numerical simulation of the operation of the gpr experiment on netlander 1 the first objective of the ground penetrating radar gpr experiment on netlander is to investigate the geological structures of the martian subsurface the aim of this paper is to present initial results obtained in the first phase of a long term effort to build a numerical model of the gpr operation on mars and test dedicated signal processing algorithms on the simulated data the simulation is based on the use of a finite difference time domain method and we have pointed out some of its advantages that allow us to take into account complex features of the underground this model has given reliable estimates of the power budget of the radar for a simple but still representative model of the martian subsurface in addition several detailed features such as gradients and roughness at the interfaces were introduced to appraise their possible influence on the gpr performances in the frame of a simplified geometry of both the gpr antennas and the various underground interfaces a simple and first order method was developed and tested on simulated data to show the ability of the gpr to retrieve a three dimensional distribution of the underground reflectors based on this model and even with some rather crude hypothesis on the subsurface electromagnetic characteristics information on the direction and distances of the reflectors has been retrieved with a satisfactory approximation

performances of ground penetrating radars in arid volcanic regions consequences for mars subsurface exploration a gpr field experiment in the republic of djibouti provides evidence for very low radar penetration in arid volcanic materials in the range 100 500 mhz this phenomenon is attributed to the high iron oxide and evaporite concentration in soils which significantly increases the conductivity thus leading to poor subsurface imaging performances the geologic context in djibouti is shown to provide a good terrestrial analogue to mars geology results of this study show that the future sounding radar missions to mars may not reach the penetration depths previously anticipated

frictional melting processes in planetary materials from hypervelocity impact to earthquakes frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip within planetary materials which are mainly natural ceramics frictional melting occurs at high strain rates typically 10 2 s 1 and at slip velocities greater than 0 1 m s 1 t he pathway to friction melting is controlled by the mechanical properties of a rock s constituent minerals especially fracture toughness minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt the product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix this cools to form the rock type known as pseudotachylyte and at even higher strain rates it forms shock veins in meteorites and in impact craters which may contain high pressure mineral polymorphs the generation of melt on sliding surfaces can lubricate earthquake faults facilitate the post shock modification of impact craters and make landslides more hazardous

ejecta from impact craters an important feature of impacts into solar system bodies is the fate of crater ejecta the near surface material launched during the highly dynamic crater formation process laboratory measurements of impact crater ejecta from 18 studies are summarized the data are examined and used to assess our understanding of how the ejecta velocity and mass distributions depend on the conditions of an impact event the effects of impact speed on the ejecta are reasonably well understood but the dependences on target properties such as strength and porosity are only poorly constrained a point source scaling model for the ejecta mass and velocity distributions is developed and fit to the data for several classes of materials distinguished by porosity

shock and annealing in aubrites implications for parent body history several aubrites e g lap 03719 bishopville khor temiki alh 83015 contain orthopyroxene grains that exhibit more pronounced shock effects than associated olivine grains the orthopyroxene grains in these samples have clinoenstatite lamellae on 100 and exhibit weak mosaic extinction characteristic of shock stage s4 the olivine grains exhibit either sharp optical extinction characteristic of shock stage s1 as in lap 03719 or undulose extinction shock stage s2 as in bishopville and alh 83015 the khor temiki regolith breccia contains s1 and s2 olivine grains because literature data show that diffusion is much slower in orthopyroxene than in olivine it seems likely that aubrites experienced postshock impact induced annealing after differentiation the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near surface materials and damaged orthopyroxene and olivine crystal lattices as a result of these impact events some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor entombed olivine crystal lattices healed and became unstrained reaching shock stage s1 but orthopyroxene lattices retained their s4 level shock damaged features aubrites with s4 orthopyroxene and s2 olivine were probably very weakly shocked again after olivine was annealed to s1

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

hidden secrets of deformation impact induced compaction within a cv chondrite the cv3 allende is one of the most extensively studied meteorites in worldwide collections it is currently classified as s1 essentially unshocked using the classification scheme of stoffler et al 1991 however recent modelling suggests the low porosity observed in allende indicates the body should have undergone compaction related deformation in this study we detail previously undetected evidence of impact through use of electron backscatter diffraction mapping to identify deformation microstructures in chondrules aoas and matrix grains our results demonstrate that forsterite rich chondrules commonly preserve crystal plastic microstructures particularly at their margins that low angle boundaries in deformed matrix grains of olivine have a preferred orientation and that disparities in deformation occur between chondrules surrounding and non adjacent matrix grains we find heterogeneous compaction effects present throughout the matrix consistent with a highly porous initial material given the spatial distribution of these crystal plastic deformation microstructures we suggest that this is evidence that allende has undergone impact induced compaction from an initially heterogeneous and porous parent body we suggest that current shock classifications stoffler et al 1991 relying upon data from chondrule interiors do not constrain the complete shock history of a sample

coherent and subsequent incoherent ringwoodite growth in olivine of shocked l6 chondrites abstract pervasive replacement of original olivine grains by ringwoodite adjacent to a shock melt vein of yamato 791384 l6 chondrite was observed with a feg sem in these olivines we observed three distinct olivine ringwoodite textures arranged in a spatially successive arrangement from the wall of the shock melt vein into the olivine grains 1 polycrystalline ringwoodite 2 oriented several sets of ringwoodite lamellae and 3 oriented single lamellae tem images show that the polycrystalline and oriented several sets of lamellae parts consist of polycrystalline ringwoodite indicating that phase transformation from olivine to ringwoodite is controlled by incoherent growth mechanism on the other hand the set of oriented single lamellae consists of thin ringwoodite platelets

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

shock induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the ries impact crater germany the microtextures of stishovite and coesite in shocked non porous lithic clasts from suevite of the ries impact structure were studied in transmitted light and under the scanning electron microscope both high pressure silica phases were identified in situ by laser raman spectroscopy they formed from silica melt as well as by solid state transformation in weakly shocked rocks stage i fine grained stishovite 1 8 m occurs in thin pseudotachylite veins of quartz rich rocks where it obviously nucleated from high pressure frictional melts generally no stishovite was found in planar deformation features pdfs within grains of rock forming quartz the single exception is a highly shocked quartz grain trapped between a pseudotachylite vein and a large ilmenite grain in which stishovite occurs within two sets of lamellae it is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation in strongly shocked rocks stages ib iii above 30 gpa grains of former quartz typically contain abundant and variably sized stishovite 6 m embedded within a dense amorphous silica phase in the interstices between pdfs the formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and pdfs to diaplectic glass in the solid state coesite formed during unloading occurs in two textural varieties granular micrometre sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries these former high pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite the latter is mostly replaced by amorphous silica

