contrary to previous suggestions the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites

on the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite alha77307 abstract the origin of isolated olivine grains in carbonaceous chondrites has been suggested to be either by direct condensation or by fragmentation of chondrules in an attempt to resolve this debate isolated olivine grains in the carbonaceous chondrite alha77307 have been studied in detail zoning characteristics and minor element compositions of olivines from all isolated and chondrule occurrences are described the data show that a strong relationship exists between isolated olivines and the chondrule population chondrule olivine compositions and zoning properties are consistent with olivine crystallization from the molten chondrules because properties of isolated grains are very similar to chondrule olivines an igneous origin for most if not all isolated grains is proposed large isolated forsterite grains may be derived from fragmentation of macroporphyritic type i chondrules which contain one or very few large olivine grains relic forsterite grains in type ii chondrules and complex isolated olivine grains with forsteritic cores and fayalite rich rims provide evidence that fragmentation and recycling of chondrules and their components was a common and efficient process in the chondrule forming region

sks splitting and the seismic anisotropy of the mantle beneath the hikurangi subduction zone new zealand abstract the analysis of 12 sks phases recorded on broadband stations above the hikurangi subduction zone in new zealand shows clear evidence of mantle anisotropy with the fast direction 28 5 almost parallel to the strike of subduction and the dominant geology of the region the slow shear wave delay times show a systematic change with the azimuth of the arrivals which if hexagonal symmetry is assumed indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal or that more than one anisotropic layer is present the magnitude of the delays 1 5 0 4s suggests that the anisotropy is most probably confined to the top 300 km of the mantle

igneous rims on micrometeorites abstract melting of micrometeorites mms due to atmospheric entry heating causes significant changes in the textures mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials despite the abundance of melted mms the nature of melting processes in these materials is poorly constrained in this study mineralogical textural and compositional data on 77 mms with igneous rims are presented which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process the compositions of igneous rims on fine grained mms fgmms are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine grained matrix core enrichments in fe si ni si and mn si in igneous rims compared with unmelted cores within fgmms are suggested to occur due to the migration of fe s eutectic liquids from the core of the particle into the surface melt layer the sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials the presence of igneous rims on fgmms is enabled by the thermal decomposition of phyllosilicates and therefore indicates that fgmms were hydrated particles prior to atmospheric entry in contrast the compositions of igneous rims on coarse grained mms cgmms are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgmms the igneous rims of cgmms are therefore suggested to form by melting of fine grained matrix that was present on the exterior of these particles prior to atmospheric entry coarse grained mms with igneous rims thus were originally composite micrometeoroids which consist of both fine grained and coarse grained materials and are thought to be samples of chondrule like objects the abundance of composite mms to cgmms allows a first estimate of the mean chondrule radius within the parent bodies of mms of 625 m to be made

numbers types and compositions of an unbiased collection of cosmic spherules abstract abstract micrometeorites collected from the bottom of the south pole water well spww may represent a complete well preserved sample of the cosmic dust that accreted on earth from 1100 1500 a d we classified 1588 cosmic spherules in the size range 50 800 pm the collection has 41 barred olivine spherules 17 glass spheres 12 cryptocrystalline spherules 11 porphyritic olivine spherules 12 relic grain bearing spherules 3 scoriaceous spherules 2 i type spherules 1 ca al ti rich cat spherules and 1 g type spherules we also found bubbly glass spherules spherules with glass caps and ones with sulfide coatings particles that are absent from other collections a classification sequence of the stony spherules scoriaceous relic grain bearing porphyritic barred olivine cryptocrystalline glass and cat is consistent with progressive heating and evaporation of fe from chondritic materials the modem day accretion rate and size distribution measured at the spww can account for the stony spherules present in deep sea collection through preferential dissolution of glass and small stony spherules however weathering alone cannot account for the high accretion rate of i type spherules determined for two deep sea collections the spww collection provides data to constrain models of atmospheric entry heating and to assess the effects of terrestrial weathering

