petrologic evidence for low temperature possibly flood evaporitic origin of carbonates in the alh84001 meteorite high temperature models for origin of the carbonates in martian meteorite alh84001 are implausible the impact metasomatism model invoking reaction between co2 rich fluid and the host orthopyroxenite requires conversion of olivine into orthopyroxene yet olivine in alh84001 shows no depletion in carbonate rich areas or else conversion of orthopyroxene into silica which should have yielded a higher silica carbonate ratio the impact melt model implies that the fracture linked carbonates as products of melt injection should appear as continuous planar veins but in many areas they do not both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates the popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in alh84001 prior brecciation should have facilitated alteration hydrothermal fluids would be warm and rate of hydration of mafic silicates obeys an arrhenius law at least up to 100 c most important hydrothermal episodes tend to last for many years many areas of the ancient martian crust show evidence for massive flooding i propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of alh84001 but only briefly as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode the setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater analogous to a playa lake carbonate precipitation would occur in response to evaporative concentration of the water to explain the scarcity of sulfates in alh84001 the water table must be assumed to recede quickly relative to the rate of evaporation during the period when alh84001 was above the water table evaporation would have slowed as the evaporation front passed beneath the surface of the debris layer and possibly earlier if the shrinking pool of surface water developed a porous sulfate crust alternatively alh84001 may have developed as a martian form of calcrete i e the evaporating flood s may have been entirely below ground as it they passed slowly through alh84001 the greatest advantage of the flood evaporite model is that it exposes alh84001 to carbonate precipitation without prolonged exposure to aqueous alteration the model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates however this hypothesis seems no more than marginally consistent with the suggestion of mckay et al 1996 that the carbonates are biogenic

principles of early development of karst conduits under natural and man made conditions revealed by mathematical analysis of numerical models numerical models of the enlargement of primary fissures in limestone by calcite aggressive water show a complex behavior if the lengths of the fractures are large and hydraulic heads are low as is the case in nature dissolution rates at the exit of the channel determine its development by causing a slow increase of water flow which after a long gestation time by positive feedback accelerates dramatically within a short time span mathematical analysis of simplified approximations yields an analytical expression for the breakthrough time when this happens in excellent agreement with the results of a numerical model this expression quantifies the geometrical hydraulic and chemical parameters determining such karst processes if the lengths of the enlarging channels are small but hydraulic heads are high as is the case for artificial hydraulic structures such as dams it is the widening at the entrance of the flow path which determines the enlargement of the conduit within the lifetime of the dam this can cause serious water losses this can also be explained by mathematical analysis of simplified approximations which yield an analytical threshold condition from which the safety of a dam can be judged thus in both cases the dynamic processes of karstification are revealed to gain a deeper understanding of the early development of karst systems as a further important result one finds that minimum conditions below which karstification cannot develop do not exist

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

records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

pre 4 0 billion year weathering on mars constrained by rb sr geochronology on meteorite alh84001 abstract the timing and nature of aqueous alteration of meteorite alh84001 has important implications for the history of water on early mars the evolution of the martian atmosphere and the potential for early mars habitability rubidium sr isotope analyses of mineral separates from igneous textured and carbonate rich aliquots of martian meteorite alh84001 constrain the age of alteration and the source of fluids the carbonate rich aliquot defines a precise rb sr isochron between maskelynite orthopyroxene and chromite of 3952 22 ma and this is interpreted to represent a shock resetting event that was broadly coeval with carbonate precipitation carbonate bulk rock and multi mineral separates all have high 87 sr 86 sr ratios that can only have been produced by alteration via a fluid derived through interaction with high rb sr phyllosilicates that were produced prior to 3950 ma these data confirm that the source of sr in the fluids was previously altered crustal rock consistent with fluids that underwent low temperature water rock interaction eiler et al 2002 halvey et al 2011 these results therefore provide evidence for wet clay rich conditions on the surface of mars prior to 4 2 ga