high pressure phases in a shock induced melt vein of the tenham l6 chondrite constraints on shock pressure and duration abstract the microtexture and mineralogy of a 580 m wide melt vein in the tenham l6 chondrite were investigated using field emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide grains that crystallized from immiscible melts and sub rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high pressure silicates the melt vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior the 30 m wide vein edge consists of vitrified silicate perovskite ringwoodite akimotoite majorite with minor metal sulfide the 520 m wide vein interior consists of majorite magnesiowustite with irregular metal sulfide blebs although these mineral assemblages are distinctly different the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions the melt vein edge crystallized at about 23 25 gpa and the vein interior crystallized at about 21 25 gpa this relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of 25 gpa or during a relatively slow pressure release using a finite element heat transfer program to model the thermal history of this melt vein during shock we estimate that the time required to quench this 580 m wide vein was 40 ms because the entire vein contains high pressure minerals that crystallized from the melt the shock pressure duration was at least 40 ms using a synthetic hugoniot for tenham and assuming that the sample experienced a peak shock pressure of 25 gpa near the impact site we estimate that the tenham parent body experienced an impact with collision velocity 2 km s based on a one dimensional planar impact model we estimate that the projectile size was 150 m in thickness

cristobalite inclusions in the tatahouine achondrite implications for shock conditions the mineralogy of the tatahouine diogenite was investigated by optical microscopy raman micro spectrometry and scanning and transmission electron microscopies inclusions of alpha cristobalite in orthopyroxenes locally in symplectic association with chromites or associated with metal have been characterized for the first time in a diogenite mosaicism of the orthopyroxenes indicates shock effects in the meteorite the shock history of the meteorite must be consistent with the presence of vein like structures containing inclusions of well crystallized cristobalite it low pressure high temperature phase several possible mechanisms to account for these observations are discussed the simplest one consistent with all observations is that a shock event would have occurred in it hot orthopyroxenite either before extensive cooling of the asteroid or in materials heated by previous impacts and maintained hot under an ejecta blanket

martian gases in an antarctic meteorite significant abundances of trapped argon krypton and xenon have been measured in shock altered phases of the achondritic meteorite elephant moraine 79001 from antarctica the relative elemental abundances the high ratios of argon 40 to argon 36 2000 and the high ratios of xenon 129 to xenon 132 2 0 of the trapped gas more closely resemble viking data for the martian atmosphere than data for noble gas components typically found in meteorites these findings support earlier suggestions made on the basis of geochemical evidence that shergottites and related rare meteorites may have originated from the planet mars

mineralogical study of brown olivine in northwest africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles martian meteorites in particular shergottites contain darkened olivine so called brown olivine whose color is induced by iron nanoparticles formed in olivine during a shock event the formation process and conditions of brown olivine have been discussed in the northwest africa 2737 nwa 2737 chassignite however formation conditions of brown olivine in nwa 2737 cannot be applied to shergottites because nwa 2737 has a different shock history from that of shergottites therefore this study observed brown olivine in the nwa 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites our observation of nwa 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in nwa 2737 xanes analysis showed that brown olivine contains small amounts of fe3 and tem stem observation revealed that there is no sio rich phase around iron metal nanoparticles these observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine 3fe2 olivine fe0metal 2fe3 olivine volivine where volivine means a vacancy in olivine some parts of brown olivine show lamellar textures in sem observation and raman peaks in addition to those expected for olivine implying that brown olivine experienced a phase transition to e g ringwoodite in order to induce heterogeneous darkening heterogeneous high temperature of about 1500 1700 k and shock duration of at least 90 ms are required this heterogeneous high temperature resulted in high postshock temperature 900 k inducing back transformation of most high pressure phases therefore in spite of lack of high pressure phases nwa 1950 martian meteorites with brown olivine experienced higher pressure and temperature compared to other highly shocked meteorite groups

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

the violent collisional history of asteroid 4 vesta vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids the dawn spacecraft extensively imaged vesta s surface revealing a collision dominated history results show that vesta s cratering record has a strong north south dichotomy vesta s northern heavily cratered terrains retain much of their earliest history the southern hemisphere was reset however by two major collisions in more recent times we estimate that the youngest of these impact structures about 500 kilometers across formed about 1 billion years ago in agreement with estimates of vesta asteroid family age based on dynamical and collisional constraints supporting the notion that the vesta asteroid family was formed during this event

shock induced compaction melting and entrapment of atmospheric gases in martian meteorites 1 the strongest evidence for a martian origin of the snc meteorites is the strong correlation between the rare gas abundances and isotopic compositions in shock induced melt pockets and those measured for the martian atmosphere however the formation of melt pockets and the entrapment of atmospheric gases remain poorly understood here we report the presence in the melt pockets of three martian meteorites of the hollandite structured high pressure polymorph of feldspar these occurrences set constraints on the continuum pressure 21 25 gpa the local temperature increase 2000 2500 k and the energy delivered during shock we then test a mechanism for melt pocket formation by compaction of pre existing porous precursors the model explains the local temperature increase required for melting and the presence of an atmospheric component in pores that were connected with the martian atmosphere prior to the shock event

high pressure minerals in shocked meteorites heavily shocked meteorites contain various types of high pressure polymorphs of major minerals olivine pyroxene feldspar and quartz and accessory minerals chromite and ca phosphate these high pressure minerals are micron to submicron sized and occur within and in the vicinity of shock induced melt veins and melt pockets in chondrites and lunar howardite eucrite diogenite hed and martian meteorites their occurrence suggests two types of formation mechanisms 1 solid state high pressure transformation of the host rock minerals into monomineralic polycrystalline aggregates and 2 crystallization of chondritic or monomineralic melts under high pressure based on experimentally determined phase relations their formation pressures are limited to the pressure range up to 25 gpa textural crystallographic and chemical characteristics of high pressure minerals provide clues about the impact events of meteorite parent bodies including their size and mutual collision velocities and about the mineralogy of deep planetary interiors the aim of this article is to review and summarize the findings on natural high pressure minerals in shocked meteorites that have been reported over the past 50 years

