carbonates in fractures of martian meteorite allan hills 84001 petrologic evidence for impact origin carbonates in martian meteorite allan hills 84001 occur as grains on pyroxene grain boundaries in crushed zones and as disks veins and irregularly shaped grains in healed pyroxene fractures some carbonate disks have tapered mg rich edges and are accompanied by smaller thinner and relatively homogeneous magnesite microdisks except for the microdisks all types of carbonate grains show the same unique chemical zoning pattern on mgco3 feco3 caco3 plots this chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process the heterogeneous distribution of carbonates in fractures tapered shapes of some disks and the localized occurrence of mg rich microdisks appear to be incompatible with growth from an externally derived co2 rich fluid that changed in composition over time these features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual mg rich fluid that was squeezed along fractures carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate we infer that carbonates in fractures formed from grain boundary carbonates associated with plagiociase that were melted by impact and dispersed into the surrounding fractured pyroxene carbonates in fractures which include those studied by mckay et al 1996 could not have formed at low temperatures and preserved mineralogical evidence for martian organisms

life on mars evaluation of the evidence within martian meteorites alh84001 nakhla and shergotty abstract analyses both support and are in opposition to the hypothesis that the martian meteorite alh84001 contains evidence for possible biogenic activity on mars new observations in two additional martian meteorites nakhla 1 3 ga old and shergotty 300 165 ma old indicate possible biogenic features features in the three martian meteorites compare favorably with the accepted criteria for terrestrial microfossils and evidence for early life on the earth there is strong evidence for the presence of indigenous reduced carbon biogenic magnetite and the low temperature formation of carbonate globules the morphological similarities between terrestrial microfossils biofilms and the features found in the three martian meteorites are intriguing but have not been conclusively proven every investigation must recognize the possibility of terrestrial contamination of the meteorites whether or not the meteorites are martian the search for evidence of ancient life in martian meteorites has emphasized the difficulties confronting the scientific community with the respect to the positive identification of evidence of past biogenic activity

petroleum geochemistry in the neogene sedimentary basin of japan with special reference to primary migration this paper presents geochemical and some geological features of the presumed source beds of the neogene tertiary in japan distribution patterns of total organic carbon bitumen and hydrocarbon contents in the presumed source beds variation of bitumen and hydrocarbon ratios vitrinite reflectance with burial depth the time temperature relationship to the threshold of intense oil generation negative evidence for primary migration in water solution and the depth porosity curves of the japanese oil producing sedimentary basins were examined the results show that primary migration by oil phase transport of immature or mild mature hydrocarbons derived from primary bitumen takes place at a relatively shallow depth c 800 1300 m in japanese neogene oil fields

petrography and bulk chemistry of martian orthopyroxenite alh84001 implications for the origin of secondary carbonates abstract new petrologic and bulk geochemical data for the snc related martian meteorite alh84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes alh84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates rare earth element ree patterns for several splits of the meteorite reveal substantial heterogeneity in ree abundances and significant fractionation of the rees between crushed and uncrushed domains within the meteorite complex zoning in carbonates indicates nonequilibrium processes were involved in their formation suggesting that co2 rich fluids of variable composition infiltrated the rock while on mars we interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution replacement reactions between co2 charged fluids and feldspathic glass in the meteorite carbonate formation clearly postdated processes that last redistributed the ree in the meteorite

geochemical correlation of windalia oil and extracts of winning group cretaceous potential source rocks barrow subbasin western australia windalia oil is a commercial accumulation in a cretaceous reservoir at barrow island western australia a chemical correlation study has been made between this oil and hydrocarbons extracted from 37 cretaceous sedimentary rock samples from different stratigraphic levels in seven wells on and around barrow island seven established correlation parameters and one new parameter have been applied to establish oil source rock relations the new parameter is the ratio of pristane heptadecane to phytane octadecane the influence of maturation on several correlation parameters has been investigated by pyrolyzing source rocks at 300 c for varying times the changes observed in these experiments have been used to infer the effects which maturation processes have on the co relation parameters to assess the overall degree of correlation between source rock extracts and oil each method has been assigned a weighting which reflects its power as a correlation technique an overall correlation score was obtained for each potential source rock by totaling the contributions from the eight individual methods it appears likely that the windalia oil had its source within the winning group of sedimentary rocks the most probable source formations being the middle shale of the windalia sandstone member the muderong shale and the windalia radiolarite several mechanisms of formation of windalia oil from these source beds are possible