koronis asteroid dust within antarctic ice here i report the first discovery of large numbers of ordinary chondrite derived micro meteorites mms recovered from antarctic ice ordinary chondrite derived mms comprise 70 coarse grained igneous particles and are identified on the basis of their distribution of petrologic types accessory mineralogy and minor element compositions all of which are very similar to those of millimeter sized igneous objects known as chondrules from ordinary chondrites and largely distinct from those of other meteorite groups the majority of ordinary chondrite derived mms are unequilibrated materials however 15 are equilibrated indicating parent body metamorphism and have compositions consistent with both h high iron and l low iron chondrites the total abundance of ordinary chondrite derived mms of 18 is similar to predictions by numerical calculations for the abundance of dust generated by the recent breakup of the karin group asteroids of the koronis asteroid family suggesting that these asteroids are the main source of the particles these mms represent the first to be associated with a known asteroid and imply that the karin group progenitor asteroid was a rubble pile asteroid that sampled different depths within the original internally metamorphosed koronis parent body

feldspar in type 4 6 ordinary chondrites metamorphic processing on the h and ll chondrite parent bodies abstract we have carried out a study of feldspar compositions in a suite of h and ll ordinary chondrites of petrologic types 4 5 and 6 in order to examine the process of recrystallization and equilibration of feldspar as the degree of metamorphism increases in the h chondrites there is little variation in feldspar compositions among the petrologic types suggesting that homogenization of chondrule mesostasis from which feldspar is presumed to have crystallized occurred before feldspar crystallization began the ll chondrites we studied are more complex in bjurbole l ll4 plagioclase in individual relict chondrules has distinct compositions with a range of an ab ratios and low or contents this heterogeneity is most likely attributable to original compositional heterogeneity among chondrule mesostases localized recrystallization of mesostasis must have occurred before diffusional equilibration took place in tuxtuac ll5 the an ab ratio of plagioclase is more homogeneous and plagioclase includes a significant or component in addition we observe what appears to be exsolution of k feldspar from albitic host grains in saint severin ll6 the an ab ratio of plagioclase is homogeneous but plagioclase compositions show a range of or contents corresponding to a patchy distribution of k in individual feldspar grains the observations in these ll chondrites are difficult to interpret with a simple model of progressive equilibration with increasing petrologic type we suggest that the current criteria for assigning petrologic types are poorly defined it is possible that the assigned petrologic types of these chondrites do not correlate with their peak temperatures we propose that feldspar compositions might record conditions during the heating stage of metamorphism and that the early stages of metamorphism may have occurred in the presence of fluids rather than under the dry conditions that are commonly assumed

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

chondritic micrometeorites from the transantarctic mountains abstract on the basis of morphological and petrographic characteristics eight giant unmelted micrometeorites in the 300 1100 m size range were selected from the transantarctic mountain micrometeorite collection victoria land antarctica mineralogical and geochemical data obtained by means of scanning electron microscopy electron probe microanalyses and synchrotron x ray diffraction allow their classification as chondritic micrometeorites the large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions therefore allowing a better definition of their parent material a large variety of material is observed five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite one to cv chondrite one to cm chondrite and one to ci chondrite parent materials besides reporting the first occurrence of a cv like micrometeorite our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin

the textures and compositions of fine grained antarctic micrometeorites implications for comparisons with meteorites abstract micrometeorites recovered from the earth s surface constitute the most abundant interplanetary dust now falling to earth we studied eighty nine fine grained antarctic micrometeorites fg amms to evaluate their state of alteration and to identify the nature of their precursor materials fine grained amms are divided into melted and unmelted groups and subdivided on the basis of textures related to atmospheric entry heating the textures of melted particles reflect crystallisation after entry heating and those of unmelted amms broadly resemble cl cm and cr chondrite matrix the matrix compositions of the fg amms closely resemble those of cm2 chondrites matrices provide a more reliable comparison to meteorites than bulk compositions because components such as chondrules and cais are not present in representative proportions within individual amms divergences from cm like matrix compositions are generally minor and probably arose during entry heating by the dissociation of volatile bearing phases at subsolidus temperatures and evaporation and loss of immiscible metallic liquids at higher temperatures depletions in ni and mg relative to cm matrix are tentatively attributed to terrestrial weathering no conclusive evidence for contamination during atmospheric residence was observed contrary to theory textural evidence suggests that bow shocks and high thermal gradients existed during deceleration of micrometeoroids in the atmosphere