majoritic garnet grains within shock induced melt veins in amphibolites from the ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 gpa shock induced melt veins in amphibolites from the nordlinger ries often have chemical compositions that are similar to bulk rock i e basaltic but there are other veins that are confined to chlorite rich cracks that formed before the impact and these are poor in ca and na majoritic garnets within the shock veins show a broad chemical variation between three endmembers 1 text viii text m 2 3 text vi text al 2 text iv text sio 4 3 normal garnet grt 2 text viii text m 2 3 text vi text m 2 text si ti text iv text sio 4 3 majorite maj and 3 text viii text na text m 2 2 text vi text si ti text al text iv text sio 4 3 na majorite50grt50 whereby m2 mg2 fe2 mn2 ca2 in particular we observed a broad variation in vi si ti which ranges from 0 12 to 0 58 cations per formula unit cpfu all these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures those with high vi si ti 0 36 0 58 cpfu formed at high pressures and temperatures from amphibole rich melts while majoritic garnets with lower vi si ti of 0 12 0 27 cpfu formed at lower pressures and temperatures from chlorite rich melts furthermore majoritic garnets with intermediate values of vi si ti 0 24 0 39 crystallized from melts with intermediate contents of ca and na to the best of our knowledge the morb type ca na rich majoritic garnets with maximum contents of 2 99 wt na2o and calculated crystallisation pressures of 16 18 gpa are the most extreme representatives ever found in terrestrial shocked materials at the ries the duration of the initial contact and compression stage at the central location of impact is estimated to only 0 1 s we used a 200 m thick shock induced vein in a moderately shocked amphibolite to model its pressure temperature time p t t path the graphic model manifests a peak temperature of 2600 c for the vein continuum pressure lasting for 0 02 s a quench duration of 0 02 s and a shock pulse of 0 038 s the small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks majoritic garnets of chlorite provenance however are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release an extraordinary glass and majorite bearing amphibole fragment in a shock vein of one amphibolite documents the whole unloading path

the chemical composition of martian soil and rocks returned by the mobile alpha proton x ray spectrometer preliminary results from the x ray mode the alpha proton x ray spectrometer apxs on board the rover of the mars pathfinder mission measured the chemical composition of six soils and five rocks at the ares vallis landing site the soil analyses show similarity to those determined by the viking missions the analyzed rocks were partially covered by dust but otherwise compositionally similar to each other they are unexpectedly high in silica and potassium but low in magnesium compared to martian soils and martian meteorites the analyzed rocks are similar in composition to terrestrial andesites and close to the mean composition of earth s crust addition of a mafic component and reaction products of volcanic gases to the local rock material is necessary to explain the soil composition

the hf w isotopic system and the origin of the earth and moon abstract the earth has a radiogenic w isotopic composition compared to chondrites demonstrating that it formed while 182hf half life 9 myr was extant in earth and decaying to 182w this implies that earth underwent early and rapid accretion and core formation with most of the accumulation occurring in 10 myr and concluding approximately 30 myr after the origin of the solar system the hf w data for lunar samples can be reconciled with a major moon forming impact that terminated the terrestrial accretion process 30 myr after the origin of the solar system the suggestion that the proto earth to impactor mass ratio was 7 3 and occurred during accretion is inconsistent with the w isotope data the w isotope data is satisfactorily modeled with a mars sized impactor on proto earth proto earth to impactor ratio of 9 1 to form the moon at 30 myr

formation of high pressure minerals in shocked l6 chondrite yamato 791384 constraints on shock conditions and parent body size we have studied the high pressure phases observed in yamato 791384 l6 chondrite host meteorite mainly consists of olivine pyroxenes and plagioclase glass mineral fragments observed in the veins and the vein margin region of the host meteorite were partially or totally transformed into high pressure phases ringwoodite majorite akimotoite naalsi3o8 hollandite and jadeite matrix of the shock vein contains majorite pyrope solid solution the spatial distribution indicates that high pressure phases are present in the shock veins and host rocks adjacent to the shock veins investigation of the high pressure phases revealed that in y791384 fragments and adjacent matrix were subjected to pressures around of 18 23 gpa and the vein experienced temperatures around 2300 c during the shock event ringwoodite lamellae were observed in the host olivine adjacent to the vein kinetic investigation for ringwoodite lamellar growth in olivine indicates that the shocked meteorite experienced the pressure for at least 4 s such long duration of the pressure pulse indicates that the size of the parent body was greater than 10 km

gpr a ground penetrating radar for the netlander mission in the coming decade several missions are planned that will land on the surface of mars landers or instrumented geophysical stations among the scientific objectives of these projects one of the most important will be to unravel the many unknowns in the geological and hydrological history of the planet the netlander mission offers a unique opportunity to explore the interior of mars its subsurface its atmosphere and its distant environment from four landing sites that will be selected to offer a variety of different geophysical conditions we have thus proposed to fly on these four landers a ground penetrating radar gpr to explore the geological characteristics of the subsurface and search for water reservoirs down to a depth which may be sufficient to allow a possible detection of liquid water we provide in this paper a short description of this radar which is based on a new concept to allow a 3 d imaging of the subsurface by determining the range and direction of the underground reflectors in order to access to deep layers it will operate at a low frequency of 2 mhz some results obtained by a numerical modeling of the radar operation in an electromagnetic model of the martian subsurface are presented in order to illustrate the main capabilities of the radar in the last section preliminary results from an initial field test are reported in addition to its primary goal as a ground penetrating radar the gpr will also be operated on mars as an ionospheric sounder and in a passive mode as a hf receiver to measure the radio electric background

pressure induced amorphization of albitic plagioclase in an externally heated diamond anvil cell 1 high pressure and high temperature experiments of albitic plagioclase up to 41 gpa and 270 c were carried out using an externally heated diamond anvil cell raman spectroscopy and transmission electron microscopy of the recovered samples revealed that the amorphization of albite was complete at 37 gpa and room temperature the amorphization pressure at 170 c was nearly the same as that at room temperature in contrast the pressure largely decreased to 31 gpa at 270 c in comparison with the amorphization pressure of albite in laboratory shock experiments that in the present static compression experiments is significantly lower 10 gpa even at room temperature this suggests that shorter pressure duration results in a lower degree of amorphization of plagioclase the formation of maskelynite in shocked meteorites does not necessarily require the very high shock pressure 30 90 gpa that was previously estimated on the basis of shock recovery experiments