a model for meteoritic and lunar 40 ar 39 ar age spectra addressing the conundrum of multi activation energies abstract results of whole rock 40 ar 39 ar step heating analyses of extra terrestrial materials have been used to constrain the timing of impacts in the inner solar system solidification of the lunar magma ocean and development of planetary magnetic fields despite the importance of understanding these events the samples we have in hand are non ideal due to mixed provenance isotopic disturbances from potentially multiple heating episodes and laboratory artifacts such as nuclear recoil although models to quantitatively assess multi domain diffusive 40 ar loss have long been applied to terrestrial samples their use on extra terrestrial materials has been limited here we introduce a multi activation energy multi diffusion domain model and apply it to 40 ar 39 ar temperature cycling step heating data for meteoritic and lunar samples we show that age spectra of extra terrestrial materials the jilin chondrite k 4 and apollo 16 lunar breccia 67514 43 yielding seemingly non ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene are meaningful and can be understood in context of the presence of multi diffusion domains containing multiple activation energies internally consistent results from both the meteoritic and lunar samples reveal high temperature short duration thermal episodes we interpret as due to moderate shock heating

monomineralic mobilization of plagioclase by shock an experimental study abstract shock recovery experiments of plagioclase feldspar in contact with pyroxene shock loaded at 36 53 gpa showed that monomineralic glass of plagioclase was mixed with brecciated pyroxenes with virtual absence of mixed mineral melts this amorphous phase of plagioclase produced at 36 42 gpa displays enormously high ductility although the amorphous phase is less likely to be a quenched high pressure melt because both shock and post shock bulk temperature cannot exceed the melting point the results suggest that alkaline components in feldspar can be extensively mobilized even in a very short duration of shock which can never be achieved by normal diffusion in the same time scale since feldspar is a major carrier phase of alkaline volatiles shock mobilization mechanism may explain shock disturbance of rb sr and 39 ar 40 ar ages of basaltic shergottites

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

experimental shock decomposition of siderite and the origin of magnetite in martian meteorite alh 84001 shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron carbonate were initiated naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite samples were shocked in tungsten alloy holders w 90 ni 6 cu 4 to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder each sample was shocked to a specific pressure between 30 to 49 gpa transformation of siderite to magnetite as characterized by tem was found in the 49 gpa shock experiment compositions of most magnetites are 50 fe 2 in the octahedral site of the inverse spinel structure magnetites produced in shock experiments display the same range of sizes 50 100 nm compositions 100 magnetite to 80 magnetite 20 magnesioferrite and morphologies equant elongated euhedral to subhedral as magnetites synthesized by golden et al 2001 and as the magnetites in martian meteorite allan hills alh 84001 fritz et al 2005 previously concluded that alh 84001 experienced 32 gpa pressure and a resultant thermal pulse of 100 110 c however alh 84001 contains evidence of local temperature excursions high enough to melt feldspar pyroxene and a silica rich phase this 49 gpa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to 470 c therefore magnetite in the rims of carbonates in martian meteorite alh 84001 could be a product of shock devolatilization of siderite as well

comment on petrologic evidence for low temperature possibly flood evaporitic origin of carbonates in the alh84001 meteorite by paul h warren

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

ion microprobe u th pb dating of phosphates in martian meteorite alh 84001 phosphates in martian meteorites are important carriers of trace elements although they are volumetrically minor minerals po4 also has potential as a biomarker for life on mars here we report measurements of the u th pb systematics of phosphates in the martian meteorite alh 84001 using the sensitive high resolution ion microprobe shrimp installed at hiroshima university japan eleven analyses of whitlockites and 1 analysis of apatite resulted in a total pb u isochron age of 4018 81 ma in the 238u 206pb 207pb 206pb 204pb 206pb 3 d space and a 232th 208pb age of 3971 860 ma these ages are consistent within a 95 confidence limit this result is in agreement with the previously published ar ar shock age of 4 0 0 1 ga from maskelynite and other results of 3 8 4 3 ga but are significantly different from the sm nd age of 4 50 0 13 ga based on the whole rock and pyroxene taking into account recent studies on textural and chemical evidence of phosphate our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 81 ma and that since then alh 84001 has not experienced a long duration thermal metamorphism which would reset the u pb system in phosphates