unmelted cosmic metal particles in the indian ocean fe ni metal is a common constituent of most meteorites and is an indicator of the thermal history of the respective meteorites it is a diagnostic tool to distinguish between groups subgroups of meteorites in spite of over a million micrometeorites collected from various domains reports of pure metallic particles among micrometeorites have been extremely rare we report here the finding of a variety of cosmic metal particles such as kamacite plessite taenite and fe ni beads from deep sea sediments of the indian ocean a majority of which have entered the earth unaffected by frictional heating during atmospheric entry such particles are known as components of meteorites but have never been found as individual entities their compositions suggest precursors from a variety of meteorite groups thus providing an insight into the metal fluxes on the earth some particles have undergone heating and oxidation to different levels during entry developing features similar to i type cosmic spherules suggesting atmospheric processing of individual kamacites taenite grains as another hitherto unknown source for the i type spherules the particles have undergone postdepositional aqueous alteration transforming finally into the serpentine mineral cronstedtite aqueous alteration products of kamacite reflect the local microenvironment therefore they have the potential to provide information on the composition of water in the solar nebula on the parent bodies or on surfaces of planetary bodies our observations suggest it would take sustained burial in water for tens of thousands of years under cold conditions for kamacites to alter to cronstedtite

meteorite and meteoroid new comprehensive definitions abstract meteorites have traditionally been defined as solid objects that have fallen to earth from space this definition however is no longer adequate in recent decades man made objects have fallen to earth from space meteorites have been identified on the moon and mars and small interplanetary objects have impacted orbiting spacecraft taking these facts and other potential complications into consideration we offer new comprehensive definitions of the terms meteorite meteoroid and their smaller counterparts a meteoroid is a 10 m to 1 m size natural solid object moving in interplanetary space a micrometeoroid is a meteoroid 10 m to 2 mm in size a meteorite is a natural solid object larger than 10 m in size derived from a celestial body that was transported by natural means from the body on which it formed to a region outside the dominant gravitational influence of that body and that later collided with a natural or artificial body larger than itself even if it is the same body from which it was launched weathering and other secondary processes do not affect an object s status as a meteorite as long as something recognizable remains of its original minerals or structure an object loses its status as a meteorite if it is incorporated into a larger rock that becomes a meteorite itself a micrometeorite is a meteorite between 10 m and 2 mm in size meteorite a solid substance or body falling from the high regions of the atmosphere craig 1849 a mass of stone and iron that ha s been directly observed to have fallen down to the earth s surface translated from cohen 1894 a solid bod y which came to the earth from space farrington 1915 a mass of solid matter too small to be considered an asteroid either traveling through space as an unattached unit or having landed on the earth and still retaining its identity nininger 1933 a meteoroid which has reached the surface of the earth without being vaporized 1958 international astronomical union iau definition quoted by millman 1961 a solid body which has arrived on the earth from outer space mason 1962 a solid bod y which reach es the earth or the moon mars etc from interplanetary space and is large enough to survive passage through the earth s or mars etc atmosphere gomes and keil 1980 a meteoroid that survive s passage through the atmosphere and fall s to earth burke 1986 a recovered fragment of a meteoroid that has survived transit through the earth s atmosphere mcsween 1987 a solid bod y of extraterrestrial material that penetrate s the atmosphere and reach es the earth s surface krot et al 2003

ordinary chondritic micrometeorites from the indian ocean extraterrestrial particulate materials on the earth can originate in the form of collisional debris from the asteroid belt cometary material or as meteoroid ablation spherules signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry we investigated 481 micrometeorites isolated from 300 kg of deepsea sediment out of which 15 spherules appear to have retained signatures of their provenance based on their textures bulk chemical compositions and relict grain compositions seven out of these 15 spherules contain chromite grains whose compositions help in distinguishing sub groups within the ordinary chondrite sources there are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite in addition a pop porphyritic olivine pyroxene chondrule like spherule is also recovered the bulk chemical composition of all the spherules in combination with trace elements the chromite composition and presence of dusty olivines suggest an ordinary chondritic source these micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features these micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the earth