transmission electron microscopy of l group chondrites 1 natural shock effects abstract high voltage and analytical transmission electron microscopy are used to characterize fine scale deformational and thermal effects of shock in l group chondrites belonging to shock facies d and e of dodd and jarosewich 1 in facies d olivine showing optical mosaicism has a dislocation substructure exactly like that produced in shock experiments thermal effects become pervasive in olivine of facies e with recovery of dislocation distributions and local recrystallization analogous to that in facies f but less coarse orthopyroxene has behaved more inhomogeneously than in shock experiments no single deformation mechanism being predominant in any specimen twinned clinopyroxene in facies e is attributed to the sequence of inversions ortho proto clino due to heating and rapid cooling in diopside as well as the 100 twinning previously described from less intensely shocked meteorites there is 001 twinning produced by a distinctive mechanism plagioclase is converted to glass maskelynite in lamellae and patches an intimate association of chromite with glass of plagioclase composition is interpreted as a melting phenomenon the characteristic melt pockets of facies d consist of glass with metal and sulphide globules the glass is derived from more than one mineral giving it a mixed silicate composition in facies e crystallization of silicate melts occurred and metal grains crystallized from the same melts volatilization accompanying melting is indicated by vesiculation of melt pockets and thin 1 m veins with high si mg are interpreted as vapour deposition products or melts modified by vapour transport of volatile elements

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

an experimental study on kinetically driven precipitation of calcium magnesium iron carbonates from solution implications for the low temperature formation of carbonates in martian meteorite allan hills 84001 abstract spherical carbonate globules of similar composition size and radial ca mg and fe zonation to those in martian meteorite allan hills alh 84001 were precipitated from mg rich supersaturated solutions of ca mg fe co2 h2o at 150 c the supersaturated solutions ph 6 7 were prepared at room temperature and contained in teflontm lined stainless steel vessels which were sealed and heated to 150 c for 24 h experiments were also conducted at 25 c and no globules comparable to those of alh 84001 were precipitated instead amorphous fe rich carbonates were formed after 24 h and mg fe calcites formed after 96 h these experiments suggest a possible low temperature inorganic origin for the carbonates in martian meteorite alh 84001

microscale carbon isotope variability in alh84001 carbonates and a discussion of possible formation environments abstract the carbonates in martian meteorite alh84001 preserve a record of aqueous processes on mars at 3 9 ga and have been suggested to contain signatures of ancient martian life the conditions of the carbonate formation environment are critical for understanding possible evidence for life on mars the history of water on mars and the evolution of the martian atmosphere despite numerous studies of petrographic relationships microscale oxygen isotope compositions microscale chemical compositions and other minerals associated with the carbonates formation models remain relatively unconstrained microscale carbon isotope analyses of alh84001 carbonates reveal variable 13c values ranging from 27 to 64 the isotopic compositions are correlated with chemical composition and extent of crystallization such that the mg poor early formed carbonates are relatively 13c depleted and the mg rich later forming carbonates are 13c enriched these data are inconsistent with many of the previously proposed environments for carbonate formation and a new set of hypotheses are proposed specifically two new models that account for the data involve low temperature

petrographic and geochemical evidence for multiphase formation of carbonates in the martian orthopyroxenite allan hills 84001 martian meteorites can provide valuable information about past environmental conditions on mars allan hills 84001 formed more than 4 gyr ago and owing to its age and long exposure to the martian environment and this meteorite has features that may record early processes these features include a highly fractured texture gases trapped during one or more impact events or during formation of the rock and spherical fe mg ca carbonates in this study we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events the petrographic features and compositional properties of these carbonates indicate that at least two pulses of mg and fe rich solutions saturated the rock those two generations of carbonates can be distinguished by a very sharp change in compositions from being rich in mg and poor in fe and mn to being poor in mg and rich in fe and mn between these two generations of carbonate is evidence for fracturing and local corrosion