the role of dissolution kinetics in the development of karst aquifers in limestone a model simulation of karst evolution to model the development of karst aquifers from primary fissures in limestone rock a numerical model of solutional widening of such fractures by calcite agressive water is suggested the geological setting determines relevant geometrical parameters i e length of the fracture its initial width and the hydraulic gradient driving water from the input to the output to simulate the solutional widening as it proceeds in time the solution rates must be known as a function of concentration c of dissolved calcite they are given as a first order kinetic rate law for c leq k cdot c eq as f 1 alpha 0 c c eq for c k cdot c eq close to equilibrium a fourth order rate law becomes dominant f 4 beta cdot c c eq 4 the parameters alpha 0 beta and k depend on the chemistry of the carbonate system and rock chemistry the saturation concentration c eq with respect to calcite depends on the initial co 2 concentration of the inflowing solution

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

evidence for exclusively inorganic formation of magnetite in martian meteorite alh84001 magnetite crystals produced by terrestrial magnetotactic bacterium mv 1 are elongated on a 111 crystallographic axis in a so called truncated hexa octahedral shape this morphology has been proposed to constitute a biomarker i e formed only in biogenic processes a subpopulation of magnetite crystals associated with carbonate globules in martian meteorite alh84001 is reported to have this morphology and the observation has been taken as evidence for biological activity on mars in this study we present evidence for the exclusively inorganic origin of 111 elongated magnetite crystals in alh84001 we report three dimensional 3 d morphologies for 1000 magnetite crystals extracted from 1 thermal decomposition products of fe rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments 2 carbonate globules in martian meteorite alh84001 and 3 cells of magnetotactic bacterial strain mv 1 the 3 d morphologies were derived by fitting 3 d shape models to two dimensional bright field transmission electron microscope tem images obtained at a series of viewing angles the view down the 110 axes closest to the 111 elongation axis of magnetite crystals 111 110 0 provides a 2 d projection that uniquely discriminates among the three 111 elongated magnetite morphologies found in these samples 111 elongated truncated hexa octahedron 111 tho 111 elongated cubo octahedron 111 eco and 111 elongated simple octahedron 111 eso all 111 elongated morphologies are present in the three types of sample but in different proportions in the alh84001 martian meteorite and in our inorganic laboratory products the most common 111 elongated magnetite crystal morphology is 111 eco in contrast the most common morphology for magnetotactic bacterial strain mv 1 is 111 tho these results show that 1 the morphology of 111 elongated magnetite crystals associated with the carbonate globules in martian meteorite alh84001 is replicated by an inorganic process and 2 the most common crystal morphology for biogenic mv 1 magnetite is distinctly different from that in both alh84001 and our inorganic laboratory products therefore 111 elongated magnetite crystals in alh84001 do not constitute as previously claimed a robust biosignature and in fact an exclusively inorganic origin for the magnetite is fully consistent with our results furthermore the inorganic synthesis method i e the thermal decomposition of hydrothermally precipitated fe rich carbonate is a process analogue for formation of the magnetite on mars namely precipitation of carbonate globules from carbonate rich hydrothermal solutions followed at some later time by a thermal pulse perhaps in association with meteoritic impact or volcanic processes on the martian surface

a shock produced mg fe sio3 glass in the suizhou meteorite ovoid grains consisting of glass of stoichiometric mg fe sio3 composition that is intimately associated with majorite were identified in the shock veins of the suizhou meteorite the glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low ca pyroxene and majorite these ovoid grains are surrounded by a fine grained matrix composed of majorite pyrope garnet ringwoodite magnesiowstite metal and troilite this study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature while at the rim region pyroxene grains transformed to majorite due to a higher temperature after pressure release perovskite vitrified at post shock temperature the existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 gpa the post shock temperature in the meteorite could have been above 477 c this study indicates that the occurrence of high pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites

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

early development of karst systems 1 preferential flow path enlargement under laminar flow modeling of flow and solutional processes within networks of interconnected conduits in limestone aquifers indicates that enlargement occurs very selectively during the early stages of karst aquifer development under laminar flow if initial flow paths are uniform in size almost all enlargement occurs along a single set of connected conduits that lie along a direct path between recharge and discharge locations and are aligned along the hydraulic gradient with a sufficiently large variation in initial aperture widths enlargement occurs along the flow path offering the least resistance to flow but since flow rates in laminar flow are proportional to the fourth power of diameter but only linearly proportional to hydraulic gradient the preferentially enlarged set of fractures may follow an indirect path results disfavor earlier suggestions that nonselective cave patterns result from artesian flows at least under laminar flow conditions and that all passages should be competitive until the onset of turbulent flow

olivine in martian meteorite allan hills 84001 evidence for a high temperature origin and implications for signs of life olivine from martian meteorite allan hills alh 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass the inclusions are irregular in shape and range in size from 40 pm to submicrometer some of the inclusions are elongate and boudinage like the olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite the olivine exhibits a very limited range of composition from fo65 to fob6 n 25 the 6180 values of the olivine and orthopyroxene analyzed by ion microprobe range from 4 3 to 4 5 3960 and are indistinguishable from each other within analytical uncertainty the mineral chemistries 0 isotopic data and textural relationships indicate that the olivine inclusions were produced at a temperature so0 c it is unlikely that the olivines formed during the same event that gave rise to the carbonates in alh 84001 which have more elevated and variable 6180 values and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine the reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel if the olivine was formed by either reaction during a postcarbonate heating event the implications are profound with regards to the interpretations of mckay el ul 1996 due to the low diflbsion rates in carbonates this rapid high temperature event would have resulted in the preservation of the fine scale carbonate zoning while partially devolatilizing select carbonate compositions on a submicrometer scale brearley 1998a this may have resulted in the formation of the minute magnetite grains that mckay et ul 1996 attributed to biogenic activity

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

planar deformation features in shocked quartz a transmission electron microscopy investigation abstract we present a detailed investigation by transmission electron microscopy tem of the fine structure of planar deformation features pdf in quartz grains within which shock metamorphism has been detected optically and which originate from various sites slate islands la malbaie and manicouagan lake canada ries crater germany the toba caldera sumatra the vredefort complex in south africa and the k t boundary at raton basin colorado pdfs appear on tem micrographs as straight and very narrow tem reveals many more pdfs than are optically detected probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of pdfs pdfs in samples from slate islands and raton basin exhibit a peculiar morphology a high density of very tiny bubbles precipitated on the dislocation lines comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature

spiny ooids early subaerial deformation as opposed to late burial compaction a new type of ooid has been found in southeastern tunisia in holocene sands cemented in a vadose environment the sands are associated with serpulid and stromatolite bioherms and correspond to undisturbed relict lagoonal beaches they contain ooids that have cortical spines located near the points of contact between grains these coated grains resemble ooids that have been deformed by burial compaction although the geologic context and 14 c dates clearly indicate that they have never been subjected to lithostatic pressure the location of the spines and the evidence of vadose diagenesis point to postdepositional deformation and suggest that residual capillary waters control this phenomenon the spines may be related to the compressive effect of aragonite crystal growth which remained active in the external cortices after ooid deposition deformation due to burial compaction and deformation related to early diagenetic processes can be differentiated on the basis of the specific location of the compressive deformations such a distinction is important when interpreting the diagenetic history and porosity evolution of oolitic sands

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

lithification of gas rich chondrite regolith breccias by grain boundary and localized shock melting we studied the fine grained matrices 150 m of 14 gas rich ordinary chondrite regolith breccias in an attempt to decipher the nature of the lithification process that converted loose regolith material into consolidated breccias we find that there is a continuous gradation in matrix textures from nearly completely clastic class a to highly cemented class c breccias in which the remaining clasts are completely surrounded by interstitial shock melted material we conclude that this interstitial material formed by shock melting in the porous regolith in general the abundances of solar wind implanted 4he and 20ne are inversely correlated with the abundance of interstitial shock melted feldspathic material chondrites with the highest abundance of interstitial melted material class c experienced the highest shock pressures and temperatures and suffered the most extensive degassing it is this interstitial feldspathic melt that lithifies and cements the breccias together those breccias with very little interstitial melt class a are the most porous and least consolidated