carbonaceous micrometeorites from antarctica over 100 000 large interplanetary dust particles in the 50 500 micrometers size range have been recovered in clean conditions from approximately 600 tons of antarctic melt ice water as both unmelted and partially melted dehydrated micrometeorites and cosmic spherules flux measurements in both the greenland and antarctica ice sheets indicate that the micrometeorites deliver to the earth s surface approximately 2000x more extraterrestrial material than brought by meteorites mineralogical and chemical studies of antarctic micrometeorites indicate that they are only related to the relatively rare cm and cr carbonaceous chondrite groups being mostly chondritic carbonaceous objects composed of highly unequilibrated assemblages of anhydrous and hydrous minerals however there are also marked differences between these two families of solar system objects including higher c o ratios and a very marked depletion of chondrules in micrometeorite matter hence they are chondrites without chondrules thus the parent meteoroids of micrometeorites represent a dominant and new population of solar system objects probably formed in the outer solar system and delivered to the inner solar system by the most appropriate vehicles comets one of the major purposes of this paper is to discuss applications of micrometeorite studies that have been previously presented to exobiologists but deal with the synthesis of prebiotic molecules on the early earth and more recently with the early history of the solar system

bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 c small particles 200 m in diameter from the hydrous carbonaceous chondrites orgueil ci murchison cm2 and tagish lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry the three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures and thus the brief heating up to 1000 c drastically changed the mineralogy changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400 600 c serpentine and saponite decomposition to amorphous phases at 600 and 700 c respectively decomposition of mg fe carbonate at 600 c recrystallization of secondary olivine and fe oxide or metal at 700 800 c and recrystallization of secondary low ca pyroxene at 800 c the ambient atmospheric pressures controlled species of secondary fe phase taenite at pressures lower than 10 2 torr magnesiowstite from 10 3 to 10 1 torr and magnetite from 10 2 to 1 torr the abundance of secondary low ca pyroxene increases in the order of murchison orgueil and tagish lake and the order corresponds to saponite abundance in samples prior to heating mineralogy of the three unmelted micrometeorites f96ci024 kw740052 and kw740054 were investigated in detail in order to estimate heating conditions the results showed that they might have come from different parental objects carbonaterich tagish lake type carbonate poor tagish lake or ci type and cm type respectively and experienced different peak temperatures 600 700 and 800 900 c respectively at 60 80 km altitude upon atmospheric entry

fayalite in the vigarano cv3 carbonaceous chondrite occurrences formation age and conditions abstract we have performed petrographic characterization 53mn 53cr age determination and thermodynamic stability evaluations of fayalite in vigarano meteorite that belongs to the reduced subgroup of cv3 chondrites vigarano is a breccia consisting of clasts which are separate chondrules surrounded by olivine rich fine grained materials four out of twenty three explored clasts contain fayalites that represent materials of the bali like oxidized subgroup of cv3 chondrites the fayalites fa 80 with grain sizes typically

seismic anisotropy of the french massif central and predisposition of cenozoic rifting and volcanism by variscan suture hidden in the mantle lithosphere 1 we model the structure of the lithosphere its thickness and seismic anisotropy caused mainly by systematic orientation of olivine crystals from spatial variations of p wave delay times and the shear wave splitting observed at a dense network of mobile and permanent stations within the french massif central mc three major lithosphere domains with different seismic anisotropy are distinguished a suture in the deep lithosphere limits the thick lithosphere of the limousin western mc characterized by a consistent anisotropic pattern with high velocities dipping to the west the eastern mc comprising two different domains with a weakened mantle fabric in the south shows the high velocities dipping systematically to the east the suture between the western and eastern mc parallels the major crustal boundary the late variscan sillon houiller transfer fault in the south and the tauve aigueperse fault in the north with an offset of 10 20 km to the east the offset of the crustal and mantle parts of the same suture indicates that the rigid upper crust might be detached from the mantle lithosphere we suggest that the mantle suture hidden beneath an allochthonous crust was reactivated during the cenozoic extension of the weakened and thinned lithosphere in the south of the eastern mc and predestined a space for the major volcanism mont dore cantal the rigid northern domain characterized by a thicker lithosphere with a well preserved mantle fabric reacted to the extension predominantly by rifting e g limagne and roanne grabens