shock induced metallic iron nanoparticles in olivine rich martian meteorites magnetic anomalies observed by the mars global surveyor mission are attributed to crustal remanence snc shergotty nakhla chassigny meteorites are likely samples of the martian crust and are amenable to mineralogical and magnetic measurements essential to the understanding of the origin of magnetic anomalies the recently discovered chassignite nwa 2737 and lherzolitic shergottite nwa 1950 display unusual magnetic characteristics that argue for a different magnetic carrier than the oxides and sulfides previously invoked in snc meteorites nwa 2737 the second member of the chassignite group is a dunite with unusually dark brown olivines and large magnetic susceptibility while chassigny contains green olivines and is nearly a pure paramagnet dark olivines are also found in nwa 1950 a lherzolitic shergottite which has singular magnetic properties when compared with other shergottites the dark olivine color is due to the presence of fe and feni metal nanoparticles identified both by tem and by magnetic measurements their size distribution encompasses the superparamagnetic to single domain transition at 30 k 10 nm range and explains the magnetic properties of the bulk rocks the formation of these nanoparticles is attributed to heating during the shock events that affected nwa 2737 and nwa 1950 the production of metal particles by shock induced reduction of olivine has been invoked on surfaces deprived of atmosphere but never observed on earth or mars therefore metal formed by shock in the heavily cratered noachian crust is a possible carrier for crustal magnetic remanence widespread surface formation of metal nanoparticles could provide the precursor for the oxidized particles goethite hematite observed in the martian soils c 2007 elsevier b v all rights reserved

thermodynamic properties of mg2sio4 liquid at ultra high pressures from shock measurements to 200 gpa on forsterite and wadsleyite polycrystalline samples of mg 2sio 4 forsterite and wadsleyite were synthesized and then dynamically loaded to pressures of 39 200 gpa differences in initial density and internal energy between these two phases lead to distinct hugoniots each characterized by multiple phase regimes transformation to the high pressure phase assemblage mgo mgsio 3 perovksite is complete by 100 gpa for forsterite starting material but incomplete for wadsleyite the datum for wadsleyite shocked to 136 gpa however is consistent with the assemblage mgo mgsio 3 post perovksite marked increases in density along the hugoniots of both phases between 130 and 150 gpa are inconsistent with any known solid solid phase transformation in the mg 2sio 4 system but can be explained by melting density increases upon melting are consistent with a similar density increase observed in the mgsio 3 system this implies that melts with compositions over the entire mg si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the core mantle boundary supporting the partial melt hypothesis to explain the occurrence of ultra low velocity zones at the base of the mantle from the energetic difference between the high pressure segments of the two hugoniots we estimate a gruneisen parameter of 2 6 0 35 for mg 2sio 4 liquid between 150 and 200 gpa comparison to low pressure data and fitting of the absolute pressures along the melt hugoniots both require that for the melt increases with increasing density similar behavior was recently predicted in mgsio 3 liquid via molecular dynamics simulations this result changes estimates of the temperature profile and hence the dynamics of a deep terrestrial magma ocean

new lunar meteorites impact melt and regolith breccias and large scale heterogeneities of the upper lunar crust we have analyzed nine highland lunar meteorites lunaites using mainly inaa several of these rocks are difficult to classify dhofar 081 is basically a fragmental breccia but much of its groundmass features a glassy fluidized texture that is indicative of localized shock melting also much of the matrix glass is swirly brown suggesting a possible regolith derivation we interpret dar al gani dag 400 as an extremely immature regolith breccia consisting mainly of impact melt breccia clasts we interpret dhofar 026 as an unusually complex anorthositic impact melt breccia with scattered ovoid globules that formed as clasts of mafic subophitic impact melt the presence of mafic crystalline globules in a lunar material even one so clearly impact heated suggests that it may have originated as a regolith our new data and a synthesis of literature data suggest a contrast in al2o3 incompatible element systematics between impact melts from the central nearside highlands where apollo sampling occurred and those from the general highland surface of the moon impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given al2o3 concentration than those from apollo 16 in the case of dhofar 026 both the bulk rock and a comparatively al poor composition 14 wt al2o3 7 g g sm extrapolated for the globules manifest incompatible element contents well below the apollo 16 trend impact melts from luna 20 57e distribute more along the general highland trend than along the apollo 16 trend siderophile elements also show a distinctive composition for apollo 16 impact melts ni ir averaging 1 8x chondritic in contrast lunaite impact melt breccias have consistently chondritic ni ir impact melts from luna 20 and other apollo sites show average ni ir almost as high as those from apollo 16 the prevalence of this distinctive ni ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across highland polymict breccia lunaites and other kreep poor highland regolith samples manifest a strong anticorrelation between al2o3 and mg the magnesian component probably represents the chemical signature of the mg suite of pristine nonmare rocks in its most pure form unaltered by the major kreep assimilation that is so common among apollo mg suite samples the average composition of the ferroan anorthositic component is now well constrained at al2o3 29 30 wt implying about 17 19 wt modal mafic silicates in good agreement with the composition predicted for flotation crust over a ferroan magma ocean warren 1990

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

numerical simulation of temperature effects at fissures due to shock loading the localized appearance of specific shock features in target rocks and meteorites such as melt veins and high pressure polymorphs suggests that regions with a local increase in pressure and temperature exist as a shock wave propagates through an inhomogeneous rock in this paper we investigate the effect of planar fissures on the local temperature distribution using numerical simulations time dependent parameters such as temperature pressure and displacement are evaluated the simulation model is based on a shock equation of state for the involved materials dunite and quartzite and simulates geometries that were also used in shock loading experiments an artificial gap between the materials simulates an open fissure at the interface a strong temperature increase occurs at a gap size of 0 1 mm which potentially can cause melting in a thin layer at the interfaces the temperature decreases with decreasing gap size temperature and pressure excursions at the interface are induced by the closure of the gap which causes a second shock wave to superpose the primary wave open fissures and fractures which occur ubiquitously in shallow buried target rocks and projectiles thus act as local pressure and temperature amplyfiers and may be responsible for thin melt vein formation in shocked rocks

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

a natural shock induced dense polymorph of rutile with α pbo2 structure in the suevite from the ries crater in germany a dense post rutile titanium dioxide tio2 phase was discovered in shocked garnet gneisses from the ries crater by reflected light microscopy laser rnicroraman spectroscopy and micro beam x ray diffraction the raman spectrum consists of nine bands at wave numbers 152 175 285 315 340 358 428 532 and 575 cm 1 these bands are identical to those of the alpha pbo2 structured polymorph of tio2 synthesized in a dynamic laboratory experiment the diffraction pattern of the natural mineral revealed an orthorhombic lattice similar to the alpha pbo2 polymorph with the cell parameters a 4 535 2 angstrom b 5 499 2 angstrom c 4 900 2 angstrom space group phen columbite type structure density rho 4 34g cm 2 where the numbers in parentheses are standard deviations in the last significant digits this new polymorph is 2 denser than rutile the rutile alpha pbo2 polymorph phase transformation occurs exclusively at the grain boundaries between rutile and the shock compacted host biotite and advances inwards in rutile this textural relation establishes phase boundaries as the preferable fabric settings for dynamic induced high pressure phase transitions ab initio calculations negates the formation of the alpha pbo2 polymorph by inversion of a parental fluorite structured polymorph during decompression heating of the experimentally produced alpha pbo2 polymorph above 500 degreesc shows that it inverts back in a laboratory time scale to rutile hence the survival of the alpha pbo2 structured polymorph in naturally shocked rocks constrains the post shock temperature of the tio2 bearing assemblage at an upper bound of 500 degreesc the presence of this dense phase is expected in the earth s upper mantle below 123 km the rutile alpha pbo2 structured polymorph phase transition in subducted crustal limbs in the upper mantle should then be accompanied by considerable changes in the partitioning and fractionation of nb and ta between this tio2 polymorph and the coexisting dense silicates c 2001 elsevier science b v all rights reserved