shock induced melting and vaporization of shatter cone surfaces evidence from the sudbury impact structure analytical scanning electron microscopy has been used to investigate the surface textures and compositions of newly exposed shatter cones from the 1 85 ga sudbury impact structure canada unusual surface microstructures are observed at the micron scale including silicate melt smears melt fibres and melt splats silicate and ni rich spherules up to 5 m in diameter adorn earlier formed surface features and we interpret these to be condensates formed due to shock induced vaporization of the shatter cone surfaces the development of striations on the shatter cones is attributed to shock related fracture and slip formation of melts and spherules indicates that the highest ranks of shock metamorphism stages iv and v were realized but only on a very localized scale shatter cone surfaces are therefore likely sites for the development of high pressure polymorphs and if the chemistry is appropriate fullerenes as such they may be equivalent to type a pseudotachylytes and shock veins in meteorites

hydrothermal origin for carbonate globules in martian meteorite alh84001 a terrestrial analogue from spitsbergen norway carbonate minerals in the ancient martian meteorite alh84001 are the only known solid phases that bear witness to the processing of volatile and biologically critical compounds co2 h2o on early mars similar carbonates have been found in xenoliths and their host basalts from quaternary volcanic centers in northern spitsbergen norway these carbonates were deposited by hot i e hydrothermal waters associated with the volcanic activity by analogy with the spitsbergen carbonates the alh84001 carbonates were probably also deposited by hot water hydrothermal activity was probably common and widespread on early mars which featured abundant basaltic rocks water as ice or liquid and heat from volcanos and asteroid impacts on earth descendants of the earliest life forms still prefer hydrothermal environments which are now shown to have been present on early mars

petrological evidence for shock melting of carbonates in the martian meteorite alh84001 the meteorite alh84001 a shocked igneous rock of probable martian origin contains chemically and isotopically heterogeneous carbonate globules associated with which are organic and inorganic structures that have been interpreted as possible fossil remains of ancient martian biota a critical assumption underlying this suggestion is that the carbonates formed from low temperature fluids penetrating the cracks and voids of the host rock here we report petrological studies of alh84001 which investigate the effects of shock on the various mineralogical components of the rock we find that carbonate plagioclase and silica were melted and partly redistributed by the same shock event responsible for the intense local crushing of pyroxene in the meteorite texture and compositional data show that during the period of shock decompression monomineralic melts were injected into pyroxene fractures that were subsequently cooled and resealed within seconds our results therefore suggest that the carbonates in alh84001 could not have formed at low temperatures but instead crystallized from shock melted material this conclusion weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activity

stress fields a key to oil migration a coherent oil migration model based on geomechanical considerations includes both the high molecular kerogen structure and the capillary properties of source rocks oil is squeezed from kerogen by compaction following oil generation this squeezing effect should be created by the differential stress maximum compressive stress minus least compressive stress acting on the kerogen which has been chemically broken up by oil formation in sedimentary bodies whose water is at hydrostatic pressure the migration of oil seems to involve two processes 1 lateral transfer by channeling into the more coarsely microporous layers of the source rocks from the oil generation site toward the geologic structure or lower pressured zone and 2 vertical transfer from source rock to eservoir by the opening or reopening of vertical fractures in the few areas such as structural tops where the least compressive stress is slightly greater than or equal to the pore pressure and where the capillary pressure increment 2 ggr r of oil in the microporosity exceeds the tensile strength of the rock in sedimentary bodies whose water is overpressured the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures and eventually in larger fractures the theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions negative effective compressive stress and thus explains the large asphaltic veins of southeastern turkey and the well known bitumen veins of the uinta basin