oxygen isotopic composition of relict olivine grains in cosmic spherules links to chondrules from carbonaceous chondrites most olivine relict grains in cosmic spherules selected for the present study are pristine and have not been disturbed during their atmospheric entry thereby preserving their chemical mineralogical and isotopic compositions in order to understand the origin of the particles oxygen isotope compositions of relict olivine grains in twelve cosmic spherules collected from deep sea sediments of the indian ocean were studied using secondary ion mass spectrometry most of the data lie close to the ccam carbonaceous chondrite anhydrous mineral line with 17 o ranging from 5 to 0 the data overlap oxygen isotopic compositions of chondrules from carbonaceous chondrites such as cv ck cr and cm which suggests that chondrules from carbonaceous chondrites are the source of relict grains in cosmic spherules chemical compositions of olivine in cosmic spherules are also very similar to chondrule olivine from carbonaceous chondrites several olivine relict grains in three cosmic spherules are 16 o rich 17 o 21 9 to 18 7 similar to oxygen isotopic compositions observed in calcium aluminium rich inclusions cais amoeboid olivine aggregates aoas and some porphyritic chondrules from carbonaceous chondrites these grains appear to have recorded the initial oxygen isotopic composition of the inner solar nebula three olivine grains from two cosmic spherules have 18 o values 20 which could be interpreted as mixing with stratospheric oxygen during atmospheric entry

the fusion crusts of stony meteorites implications for the atmospheric reprocessing of extraterrestrial materials fusion crusts develop on all meteorites during their passage through the atmosphere but have been little studied we have characterized the textures and compositions of the fusion crusts of 73 stony meteorites to identify the nature of meteorite ablation spheres mas and constrain the processes operating during the entry heating most chondrite fusion crusts are porphyritic and are dominated by olivine glass and accessory magnetite whereas those of the achondrites are mainly glassy chondrite fusion crusts contain sulphide droplets with high ni contents 55 wt the partially melted substrate of ordinary chondrites underlying the outer melted crusts are dominated by silicate glass and composite metal sulphide and cr bearing fe oxide droplets that form as coexisting immiscible liquids enstatite chondrite substrates contain cr and mn bearing sulphides the substrates of the carbonaceous chondrites comprise a sulphide enriched layer of matrix the compositions of melted crusts are similar to those of the bulk meteorite however differences from whole rock suggest that three main processes control their chemical evolution 1 the loss and reaction of immiscible fe rich liquids 2 mixing between substrate partial melts and bulk melts of the melted crust and 3 the loss of volatile components by evaporation and degassing data from fusion crusts suggest that mas produced at low altitude have compositions within the range of those of silicate dominated cosmic spherules that are formed by the melting dust particles meteorite ablation spheres produced at high altitude probably have compositions very different from bulk meteorite and will resemble cosmic spherules derived from coarse grained precursors

a search for h chondritic chromite grains in sediments that formed immediately after the breakup of the l chondrite parent body 470 ma ago abstract a large abundance of l chondritic material mainly in the form of fossil meteorites and chromite grains from micrometeorites has been found in mid ordovician 470 ma old sediments globally the material has been determined to be ejecta from the l chondrite parent body breakup event a major collision in the asteroid belt 470 ma ago in this study we search the same sediments for h chondritic chromite grains in order to improve our understanding of the extraterrestrial flux to earth after the asteroid breakup event we have used sims in conjunction with quantitative sem eds to determine the three oxygen isotopic and elemental compositions respectively of a total of 120 randomly selected sediment dispersed extraterrestrial chromite grains mainly representing micrometeorites from 470 ma old post breakup limestone from the thorsberg quarry in sweden and the lynna river site in russia we show that 99 or more of the grains are l chondritic whereas the h chondritic fraction is 1 or less the l h chondrite ratio after the breakup thus was 99 compared to 1 1 in today s meteoritic flux this represents independent evidence in agreement with previous estimates based on sediment dispersed extraterrestrial chromite grain abundances and sedimentation rates of a two orders of magnitude higher post breakup flux of l chondritic material in the micrometeorite fraction finally we confirm the usefulness of three oxygen isotopic sims analyses of individual extraterrestrial chromite grains for classification of equilibrated ordinary chondrites the h and l chondritic chromites differ both in their three oxygen isotopic and elemental compositions but there is some overlap between the groups in chromite tio 2 is the oxide most resistant to diagenesis and the combined application of tio 2 and oxygen three isotope analysis can resolve uncertainties arising from the compositional overlaps