impact induced melting and the development of large igneous provinces abstract we use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater this mechanism may generate sufficient quantity of melt to auto obliterate the crater melting would initiate almost instantaneously but the effects of such massive mantle melting may trigger long lived mantle up welling that could potentially resemble a mantle hotspot decompression melting is well understood it is the main method advocated by geophysicists for melting on earth whether caused by thinned lithosphere or hot rising mantle plumes the energy released is largely derived from gravitational energy and is outside but additive to the conventional calculations of impact modelling where energy is derived solely from the kinetic energy of the impacting projectile be it comet or asteroid the empirical correlation between total melt volume and crater size will no longer apply but instead there will be a discontinuity above some threshold size depending primarily on the thermal structure of the lithosphere we estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces 106 km3 similar melting of the mantle beneath an oceanic impact was also modelled by roddy et al int j impact eng 5 1987 525 the mantle melts will have plume like geochemical signatures and rapid mixing of melts from sub horizontal sub crater reservoirs is likely direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record we suggest that the end permian siberian traps should be reconsidered as the result of a major impact at 250 ma auto obliteration by volcanism of all craters larger than 200 km would explain their anomalous absence on earth compared with other terrestrial planets in the solar system

back transformation of high pressure minerals in shocked chondrites low pressure mineral evidence for strong shock post shock annealing of meteorites can destroy their shock induced features particularly high pressure minerals and complicate the estimation of impact pressure temperature conditions however distinguishing post shock annealing features from thermal metamorphism effects can be practically difficult here we report results from mbale a highly shocked l chondrite to investigate the mechanisms kinetics and identification criteria for post shock annealing of high pressure signatures olivine fragments within shock melt veins in mbale occur as chemically heterogeneous nanocrystalline aggregates that contain trace wadsleyite and ringwoodite their strong variation in fayalite content provides evidence of iron partitioning during transformation of olivine to wadsleyite followed by back transformation to olivine after decompression experimental studies of transformation kinetics show that wadsleyite transforms to olivine in seconds at temperatures above 1200 k and in hours at temperatures between 900 and 1200 k thermal models of shock melt cooling show that shock veins in mbale cooled to 1200 k in 1 s the shock pulse must have been shorter than 1 s to provide the high temperature conditions for post shock back transformation of wadsleyite many highly shocked l chondrites which have abundant high pressure minerals must have experienced relatively long shock durations combined with rapid cooling of shock melt regions to preserve high pressure phases the most highly shocked samples such as impact melt breccias lack high pressure phases because of post shock back transformations

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

40ar 39ar impact ages and time temperature argon diffusion history of the bunburra rockhole anomalous basaltic achondrite abstract the bunburra rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse medium and fine grained lithologies petrographic observations constrain the limited shock pressure to between ca 10 gpa and 20 gpa in this study we carried out nine 40 ar 39 ar step heating experiments on distinct single grain fragments extracted from the coarse and fine lithologies we obtained six plateau ages and three mini plateau ages these ages fall into two internally concordant populations with mean ages of 3640 21 ma n 7 p 0 53 and 3544 26 ma n 2 p 0 54 respectively based on these results additional 40 ar 39 ar data of fusion crust fragments argon diffusion modelling and petrographic observations we conclude that the principal components of the bunburra rockhole basaltic achondrite are from a melt rock formed at 3 64 ga by a medium to large impact event the data imply that this impact generated high enough energy to completely melt the basaltic target rock and reset the ar systematics but only partially reset the pb pb age we also conclude that a complete 40 ar resetting of pyroxene and plagioclase at this time could not have been achieved at solid state conditions comparison with a terrestrial analog lonar crater shows that the time temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve ar data also suggest that a second medium size impact event occurred on a neighbouring part of the same target rock at 3 54 ga concordant low temperature step ages of the nine aliquots suggest that at 3 42 ga a third smaller impact excavated parts of the 3 64 ga and 3 54 ga melt rocks and brought the fragments together the lack of significant impact activity after 3 5 ga as recorded by the bunburra rockhole suggests that 1 either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts or 2 it was covered by a porous heat absorbing regolith blanket which when combined with the diminishing frequency of large impacts in the solar system protected bunburra from subsequent major heating events finally we note that the total k ar resetting impact event history recorded by some of the brecciated eucrites peak at 3 8 3 5 ga is similar to the large impact history recorded by the bunburra rockhole parent body ca 3 64 3 54 ga this study and could indicate a similar position in the asteroid belt at that time

shocked chromites in fossil l chondrites a raman spectroscopy and transmission electron microscopy study chromites from middle ordovician fossil l chondrites and from matrix and shock melt veins in catherwood tenham and coorara l chondrites were studied using raman spectroscopy and tem raman spectra of chromites from fossil l chondrites showed similarities with chromites from matrix and shock melt veins in the studied l chondrite falls and finds chromites from shock melt veins of l chondrites show polycrystallinity while the chromite grains in fossil l chondrites are single crystals in addition chromites from shock melt veins in the studied l chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite matrix chromite of tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite sextummen 003 using this observation and knowing that the matrix of tenham experienced 20 22 gpa and 1000 c an upper limit for the p t conditions of chromite from golvsten 001 and sextummen 003 can be estimated to be 20 22 gpa and 1000 c shock stage s3 s6 and 20 gpa and 1000 c s3 s5 respectively and we conclude that the studied fossil meteorite chromites are from matrix

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

shock veins in the central uplift of the manicouagan impact structure context and genesis we describe the development of shock veins that penetrate the anorthositic central uplift of the manicouagan impact structure they occur as thin 2 5 mm wide linear micro fault systems that can be traced for several meters in length and which predominantly trend radially from the point of impact the shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices which are otherwise absent in non veined regions these phases define a shock excursion of up to 30 gpa in contrast to bulk shock effects of 12 gpa defined by development of shatter cones planar fractures and planar deformation features in various minerals the shock veins at manicouagan share many similarities with vein systems developed in meteorites they also provide an in situ context with which to better understand meteoroid source and lofting conditions in addition to containing high pressure phases the shock veins exhibit evidence for high temperature partial melting of host silicate clasts with the generation of flow textured fragments and glasses the formation of microcrystallites and dendrites from some melts indicates rapid cooling we propose a two stage generation mechanism comprising an initial high pressure shock excursion estimated to last 0 5 s based on projectile size considerations followed by a longer high temperature pulse of a few seconds duration we suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front this results in radially oriented tearing and vein formation with shock amplification occurring via intra vein shock reverberation high speed displacement along the veins is driven by stress release on rarefaction which results in frictional melting via adiabatic heating