friction melt distribution in a multi ring impact basin it is generally accepted that multi ring basins are the consequence of very large impacts but the mechanism by which they form is still a matter of contention1 2 most of what is currently known about multi ring basins is based on remote studies of the moon3 5 and to a lesser extent mars and mercury4 6 but at least two multi ring impact basins have been recognized on earth7 the sudbury8 canada and vredefort9 south africa impact structures providing an opportunity to study their properties directly here we describe the distribution of friction melt pseudotachylyte in the floor of the sudbury impact basin although the veins and dykes of pseudotachylyte decrease in both thickness and frequency of occurrence towards the basin periphery the greatest volumes of friction melt appear to define four rings around the central impact melt sheet field evidence indicates that the rings originated as zones of large displacement which facilitated localized frictional melting of the basin floor during the modification collapse stage of the cratering process by analogy we argue that the rings of other multi ring impact basins are also likely to be the remnants of such large displacement fault zones

shock temperature in calcite caco3 at 95 160 gpa the temperatures induced in crystalline calcite caco 3 upon planar shock compression 95 160 gpa are reported from two stage light gas gun experiments temperatures of 3300 5400 k are obtained by fitting six channel optical pyrometer radiances in the 450 900 nm range to the planck gray body radiation law thermodynamic calculations demonstrate that these temperatures are some 400 1350 k lower than expected for vibronic excitations of the lattice with a 3r mole atom specific heat r is gas constant the temperature deficit along the hugoniot is larger than that expected from only melting in addition to melting it appears likely that shock induced decomposition of calcite occurs behind the shock front we modeled disproportionation of calcite into cao solid plus co 2 gas for temperature calculations specific heat at constant volume for 1 mole of co 2 is taken to be 6 7r as compared to 9r in the solid state whereas a mole of calcite and a mole of cao have their solid state values 15r and 6r respectively calculations suggest that the calcite decomposes to cao and co 2 at 110 10 gpa along the hugoniot recent reanalysis of earlier visar measurements of particle velocity profiles 1 indicates that calcite shocked to 18 gpa undergoes disproportionation at much lower pressures upon isentropic expansion

paleomagnetic evidence of a low temperature origin of carbonate in the martian meteorite alh84001 indirect evidence for life on mars has been reported from the study of meteorite alh84001 the formation temperature of the carbonates is controversial some estimates suggest 20 to 80 c whereas others exceed 650 c paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals study of two adjacent pyroxene grains from the crushed zone of alh84001 shows that each possesses a stable natural remanent magnetization nrm implying that mars had a substantial magnetic field when the grains cooled however nrm directions from these particles differ implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago the carbonate globules postdate this brecciation and thus formed at low temperatures

experimental hydrothermal alteration of a martian analog basalt implications for martian meteorites a number of martian meteorite samples contain secondary alteration minerals such as ca mg fe carbonates fe oxides and clay minerals these mineral assemblages hint at hydrothermal processes occurring in the martian crust but the alteration conditions are poorly constrained this study presents the results of experiments that examined the alteration of a high fe basalt by co2 saturated aqueous fluids at 23 and 75 c and by mixed h2o co2 vapors at 200 and 400 c and water rock ratios of 1 1 and 1 10 results indicate that observable alteration of the basalt takes place after runs of only seven days this alteration includes mobilization of silica into phases such as opal ct and quartz as well as the formation of carbonates oxides and at some conditions zeolites and hydrous silicates the degree of alteration increases with run temperature and in high temperature vapor experiments with increasing water content of the vapor the degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time nakhla and lafayette probably interacted with water for relatively brief periods of time if so silica may have been leached from the parent rocks by the altering fluids allan hills 84001 shows possible evidence for very limited interaction with an aqueous fluid but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water bearing fluid elephant moraine a79001 may not have been altered by aqueous fluids at all the results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long term reaction with large volumes of water our experiments studied alteration of a high fe basalt by dilute co2 saturated aqueous solutions at 23 and 75 c and by mixed h2o co2 vapors at 200 and 400 c the results suggest that alteration of the parent rock takes place even after very short reaction times of seven days all experiments produced carbonate minerals including calcite and in some cases magnesite siderite and ankerite a free silica phase either opal quartz or hydrated silica formed in most experiments more altered experiments also contained minerals such as zeolites and hydrous phyllosilicates clay minerals were not observed to form in any experiments in aqueous fluids higher temperature corresponded with a higher degree of alteration whereas changing fluid composition had no observable effect in high temperature vapors the degree of alteration was controlled by temperature and the proportion of h2o to co2 with water rock ratio also playing a role in transport of silica application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids nakhla and lafayette contain clay minerals which suggests that they interacted with water to some extent possibly at elevated temperatures although alh84001 shows possible evidence of very limited interaction with aqueous fluids eeta79001 does not these results support models for the alteration of these meteorites that do not invoke long term interaction with water or reaction with large volumes of water except for some models for alteration of alh84001 this conclusion agrees with most of the literature on alteration of martian meteorites