major and trace element geochemistry of s type cosmic spherules micrometeorites that pass through the earth s atmosphere undergo changes in their chemical compositions thereby making it difficult to understand if they are sourced from the matrix chondrules or calcium aluminum rich inclusions cais these components have the potential to provide evidence toward the understanding of the early solar nebular evolution the variations in the major element and trace element compositions of 155 different type scoriaceous relict bearing porphyritic barred cryptocrystalline and glass of s type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma mass spectrometry the s type cosmic spherules appear to show a systematic depletion in volatile element contents but have preserved their refractory trace elements the trends in their chemical compositions suggest that the s type spherules comprise of components from similar parent bodies that is carbonaceous chondrites large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites furthermore four spherules two of these spherules enclose spinels and one comprised entirely of a ca al rich plagioclase show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules the information on the chemical composition and rare earth elements rees on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies the ce eu and tm anomalies found in the cosmic spherules have similar behavior as that of chondrites distinct correlations observed between different rees and types of cosmic spherules reflect the inherited properties of the precursors

spinel group minerals in ll3 00 6 chondrites indicators of nebular and parent body processes we carried out a systematic study of spinel group minerals in ll3 00 3 9 and ll4 6 chondrites with increasing petrologic type the size and abundance of spinel increase the compositions of spinel group minerals in type 3 chondrites depend on the occurrence mg al rich spinel occurs mainly in chondrules some chromite occurs in chondrules and matrix and nearly pure chromite is exclusively encountered in the matrix the occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3 00 3 3 from those in the other types spinel group minerals in types 3 5 3 9 show a narrower range of compositions and those in types 4 6 are homogeneous the changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism therefore the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions not only for type 3 6 but also 3 00 3 9 they can be applied to identify the most primitive least metamorphosed chondrites the bulk compositions of spinel bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts however some spinel grains especially those enclosed in olivine phenocrysts can not be explained by in situ crystallization in the chondrule we interpret these spinel grains to be relic phases that survived chondrule melting this is supported by the oxygen isotopic composition of a spinel grain which has significantly lighter oxygen than the coexisting olivine the oxygen isotopic composition of this spinel is similar to those of al rich chondrules our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion through al rich chondrule to ferromagnesian chondrules and attests to the recycling of earlier formed materials into the precursors of later formed materials the characteristic features of spinel group minerals are not only sensitive to thermal metamorphism but also shed light on chondrule formation processes

fission track age of transantarctic mountain microtektites abstract we determined the fission track age of transantarctic mountain microtektites the plateau method yielded a formation age of 0 85 0 17 ma this age overlaps within error with that of the catastrophic impact that produced the australasian tektite microtektite strewn field ca 0 8 ma ago this provides further evidence that transantarctic mountain microtektites belong to the australasian tektite microtektite strewn field as previously suggested on the basis of geochemical evidence sr nd isotope systematics and poorly resolved radiometric data

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

seismic characterization of mantle flow in subduction systems can we resolve a hydrated mantle wedge this study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy we are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries changes in the deformation state of mantle minerals or a combination of both while shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension experimental studies suggest that olivine slip systems change under higher stress and hydration states conditions likely appropriate for subduction systems in this study we predict shear wave splitting as a result of mantle silicate lattice preferred orientation development in steady state two dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization we utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge the results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling including tonga japan and kamchatka the implications of our work suggest that provided adequate data sampling shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

subglacial lake ellsworth a candidate for in situ exploration in west antarctica radio echo sounding reveals a 10 km long lake beneath 3 4 km of ice near the ellsworth mountains in west antarctica 20 km from the ice divide subglacial lake ellsworth is located within a distinct topographic hollow which is 1 5 km deeper than the surrounding bed judging by bed slopes flanking the lake the water depth is at least 10s of metres calculations of basal temperature reveal the ice base to be warm both now and during full glacial periods as the environments of subglacial lakes are broadly similar life may be expected in lake ellsworth as in any other given this its physical characteristics and the fact that the west antarctic ice sheet has been accessed on several occasions lake ellsworth is an excellent candidate for in situ examination