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

a new natural high pressure na ca hexaluminosilicate caxna1 x al3 xsi3 xo11 in shocked martian meteorites a ca na hexaluminosilicate whose ca end member was previously synthesized in numerous high pressure experiments has been identified by raman spectroscopy in heavily shocked martian meteorites this mineral has a structural formula close to caxna1 x al3 xsi3 xo11 and is similar to the calcium aluminum silicate phase previously synthesized in high pressure experiments performed on anorthite and rocks of basaltic composition this new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with sio2 stishovite the first setting encountered in zagami consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition the second setting contains the na ca hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures the mineralogical association na ca hexaluminosilicate stishovite was observed in shock melt pockets which have distinct bulk compositions in seven martian shergottites this new mineral represents after majorite the second natural occurrence of a silicate mineral with silicon in both four and six coordination the assemblage stishovite na ca hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites the na ca hexaluminosilicate mineral is a potential carrier of al and na during subduction of oceanic crust in the lower mantle of the earth c 2003 elsevier b v all rights reserved

shock events in the solar system the message from minerals in terrestrial planets and asteroids impacts are central to the origin and evolution of planets of the solar system the shapes of craters which can reach up to 1 000 km in diameter on the moon provide critical information on the large scale dynamics of the impact and related shock minerals formed at high pressure and temperature found in shocked terrestrial rocks and meteorites give additional and complementary insights on the shock process at a smaller scale typically from a few micrometers to a few millimeters local flaws in rocks such as voids and mineral interfaces are the preferential sites for the formation of high pressure melts and minerals calculations based on the physics of shocks and the thermodynamics and kinetics of mineral transformations provide orders of magnitude for the duration transient pressure and prevailing temperature conditions of shock events case studies on shocked terrestrial and extraterrestrial materials illustrate the links between these parameters and impact duration many of the high pressure min

impact spherules as a record of an ancient heavy bombardment of earth the fossilized remnants of vaporized asteroids called spherules can be used to infer that the flux of asteroid impacts on earth 3 5 billion years ago was much greater than it is now the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets impact craters are the most obvious indication of asteroid impacts but craters on earth are quickly obscured or destroyed by surface weathering and tectonic processes1 earth s impact history is inferred therefore either from estimates of the present day impactor flux as determined by observations of near earth asteroids or from the moon s incomplete impact chronology2 3 4 asteroids hitting earth typically vaporize a mass of target rock comparable to the projectile s mass as this vapour expands in a large plume or fireball it cools and condenses into molten droplets called spherules5 for asteroids larger than about ten kilometres in diameter these spherules are deposited in a global layer spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found1 here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers the impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3 5 billion years ago than it is now this conclusion is consistent with a gradual decline of the impactor flux after the late heavy bombardment

the nature of the l chondrite parent body s disruption as deduced from high pressure phases in the sixiangkou l6 chondrite the disruption of the l chondrite parent body lcpb at similar to 470ma is currently the best documented catastrophic celestial impact event based on the large number of l chondritic materials associated with this event uranium lead u pb dating of apatite and its high pressure decomposition product tuite in the sixiangkou l6 chondrite provides a temporal link to this event the u pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the u pb system correlates closely with the extent of apatite decomposition this suggests that the u pb system of apatite could be substantially disturbed by high temperature pulse during shock compression from natural impacts at least on the scale of mineral grains although many l chondrites can be temporally related to the catastrophic lcpb impact event the shock conditions experienced by each individual meteorite vary this could be due to the different geologic settings of these meteorites on their parent body the shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions although they could provide lower limits to the impact conditions the sixiangkou shock duration similar to 4s estimated from high pressure transformation kinetics provides a lower limit to the high pressure pulse of the lcpb disruption impact combined with available literature data of l chondrites associated with this impact event our results suggest that the lcpb suffered a catastrophic collision with a large projectile with a diameter of at least 18 22km at a low impact velocity 5 6kms 1 this is consistent with astronomical estimates based on the dynamical evolution of l chondritic asteroids

shock conditions recorded in nwa 8159 martian augite basalt with implications for the impact cratering history on mars nwa 8159 is an augite rich martian basalt formed by cooling of a relatively evolved ca rich ti poor and lree depleted lava under relatively oxidizing conditions during the early amazonian in addition to its distinct igneous petrogenesis and high fo 2 nwa 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase an 50 65 in this study mineral transformations within and adjacent to shock veins in nwa 8159 were investigated using scanning electron microscopy raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite the transformation of olivine to ahrensite fe ringwoodite along shock vein margins and tissintite and coesiteformed from igneous mineral labradorite and silica grains entrained as clasts within shock veins has been documented in this study we report on a previously unidentified mineral assemblage of ca na majoritic garnet sodic clinopyroxene and stishovite crystallized from shock melt this mineral assemblage indicates a crystallization pressure of approximately 16 gpa which is within the range of previous shock pressure estimates for this meteorite 15 23 gpa the presence of a majoritic garnet bearing assemblage throughout veins up to 0 6 mm wide indicates that the sample remained at high pressure throughout the melt vein quench based on thermal models the sample must have remained at high pressure for 100 ms this shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as tissint or zagami 30 gpa 10 20 ms and would seem to imply a relatively large impact event recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact the shock conditions experienced by nwa 8159 therefore do not require an impact event distinct from other shergottites rather our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site the shock history of nwa 8159 provides a picture of mars consistent with that derived from remote observation that of a random cratering process that samples a geologically long lived and complex planet

in situ discovery of shock induced graphite diamond phase transition in gneisses from the ries crater germany reflected light microscopy and fine scale lasts microraman spectroscopy of shocked garnet cordierite sillimanite gneisses in suevites of the ries meteorite impact crater germany led to the discovery of impact diamonds in their pristine graphite diamond assemblages graphite diamond textural relations permit a clear determination of the solid state nature of the formation of diamond from graphite which is estimated to have occurred at a peak shock pressure between 30 and 40 gpa shock induced transformations were promoted only in unkinked and undeformed graphite booklets at the graphite garnet graphite sillimanite or graphite rutile interfaces where the difference in shock impedance is very high reverberations of shock waves with short wavelengths similar to the grain sizes at the phase boundaries are probably important constraints for dynamic graphite diamond phase transformation raman spectroscopic investigations of hard transparent carbon platelets intercalated between fine grained diamond and deformed graphite revealed the platelets to be raman inactive the platelets are either dense amorphous carbon or an unknown dense crystalline carbon phase that is raman inactive