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

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

evidence for magma carbonate interaction beneath syrtis major mars 1 we used three methods spectral index mapping linear spectral mixture analysis and factor analysis and target transformation to determine if carbonate decomposition products including lime cao periclase mgo portlandite ca oh 2 and brucite mg oh 2 are present on the surface of mars using an expanded spectral library that includes decomposition products of calcite and magnesite we deconvolved a binned emissivity data set from the mars global surveyor thermal emission spectrometer mgs tes the deconvolution model indicates that anhydrous carbonates are not present at kilometer scales above the tes detection limit however carbonate decomposition products may be present at or above the tes detection limit in northeast syrtis major and several other small regions on mars the model results are supported by spectral index mapping and factor analysis and target transformation of tes data in this region the volcanic setting of the detections may indicate the interaction of syrtis major lavas with subsurface carbonates or alternatively the eruption of carbonate bearing lavas that were subsequently devolatilized by impacts or later lava flows

vibrational spectroscopic study of minerals in the martian meteorite alh 84001 micro raman spectra of carbonates silica and amorphous plagioclase and both micro raman and ir reflectance spectra of phosphates in alh84001 are reported data from these vibrational techniques combined with electron microprobe analyses show that 1 the carbonates exhibit complex compositional heterogeneity on a sub micrometer scale 2 the phosphates chlorapatite and merrillite are largely anhydrous 3 amorphous silica and plagioclase experienced peak shock pressures 32 gpa and 50 gpa respectively and 4 vitreous plagioclase was quenched from a shockinduced melt after relaxation of the peak shock pressure the observed general raman band broadening of lattice and internal modes of carbonates in alh84001 indicates complex sub microscopic compositional heterogeneity and possibly structural disorder any search for biogenic markers in alh84001 must recognize the complex shock and thermal history of these minerals

chromite plagioclase assemblages as a new shock indicator implications for the shock and thermal histories of ordinary chondrites abstract chromite in ordinary chondrites oc can be used as a shock indicator a survey of 76 equilibrated h l and ll chondrites shows that unshocked chromite grains occur in equant subhedral and rounded morphologies surrounded by silicate or intergrown with metallic fe ni and or troilite some unmelted chromite grains are fractured or crushed during whole rock brecciation others are transected by opaque veins the veins form when impacts cause localized heating of metal troilite intergrowths above the fe fes eutectic 988 c mobilization of metal troilite melts and penetration of the melt into fractures in chromite grains chromite plagioclase assemblages occur in nearly every shock stage s3 s6 oc the assemblages range in size from 20 300 m and consist of 0 2 20 m size euhedral subhedral anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition plagioclase has a low impedance to shock compression heat from shock melted plagioclase caused adjacent chromite grains to melt chromite grains crystallized from this melt those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in al 2 o 3 than unmelted matrix chromite grains in the same meteorite chromite veinlets typically 0 5 2 m thick and 10 300 m long occur typically in the vicinity of chromite plagioclase assemblages the veinlets formed from chromite plagioclase melts that were injected into fractures in neighboring silicate grains chromite crystallized in the fractures and the residual plagioclase rich melt continued to flow eventually pooling to form plagioclase rich melt pockets chromite rich chondrules consisting mainly of olivine plagioclase normative mesostasis and 5 15 vol chromite occur in many shocked oc and oc regolith breccias but they are absent from primitive type 3 oc they may have formed by impact melting chromite plagioclase and adjacent mafic silicates during higher energy shock events the melt was jetted from the impact site and formed droplets due to surface tension crystallization of these droplets may have commenced in flight prior to landing on the parent body surface chromite plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock stage s1 oc of petrologic type 5 and 6 that i examined although these rocks contain unstrained olivine with sharp optical extinction most possess other shock indicators such as extensive silicate darkening numerous occurrences of metallic cu polycrystalline troilite and opaque veins it seems likely that these rocks were shocked to levels at least as high as shock stage s3 and then annealed by heat generated during the shock event during annealing the olivine crystal lattices healed but other shock indicators survived published ar ar age data for some s1 oc indicate that many shock and annealing events occurred very early in the history of the parent asteroids the common occurrence of shocked and annealed oc is consistent with collisions being a major mechanism responsible for metamorphosing oc

experimental approach to generate shock veins in single crystal olivine by shear melting a shock experiment has been devised to produce large shear in a single crystal sample of olivine the recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine the shear faults appear as thin veins containing small grains of olivine and pockets of glass the microstructure and composition of the material in the veins point to fractional crystallization of a melt an order of magnitude calculation is consistent with the idea that the veins were produced by shear melting these results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities