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

seismic anisotropy beneath the lower half of the north island new zealand teleseismic scs and sks events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the north island new zealand this area lies above the hikurangi subduction zone and the array provides ray paths which sample the mantle both above and below the slab shear wave splitting measurements give similar fast polarizations and delay times at each station the average sks fast polarization is approximately ne sw subparallel to the strike of subduction and the major geological features with an average sks delay time of 1 6 0 1 s this lack of variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle the anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence any anisotropy in the slab is probably due to fossil mineral alignment anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow the similar ne sw fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench parallel direction in both regions

meteoritic event recorded in antarctic ice during systematic sampling of volcanic ash tephra layers at a well known antarctic meteorite collection site the allan hills main ice field a band of unusually dark and rounded many spheroidal particles was discovered this debris layer bit 58 extends parallel to the stratigraphy of the ice established from the tephra bands apparently marking a single depositional event the shapes internal texture major element composition and levels of cosmogenic nuclides of particles from within bit 58 all strongly suggest that this material represents ablation debris from the passage of a large h group ordinary chondrite preliminary cosmogenic isotope dating suggests an age of 2 8 ma implying that the east antarctic ice sheet has been stable since that time the relationship of the bit 58 layer to known impact events is not clear

heating experiments simulating atmospheric entry heating of micrometeorites clues to their parent body sources depending on their velocity entry angle and mass extraterrestrial dust particles suffer certain degrees of heating during entry into earth s atmosphere and the mineralogy and chemical composition of these dust particles are significantly changed in the present study pulse heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating fragments of the ci chondrites orgueil and alais as well as pyrrhotites from orgueil were used as analogue material the experiments show that the volatile elements s zn ga ge and se can be lost from 50 to 100 m sized ci meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles it is concluded that depletions of these elements relative to ci as observed in micrometeorites are mainly caused by atmospheric entry heating besides explaining the element abundances in micrometeorites the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry using the abundances of zn and ge and assuming their original concentrations close to ci a maximum heating of 1100 1200 c is obtained for previously analyzed antarctic micrometeroites thermal alteration also strongly influenced the mineralogy of the meteorite fragments while the unheated samples mainly consisted of phyllosilicates these phases almost completely transformed into olivine and pyroxene in the fragments heated to 800 c therefore dust particles that still contain hydrous minerals were probably never heated to temperatures 800 c in the atmosphere during continued heating the grain size of the newly formed silicates increased and the composition of the olivines equilibrated applying these results quantitatively to antarctic micrometeorites typical peak temperatures in the range of 1100 1200 c during atmospheric entry heating are deduced this temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles

alaska north slope regional gas hydrate production modeling forecasts abstract a series of gas hydrate development scenarios were created to assess the range of outcomes predicted for the possible development of the eileen gas hydrate accumulation north slope alaska production forecasts for the reference case were built using the 2002 mallik production tests mechanistic simulation and geologic studies conducted by the us geological survey three additional scenarios were considered a downside scenario which fails to identify viable production an upside scenario describes results that are better than expected to capture the full range of possible outcomes and balance the downside case an extreme upside scenario assumes each well is exceptionally productive starting with a representative type well simulation forecasts field development timing is applied and the sum of individual well forecasts creating the field wide production forecast this technique is commonly used to schedule large scale resource plays where drilling schedules are complex and production forecasts must account for many changing parameters the complementary forecasts of rig count capital investment and cash flow can be used in a pre appraisal assessment of potential commercial viability since no significant gas sales are currently possible on the north slope of alaska typical parameters were used to create downside reference and upside case forecasts that predict from 0 to 71 bm3 2 5 tcf of gas may be produced in 20 years and nearly 283 bm3 10 tcf ultimate recovery after 100 years outlining a range of possible outcomes enables decision makers to visualize the pace and milestones that will be required to evaluate gas hydrate resource development in the eileen accumulation critical values of peak production rate time to meaningful production volumes and investments required to rule out a downside case are provided upside cases identify potential if both depressurization and thermal stimulation yield positive results an extreme upside case captures the full potential of unconstrained development with widely spaced wells the results of this study indicate that recoverable gas hydrate resources may exist in the eileen accumulation and that it represents a good opportunity for continued research