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

earth impact effects program a web based computer program for calculating the regional environmental consequences of a meteoroid impact on earth we have developed a web based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on earth www lpl arizona edu impacteffects this paper details the observations assumptions and equations upon which the program is based it describes our approach to quantifying the principal impact processes that might affect the people buildings and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions the program requires six inputs impactor diameter impactor density impact velocity before atmospheric entry impact angle the distance from the impact at which the environmental effects are to be calculated and the target type sedimentary rock crystalline rock or a water layer above rock the program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse the thermal radiation emitted by the impact generated vapor plume fireball and the intensity of seismic shaking the program also approximates various dimensions of the impact crater and ejecta deposit as well as estimating the severity of the air blast in both crater forming and airburst impacts we illustrate the utility of our program by examining the predicted environmental consequences across the united states of hypothetical impact scenarios occurring in los angeles we find that the most wide reaching environmental consequence is seismic shaking both ejecta deposit thickness and air blast pressure decay much more rapidly with distance than with seismic ground motion close to the impact site the most devastating effect is from thermal radiation however the curvature of the earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon

plagioclase breakdown as an indicator for shock conditions of meteorites the collision history of asteroids in the early solar system is difficult to reconstruct a study of plagioclase breakdown using x ray diffraction measurements under increasing pressures and temperatures suggests that peak pressures in collisions that formed shocked meteorites have been overestimated shocked meteorites that were formed when their parent body underwent shock metamorphism often contain the mineral plagioclase either in an amorphous form or in its high pressure phase the peak pressures in shock metamorphism can be constrained by shock recovery experiments that determine the amorphization pressures of plagioclase1 2 3 however in these experiments temperatures have been unrealistically low and timescales much shorter than those in natural events here we present in situ x ray diffraction measurements of two kinds of plagioclase feldspar in conditions of increasing pressures and temperatures we find that the amorphization pressure of plagioclase decreases with increasing temperature suggesting that previous studies overestimated this parameter1 2 3 we also found that the mineral jadeite forms first from amorphous plagioclase whereas the nucleation of other minerals such as stishovite or garnet is significantly delayed the occurrence of jadeite in shocked meteorites that do not contain stishovite4 5 6 7 8 can therefore be explained as a result of the crystallization kinetics we conclude that the study of plagioclase breakdown can constrain the pressure temperature time history of shock events and thus help to reconstruct the collisional history of asteroids in the early solar system

petrology and chemistry of the basaltic shergottite north west africa 480 north west africa nwa 480 is a new martian meteorite of 28 g found in the moroccan sahara in november 2000 it consists mainly of large gray pyroxene crystals the largest grains are up to 5 mm in length and plagioclase converted to maskelynite excluding the melt pocket areas modal analyses indicate the following mineral proportions 72 vol pyroxenes extensively zoned 25 maskelynite 1 phosphates merrillite and chlorapatite 1 opaque oxides ilmenite ulvospinel and chromite and sulfides and 1 others such as silica and fayalite the compositional trend of nwa 480 pyroxenes is similar to that of queen alexandra range que 94201 but in nwa 480 the pyroxene cores are more mg rich en 77 en 65 maskelynites display a limited zoning an 42 50 ab 54 48 or 2 4 our observations suggest that nwa 480 formed from a melt with a low nuclei density at a slow cooling rate the texture was achieved via a single stage cooling where pyroxenes grew continuously a similar model was previously proposed for que 94201 by mcsween et al 1996 nwa 480 is an al poor ferroan basaltic rock and resembles zagami or shergotty for major elements and compatible trace element abundances the bulk rock analysis for oxygen isotopes yields delta 17 o 0 42parts per thousand a value in agreement at the high margin with those measured on other shergottites clayton and mayeda 1996 romanek et al 1998 franchi et al 1999 its ci normalized rare earth element pattern is similar to those of peridotitic shergottites such as allan hills alh a77005 suggesting that these shergottites shared a similar parent liquid or at least the same mantle source

akimotoite in the tenham meteorite crystal chemistry and high pressure transformation mechanisms the transformation of pyroxene to its high pressure polymorph akimotoite mgsio3 ilmenite type structure is documented in the shock induced melt veins of the l6 tenham chondrite four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein in one of the entrained enstatite grains the transformation to akimotoite is partial one third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite akimotoite fe0 4mg1 24ca0 07na0 12al0 14 si1 963al0 037 o6 is slightly enriched in ca cao 1 74 al and na with respect to enstatite cao 0 71 narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain in a second grain entrained in the shear melt vein tablets of akimotoite are interwoven with a pyroxene glass with a lower na and fe content and a higher mg content compared to the adjacent akimotoite this pyroxene glass is probably the product of mg fe sio3 perovskite amorphization polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein in that case akimotoite is also enriched in ca cao 2 85 na nao 1 72 and al al2o3 4 14 all these observations suggest that akimotoite is mainly formed by solid state transformation of former pyroxenes with subsequent diffusion of calcium aluminum and sodium from the chondritic melt of the shear melt vein finally the first complete x ray diffraction pattern of natural akimotoite is presented

magma oceans in the inner solar system theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early accreting planetesimals the apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet the processes of magma ocean formation and solidification therefore strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets and they form the starting point for cooling to clement habitable conditions and for the onset of thermally driven mantle convection and plate tectonics this review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean distribution and degassing of volatiles and cooling

dating phosphates of the strongly shocked suizhou chondrite impacts on undifferentiated asteroidal bodies provide invaluable information for understanding the solar system evolution nevertheless dating early and small scale impact events is technically challenging uranium lead u pb systematics of u bearing phases within shock induced melt veins smvs of chondrites may be significantly disturbed by localized heating thus providing opportunities to date these impact events as one of the major u hosts in chondrites apatite in the suizhou l6 chondrite has been compositionally and structurally modified to varying degrees by shock metamorphism apatite grains in the host remained largely intact during the impact and have recorded the initial thermal cooling time similar to 4550 ma on their parent body apatite grains in regions less than 100 mu m bordering the smvs or in relatively cold regions within the smvs were partly decomposed to tuite driven by the localized transient heating within the smvs their u pb systematics were disrupted to varying extents apatite in regions close to the center of the smvs has been completely transformed to tuite which yields an age of 4481 30 ma 2 sigma providing an upper limit to the impact event this study clearly demonstrates that by integrating in situ u pb isotope analysis with detailed microstructural and compositional analysis of phosphates it is possible to deduce the timing of early and small scale celestial impact events and hence create a more comprehensive understanding of the impact history of the solar system