petrogenesis of allan hills 84001 constraints from impact melted feldspathic and silica glasses abstract compositional and textural relationships of shock melted glasses in the allan hills alh 84001 meteorite have been examined by optical microscopy electron microprobe analysis and compositional mapping the feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in alh 84001 the feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase an27 39ab58 68or3 7 or sanidine or51ab46an3 or from a mixture of these phases mixed feldspar glasses these observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite hydrothermal precipitates or alteration products or shock melts that have undergone alkali volatilization carbonate was deposited around previously formed mixed feldspar glass clasts suggesting that carbonate deposition occurred after the shock event that formed the granular bands crushed zones in this meteorite sio2 rich glasses appear to be silica remobilized during shock with little addition of other material a petrogenetic history of alh 84001 consistent with the observations of feldspathic and silica glasses is 1 igneous crystallization and cumulate formation 2 a pre carbonate shock event that formed the granular bands crushed zones and sheared chromites and melted igneous plagioclase and sanidine to form mixed feldspar glasses 3 carbonate and silica deposition in the granular bands veining of plagioclase glasses by sio2 and deposition of carbonate around mixed feldspar and plagioclase glass clasts 4 a post carbonate shock event that resulted in invasion of carbonate by feldspathic melts shock faulting and decarbonation of carbonate high temperature mobilization of silica melts and minor dissolution of orthopyroxene by silica melts

tectonic patterns on reoriented and despun planetary bodies abstract several processes may produce global tectonic patterns on the surface of a planetary body the stresses associated with distortions of biaxial figures due to despinning or reorientation were first calculated by vening meinesz vening meinesz f a 1947 trans am geophys union 28 1 1 23 we adopt a mathematically equivalent but physically more meaningful treatment for distortions associated with rotation the new approach allows us to find analytic solutions for the general case of stresses associated with distortions of biaxial or triaxial planetary figures distortions of biaxial figures may be driven by variations in rotation rate rotation axis orientation or the combination of both distortions of triaxial figures may be driven by the same mechanisms and or variations in tidal axis orientation for tidally deformed satellites while the magnitude of the resulting stresses depends on the adopted elastic and physical parameters the expected tectonic pattern is independent of these parameters for these mechanisms reorientation of the rotation tidal axis alone is expected to produce normal thrust faulting provinces enclosing the initial rotation tidal poles and thrust normal faulting provinces enclosing the final rotation tidal poles reorientation of both the rotation and tidal axis results in a wide variety of tectonic patterns for different reorientation geometries on europa the tidal axis reorientation which generally accompanies rotation axis reorientations may provide an alternative explanation for tectonic features that have been interpreted as evidence for non synchronous rotation the observed tectonic pattern on enceladus is more easily explained by a large reorientation 90 of the rotation axis than by rotation rate variations

magnetic properties of martian meteorites implications for an ancient martian magnetic field the magnetic properties of samples of seven martian meteorites eet 79001 zagami nakhla lafayette governador valadares chassigny and alh 84001 have been investigated all possess a weak very stable primary natural remanent magnetization nrm and some have less stable secondary components in some cases the latter are associated with magnetic contamination of the samples imparted since their recovery and with viscous magnetization acquired during exposure of the meteorites to the geomagnetic field since they fell the magnetic properties are carried by a small content 1 of titanomagnetite and in alh 84001 possibly by magnetite as well the most likely source of the primary nrm is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence of a magnetic field current evidence is that this was 1 3 ga ago for the nakhlites and chassigny and 180 ma for shergottites the time of the last relevant cooling of alh 84001 is not presently known preliminary estimates of the strength of the magnetizing field are in the range 0 5 5 ut which is at least an order of magnitude greater than the present field it is tentatively concluded that the magnetic field was generated by a dynamo process in a martian core with appropriate structure and properties