connection between micrometeorites and wild 2 particles from antarctic snow to cometary ices we discuss the relationship between large cosmic dust that represents the main source of extraterrestrial matter presently accreted by the earth and samples from comet 81p wild 2 returned by the stardust mission in january 2006 prior examinations of the stardust samples have shown that wild 2 cometary dust particles contain a large diversity of components formed at various heliocentric distances these analyses suggest large scale radial mixing mechanism s in the early solar nebula and the existence of a continuum between primitive asteroidal and cometary matter the recent collection of concordia antarctic micrometeorites recovered from ultra clean snow close to dome c provides the most unbiased collection of large cosmic dust available for analyses in the laboratory many similarities can be found between antarctic micrometeorites and wild 2 samples in terms of chemical mineralogical and isotopic compositions and in the structure and composition of their carbonaceous matter cosmic dust in the form of concordia antarctic micrometeorites and primitive idps are preferred samples to study the asteroid comet continuum

experimental simulation of atmospheric entry of micrometeorites depending on their velocity entry angle and mass micrometeorites suffer different degrees of heating during their deceleration in the earth s atmosphere leading in most cases to significant textural mineralogical and chemical modifications one of these modifications is the formation of a magnetite shell around most micrometeorites which until now could not be reproduced neither theoretically nor experimentally the present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell fragments of the murchison and orgueil meteorites were used as analogue material in flash heating experiments performed in a high temperature furnace effects of temperature heating duration and oxygen fugacity were investigated these experiments were able to reproduce most of the micrometeorites textures from the vesicular fine grained micrometeorites to the totally melted cosmic spherules for the first time the formation of a magnetite shell could be observed on micrometeorite analogues we suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite furthermore with this study it is possible to estimate the atmospheric entry conditions of micrometeorites such as the peak temperature and the duration of flash heating

factors controlling compositions of cosmic spinels application to atmospheric entry conditions of meteoritic materials abstract during their deceleration through the earth s atmosphere meteoritic materials i e interplanetary dust particles micrometeorites and meteorites experience thermal shocks which may alter their pristine mineralogy texture or chemical characteristics among these changes one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material these cosmic spinels differ from terrestrial spinels by their high ni and fe3 contents and show large variations in composition in order to better understand the factors controlling their chemistry pulse heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using orgueil samples as proxies of meteoritic material covering a large range of experimental conditions temperature 500 c we also show that due to their fast crystallization kinetics cosmic spinels can record through their composition i e al2o3 contents and feo fe2o3 ratio the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the earth s surface chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the earth s atmosphere of any extraterrestrial object including altitude of deceleration entry angle and incident velocity these in turn may provide valuable information on the origin of the extraterrestrial material

extraterrestrial spherules in glacial sediment from the transantarctic mountains antarctica structure mineralogy and chemical composition silicate and glassy spherules of probably extraterrestrial origin have been discovered in glacial till from localities in the beardmore glacier region transantarctic mountains antarctica these spherules are similar in structure appearance and composition to spherules found in deep sea sediments and in greenland most spherules have a shiny and smooth dark surface while some show a characteristic brickwork pattern composed of intergrown olivine crystals and magnetite dendrites several spherules have been sectioned after trace element analysis by inaa the interiors of the spherules contain olivine crystals of varying size predominantly of forsteritic composition set in a glassy fe rich and mg poor matrix the matrix contains very fine grained dendritic magnetite crystals and rare accessory wustite sem and microprobe studies show that most of the spherules have not experienced significant weathering trace element analyses for individual particles confirm the extraterrestrial nature of the spherules they have undifferentiated chondritic rare earth element ree patterns and are enriched in ir and other siderophile elements with ir contents between 10 and 1 000 ppb some spherules have experienced loss of volatile elements and some differentiation of siderophile elements e g ni the antarctic spherules which might be micrometeorites that melted during passage through the atmosphere or ablation spherules are amore significant addition to the collection of extraterrestrial microparticles of key importance is their high abundance in the sediment and the fact that they have been concentrated by glacial action less