shock induced transformation of olivine to a new metastable mg fe 2sio4 polymorph in martian meteorites transient high pressures and temperatures generated during meteor or asteroid impacts induce mineral phase transformations that can mimic those occurring at depth within the silicate mantle of terrestrial planets olivine mg fe 2sio4 the primary constituent of the earth s upper mantle and of chondritic meteorites transforms to the high pressure polymorphs wadsleyite and ringwoodite mg fe 2sio4 that are observed in shocked chondrites the observed phase transitions place constraints on the shock p t conditions attained and they lead to models that describe the impact event we studied the olivines present within two newly catalogued martian meteorites nwa 2737 and nwa 1950 using micro raman spectroscopy and high resolution transmission electron microscopy hrtem techniques the shock conditions were not sufficient to cause melting or transformation of the olivines into wadsleyite or ringwoodite mg fe 2sio4 the shocked olivines are dark coloured in hand specimen and thin section due to the presence of fexniy metallic nanoparticles formed during the shock molecular dynamics simulations md are consistent with the observation that the shocked olivines give rise to a new orthosilicate polymorph mg fe 2sio4 that is formed metastably during the shock process and that is subsequently recovered to ambient conditions the presence of the new mg fe 2sio4 polymorph in shocked ultrabasic rocks including meteorites may have remained undetected due to its structural and spectroscopic similarities with olivine the existence of the metastable phase transition also allows rationalising previously unexplained results of shock compression experiments on olivines mg fe 2sio4 is also formed at ambient pressure as a metastable intermediate during back transformation from wadsleyite

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

origin of the eclogitic clasts with graphite bearing and graphite free lithologies in the northwest africa 801 cr2 chondrite possible origin from a moon sized planetary body inferred from chemistry oxygen isotopes and ree abundances abstract in order to clarify the origin of the eclogitic clasts found in the nwa801 cr2 chondrite kimura et al 2013 especially that of the high pressure and temperature p t condition 3 gpa and 1000 c we conducted ion microprobe analyses of oxygen isotopes and rare earth element ree abundances in the clasts oxygen isotopic compositions of the graphite bearing lithology gbl and graphite free lithology gfl show a slope 0 6 correlation slightly below the cr ch cb chondrites field in the o three isotope diagram with a large variation for the former and almost homogeneous composition for the latter the average ree abundances of the two lithologies show almost unfractionated patterns based on these newly obtained data as well as mineralogical observations bulk chemistry and considerations about diffusion timescales for various elements we discuss in detail the formation history of the clasts consistency of the geothermobarometers used by kimura et al 2013 suggesting equilibration of various elements among different mineral pairs provides a strong constraint for the duration of the high p t condition we suggest that the high p t condition lasted 102 103 years this clearly precludes a shock high pressure hp model and hence strongly supports a static hp model a static hp model requires a moon sized planetary body of 1500 km in radius furthermore it implies two successive violent collisions first at the formation of the large planetary body when the clasts were placed its deep interior and second at the disruption of the large planetary body when the clasts were expelled out of the parent body and later on transported to the accretion region of the cr chondrites we also discuss possible origin of o isotopic variations in gbl and presence absence of graphite in gbl gfl respectively in relation to smelting possibly occurred during the igneous process es which formed the two lithologies finally we present a possible formation scenario of the eclogitic clasts

eclogitic minerals in a shocked basaltic meteorite analytical transmission electron microscopy atem was used to characterize mineral parageneses occurring in melt veins of shock origin in the martian meteorite zagami we discovered a new ultra high pressure eclogite facies mineral assemblage consisting of omphacite stishovite and kalsi3o8 hollandite these tiny 200 300 nm high pressure minerals are embedded in silicate glass which forms the groundmass of the shock veins the microtexture of the veins and zoning of the clinopyroxene indicate the formation of minerals by crystallization from a high pressure melt using calibration data from shock and multi anvil experiments we infer that frictional melting occurred in veins at minimum temperatures of 2400 2500 c and at a pressure of ca 30 gpa subsequently the high pressure phases crystallized during decompression in the pressure interval from 25 5 gpa

carbon polymorphism in shocked meteorites evidence for new natural ultrahard phases a slice of the havero meteorite which belongs to the ureilite class known to contain graphite and diamond was cut and then polished as a thin section using a diamond paste we identified two carbonaceous areas which were standing out by more than 10 m in relief over the surface of the silicate matrix suggesting that the carbonaceous phases were not easily polishable by a diamond paste and would therefore imply larger polishing hardness these areas were investigated by reflected light microscopy high resolution field emission sem fesem energy dispersive x ray edx analysis raman spectroscopy and were subsequently extracted for in situ synchrotron microbeam x ray fluorescence xrf imaging and x ray diffraction xrd we report here the natural occurrences of one new ultrahard rhombohedral carbon polymorph of the r3m space group which structure is very close to diamond but with a partial occupancy of some of the carbon sites we also report the natural occurrence of the theoretically predicted 21r diamond polytype with lattice parameters very close to what has been modelized these findings are of great interests for better understanding the world of carbon polymorphs and diamond polytypes giving new natural materials to investigate these natural samples demonstrate that the carbon system is even more complex than what is currently thought based on ab initio static lattice calculations and high pressure experiments since this new ultrahard polymorph has never been predicted nor synthesized

natural high pressure polymorph of merrillite in the shock veins of the suizhou meteorite abstract a new high pressure polymorph of merrillite with the structure of trigonal ca 3 po 4 2 was found in the shock produced veins of the suizhou meteorite where it coexists with ringwoodite majorite naalsi 3 o 8 hollandite and majorite pyrope garnet the crystallographic nature of this natural ca 3 po 4 2 phase was characterized by raman spectroscopy and x ray diffraction and all data compare favorably to the same data obtained from ca 3 po 4 2 synthesized at 14 gpa and 1400 c the cell parameters of this new high pressure mineral are a 5 258 1 angstroms and c 18 727 3 angstroms space group r 3m and density 3 447 g cm 3 where the number in parentheses are standard deviations in the last significant digits the natural occurrence of the ca 3 po 4 2 phase together with other high pressure minerals constrains the pressure of the shock veins at about 23 gpa the suizhou meteorite provides the first naturally occurring example of ca 3 po 4 2 polymorph