the temperature of formation of carbonate in martian meteorite alh84001 constraints from cation diffusion we have measured the rates of chemical diffusion of mg in calcite and ca in magnesite and used these new data to constrain the formation temperature and thermal history of carbonates in the martian meteorite alh84001 our data have been collected at lower temperatures than in previous studies and provide improved constraints on carbonate formation during relatively low temperature processes 400 c mea sured log d0 values for chemical diffusion of mg in calcite and ca in magnesite are 216 0 6 1 1 and 27 8 6 4 3 m 2 s and the activation energies ea are 76 6 16 and 214 6 60 kj mol respectively measured diffusion rates of mg in calcite at temperatures between 400 and 550 c are substantially faster than expected from extrapolation of existing higher temperature data suggesting that different mechanisms may govern diffusion of mg at temperatures above and below 550 c we have used these data to constrain thermal histories which will allow the 1 mm variations in ca mg composition in alh84001 carbonates to survive homogenization by diffusion our results are generally consistent with models for formation of carbonates in alh84001 at low temperatures for initial cooling rates of between 10 21 and 10 3 ma our results demonstrate that carbonates formed at temperatures no higher than 400 c and most probably less than 200 c this range of cooling rates is similar to those observed within the terrestrial crust and suggests that formation of the carbonates by igneous metamorphic or hydrothermal or other processes in the martian crust most plausibly occurred at temperatures below 200 to 400 c models that suggest alh84001 carbonates formed during a martian impact event are also constrained by our data the thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preser vation of the observed carbonate zoning at formation temperatures in excess of 600 c 10 7 c ma occur only within the uppermost melt rich portions of an impact structure material deeper within the impact structure where cooling is dominated by uplifted crustal material and where much of the formation of hydrothermal minerals is concentrated cools much slower typically at rates of 10 2 to 10 3 ma carbonates formed within this region would also only preserve 1 mm compositional zoning at formation temperatures of less than 200 to 400 c copyright 2001 elsevier science ltd

transmission electron microscopy of minerals in the martian meteorite allan hills 84001 we have studied carbonate and associated oxides and glasses in a demountable section of allan hills 84001 alh 84001 using optical scanning and transmission electron microscopy tem to elucidate their origins and the shock history of the rock massive fracture zone and fracture filling carbonates in typical locations were characterized by tem x ray microanalysis and electron diffraction in a comprehensive study that preserved textural and spatial relationships orthopyroxene is highly deformed fractured partially comminuted and essentially unrecovered lamellae of diaplectic glass and other features indicate shock pressures 30 gpa bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene carbonate crystals are 5 m in size untwinned and very largely exhibit the r3c calcite structure evidence of plastic deformation is generally found mildly only in fracture zone and fracture filling carbonates even adjacent to highly deformed orthopyroxene and appears to have been caused by low stress effects including differential shrinkage high dislocation densities like those observed in moderately shocked calcite are absent carbonate contains impact derived glasses of plagioclase silica and orthopyroxene composition indicating brief localized impact heating stringers and lenses of orthopyroxene glass in fracture filling carbonate imply flow of carbonates and crystallization during an impact periclase mgo occurs in magnesite as 30 50 nm crystals adjacent to voids and negative crystals and as 1 m patches of 3 nm crystals showing weak preferred orientation consistent with 111 mgo 0001 carb as observed in the thermal decomposition of caco3 to cao magnetite crystals that are epitaxially oriented at voids negative crystals and microfractures clearly formed in situ fully embedded faceted magnetites are topotactically oriented in general with 111 mag 0001 carb so that their oxygen layers are aligned in optically opaque rims magnetites are more irregularly shaped and except for the smallest crystals poorly aligned all magnetite and periclase crystals probably formed by exsolution from slightly non stoichiometric co2 poor carbonate following impact induced thermal decomposition any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity

negative δ18o values in allan hills 84001 carbonate possible evidence for water precipitation on mars the martian meteorite alh84001 contains 1 by weight of carbonate formed by secondary processes on the martian surface or in the shallow subsurface the major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere this study concentrates upon carbonate regions 200 m across which possess previously unobserved magnesium rich inner cores interpreted here as rosette fragments surrounded by a later stage cement containing rare ca rich carbonates up to ca81mg07fe04mn07 intimately associated with feldspar high spatial resolution ion probe analyses of ca rich carbonate surrounding rosette fragments have 18ov smow values as low as 10 these values are not compatible with deposition from a global martian atmosphere invoked to explain alh84001 rosettes the range of 18o values are also incompatible with a fluid that has equilibrated with the martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition at martian atmospheric temperatures the small co2 gas co2 ice fractionation makes meteoric co2 an unlikely source for 10 carbonates in contrast closed system rayleigh fractionation of h2o can generate 18oh2o 30 as observed at high latitudes on earth we suggest that atmospheric transport and precipitation of h2o in a similar fashion to that on earth provides a source of suitably 18o depleted water for generation of carbonate with 18ov smow 10

shock metamorphism of carbonaceous chondrites abstract we have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites we find that sixty nine carbonaceous chondrites can be assigned to four shock stages si to s4 largely on the basis of shock effects in olivine the least shocked chondrite groups are cm2 and co3 thirty six out of thirty eight members are classified as shock stage s1 differences between the mean shock levels of certain groups of chondrites e g co3 and cv3 are probably due to stochastic differences in the sampling of their parent bodies however there is an overall tendency for the mean shock level of carbonaceous chondrites and ordinary chondrites to increase with increasing petrologic type that may reflect real differences in the shock level of near surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6 the greater porosity of types 2 3 causes higher post shock temperatures and melting at lower pressures and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure we suggest that type 2 3 material shocked above 20 30 gpa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites in the cv3 group there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism analogous to that discovered in ordinary chondrites by sneyd et al 1988 suggesting that shock rather than static overburden pressure is responsible for chondrule flattening we infer that the correlations are due to collapse of pores under shock pressures of at least 5 10 gpa and that shock is an important process affecting many physical properties of type 2 4 chondrites

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

a simple inorganic process for formation of carbonates magnetite and sulfides in martian meteorite alh84001 we show experimental evidence that the zoned mg fe ca carbonates magnetite and fe sulfides in martian meteorite alh84001 may have formed by simple inorganic processes chemically zoned carbonate globules and fe sulfides were rapidly precipitated under low temperature 150 c hydrothermal and non equilibrium conditions from multiple fluxes of ca mg fe co2 s h2o solutions that have different compositions chemically pure single domain defect free magnetite crystals were formed by subsequent decomposition of previously precipitated fe rich carbonates by brief heating to 470 c the sequence of hydrothermal precipitation of carbonates from flowing co2 rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules magnetite and fe sulfides in alh84001 in separate experiments kinetically controlled 13c enrichment was observed in synthetic carbonates that is similar in magnitude to the 13c enrichment in alh84001 carbonates

isotopic and petrographic evidence for young martian basalts radiometric age data for shergottites yield ages of 4 0 ga and 180 575 ma the interpretation of these ages has been and remains a subject of debate here we present new 39 ar 40 ar laser probe data on lherzolitic shergottites allan hills alh 77005 and northwest africa nwa 1950 these two meteorites are genetically related but display very different degrees of shock damage on a plot of 40 ar 36 ar versus 39 ar 36 ar the more strongly shocked alh 77005 45 55 gpa does not yield an array of values indicating an isochron but the data are highly scattered with the shock melts yielding 40 ar 36 ar ratios of 1600 2026 apparent ages calculated from these extractions range from 374 8183 ma with 50 of the data particularly from the shock melts yielding impossibly old ages 4 567 ga on the same plot extractions from igneous minerals in the less shocked nwa 1950 30 44 gpa yield a fitted age of 382 36 ma argon extractions from the shock melts are well distinguished from minerals with the melts exhibiting the highest 40 ar 36 ar ratios 1260 1488 and the oldest apparent ages laser step heating was also performed on maskelynite separates from nwa 1950 yielding ages of 1000 ma at the lowest release temperatures and ages of 360 and 362 ma at higher temperature steps stepped heating data from previous studies have yielded ages of 500 and 700 ma to 1 7 ga for alh 77005 maskelynite separates if the ages obtained from igneous minerals represent undegassed argon from an ancient 4 0 ga rock then the ages are expected to anticorrelate with the degree of shock heating the data do not support this inference our data support young crystallization ages for minerals and martian atmosphere as the origin of excess 40 ar in the shock melts the shock features of shergottites are also reviewed in the context of what is known of the geologic history of the martian surface through remote observation the oldest most heavily cratered surfaces of mars are thought to be 4 0 ga we contend that ancient rocks from mars noachian 3 5 ga are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era young rocks late amazonian 0 6 ga should record a relatively simple history of emplacement and ejection from the near surface we show that although shergottites are strongly shocked they are relatively pristine crystalline igneous rocks and not pervasively altered breccias the petrography of shergottites is at odds with an ancient age interpretation a model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