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

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

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

ion microprobe measurements of 18o 16o ratios of phosphate minerals in the martian meteorites alh84001 and los angeles abstract oxygen isotope ratios of merrillite and chlorapatite in the martian meteorites alh84001 and los angeles have been measured by ion microprobe in multicollector mode 18o values of phosphate minerals measured in situ range from 3 to 6 and are similar to martian meteorite whole rock values as well as the 18o of igneous phosphate on earth these results suggest that the primary abiotic igneous phosphate reservoir on mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on earth this is an important first step in the characterization of martian phosphate reservoirs for the use of 18o of phosphate minerals as a biomarker for life on mars cumulative textural major element and isotopic evidence presented here suggest a primary igneous origin for the phosphates in los angeles and alh84001 textural and chemical evidence suggests that phosphates in alh84001 were subsequently shock melted in a later event

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

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

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

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

multi generational carbonate assemblages in martian meteorite allan hills 84001 implications for nucleation growth and alteration the carbonate mineralogy of several complex carbonate rich regions in allan hills alh 84001 has been examined these regions contain familiar forms of carbonate as well as textural forms previously unreported including carbonate rosettes planiform slab carbonates distinct post slab magnesites and carbonates interstitial to feldspathic glass and orthopyroxene slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite rich the kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite siderite layers on the exterior surfaces of the carbonate post slab magnesite intimately associated with silica glass is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth feldspathic glasses had little or no thermal effect on carbonates as indicated by the lack of thermal decomposition or any compositional changes associated with glass carbonate contacts

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

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

magnetite in alh 84001 an origin by shock induced thermal decomposition of iron carbonate in martian orthopyroxenite alh 84001 pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact s transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad nanometer sized magnetite particles with cuboid irregular and teardrop morphologies frequently associated with voids the fragments of carbonate must have been incorporated into the melt at temperatures of 900 c well above the upper thermal stability of siderite feco3 which decomposes to produce magnetite and co2 below 450 c these observations suggest that most if not all of the fine grained magnetite associated with fe bearing carbonate in alh 84001 could have been formed as result of the thermal decomposition of the siderite feco3 component of the carbonate and is not due to biological activity

origin of carbonate magnetite sulfide assemblages in martian meteorite alh84001 a review of the mineralogical isotopic and chemical properties of the carbonates and associated submicrometer iron oxides and sulfides in martian meteorite alh84001 provides minimal evidence for microbial activity some magnetites resemble those formed by magnetotactic microorganisms but cubic crystals 50 nm in size and elongated grains 25 nm long are too small to be single domain magnets and are probably abiogenic magnetites with shapes that are clearly unique to magnetotactic bacteria appear to be absent in alh84001 magnetosomes have not been reported in plutonic rocks and are unlikely to have been transported in fluids through fractures and uniformly deposited where abiogenic magnetite was forming epitaxially on carbonate submicrometer sulfides and magnetites probably formed during shock heating carbonates have correlated variations in ca mg and 18o 16o magnetite rich rims and they appear to be embedded in pyroxene and plagioclase glass carbonates with these features have not been identified in carbonaceous chondrites and terrestrial rocks suggesting that the alh84001 carbonates have a unique origin carbonates and hydrated minerals in alh84001 like secondary phases in other martian meteorites have o and h isotopic ratios favoring formation from fluids that exchanged with the martian atmosphere i propose that carbonates originally formed in alh84001 from aqueous fluids and were subsequently shock heated and vaporized the original carbonates were probably dolomite magnesite siderite assemblages that formed in pores at interstitial sites with minor sulfate chloride and phyllosilicates these phases like many other volatile rich phases in martian meteorites may have formed as evaporite deposits from intermittent floods