the age of the carbonates in martian meteorite alh84001 the age of secondary carbonate mineralization in the martian meteorite alh84001 was determined to be 3 90 0 04 billion years by rubidium strontium rb sr dating and 4 04 0 10 billion years by lead lead pb pb dating the rb sr and pb pb isochrons are defined by leachates of a mixture of high graded carbonate visually estimated as approximately 5 percent whitlockite trace and orthopyroxene approximately 95 percent the carbonate formation age is contemporaneous with a period in martian history when the surface is thought to have had flowing water but also was undergoing heavy bombardment by meteorites therefore this age does not distinguish between aqueous and impact origins for the carbonates

shock and thermal history of martian meteorite allan hills 84001 from transmission electron microscopy microstructures in the allan hills 84001 meteorite were studied using optical and electron microscopy putting emphasis on shock effects which are widespread some orthopyroxene exhibits only 100 slip but more typical grains suffered extensive slip microfracturing and frequently contain 100 clino inversion lamellae in fracture zones shock deformation of orthopyroxene has produced all three effects in profusion together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae which were apparently created by shearing on low index planes usually 100 or 110 both types of plane are loci that pseudo planar fractures tend to follow thus the glass lamellae which have not been observed in other meteorites probably formed by frictional heating during the sliding of microscale corrugated surfaces one over another leading to local melting we infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation nanocrystals of eskolaite cr2o3 coating silica glass grains are probably also a result of shock induced thermal decomposition of chromite iron sulfides pyrite and pyrrhotite were identified tended to be associated with plagioclase glass a carbonate disk showing no evidence for shock deformation had a substructure of elongated slightly misoriented subcells in the exterior interior regions had more eqiaxed subcells both microstructures probably formed during growth but the conditions are undetermined chemical composition varied on a micron scale but the rim of the disk was more ferroan oxide precipitates and voids were widely distributed as in fracture filling carbonates if the fracture zones and opx glass are the result of strong shock as we deduce it is very unlikely that pores could have filled by carbonate long after the fracture zones formed we infer that the carbonate like the phosphate olivine pyrrhotite eskolaite and many euhedral submicrometer chromites crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase silica and orthopyroxene

saturate biomarkers and aromatic sulfur compounds in oils and condensates from different source rock lithologies of kazakhstan japan and russia abstract representative petroleums from the seven basins of kazakhstan russia and japan were analyzed using gas chromatography with dual fid fpd and gc ms the present study has shown that the deviation in relative concentrations of aromatic sulfur compounds in petroleums could be related to the type of source rock all oil samples were classified into three groups on the basis of the relative abundance of aromatic sulfur compounds biomarker distribution and lithology of source rock the first group of oils derived from carbonate source rocks is rich in aromatic sulfur compounds such as methylbenzothiophenes mbt and methyldibenzothiophenes mdbt and poor in dibenzothiophene dbt compared to methylated homologs the second group derived from siliceous and siliceous carbonate source rocks contains various types of benzothiophenes and dibenzothiophenes including dbt the third group originating from argillaceous rocks has lower concentrations of aromatic sulfur compounds and especially of methylated benzothiophenes