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

formation and relative ages of maskelynite and carbonate in alh84001 the morphology and stoichiometry of feldspathic glass in the martian meteorite alh84001 indicates it is maskelynite a diaplectic glass rather than a flowed glass although this glass was heterogeneously affected by a tertiary set of processes an impact event with shock pressures in excess of 31 gpa was needed to convert the original plagioclase an36ab60or4 to maskelynite carbonate is intimately associated with the maskelynite and the carbonate s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite textures also suggest carbonate was produced at the expense of maskelynite in a dissolution precipitation reaction that involved a carbonic fluid this fluid system is tentatively estimated to have been active for at least a few years at temperatures 300 c based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems which is the only analogue currently available this reaction does not need to be mitigated by microbial life neither are bacteria needed to produce the radiating textures and globular forms of carbonate which may instead reflect kinetic phenomena associated with crystal nucleation and growth because the carbonate was produced at the expense of maskelynite it is younger than maskelynite which has previously been shown to have last degassed 3 92 0 04 ga turner et al 1997 however the specific age of the carbonate and the carbonic fluid system remains unknown

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

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

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

isotopic composition of carbonate in eeta 79001 and its relation to parent body volatiles calcite a probable extraterrestrial weathering product in the shergottite eeta 79001 has 13c 9 7 and 18o 21 0 in order to account for the large excess of 18o relative to the silicate substrate 18o 4 2 a parent body surface reservoir containing both water and carbon dioxide is required the inferred h2oco2 ratio is consistent with estimates for the degassed reservoir on mars

50 myr recovery from the largest negative δ13c excursion in the ediacaran ocean sedimentary rocks deposited during the ediacaran period 630 542 ma contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from 5 to 12 followed by a sub linear recovery to positive 13c values isotopic ages u pb and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at 600 ma and duration of recovery of approximately 50 myr the excursion is widely recognized in oman and has potential correlatives in ediacaran strata elsewhere and may thus represent a characteristic feature of the ediacaran period the amplitude of this carbon isotope excursion far exceeds those of other neoproterozoic anomalies the isotopic trend of negative excursion and long term recovery spanned at least one short lived glacial episode at 580 ma but appears unrelated to glaciation which indicates that negative anomalies in the neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages

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

hosts of hydrogen in allan hills 84001 evidence for hydrous martian salts in the oldest martian meteorite the martian meteorite allan hills alh 84001 contains d rich hydrogen of plausible martian origin leshin et al 1996 the phase identity of the host s of this hydrogen are not well known and could include organic matter mckay et al 1996 phlogopite brearley 2000 glass mittlefehldt 1994 and or other unidentified components of this rock previous ion microprobe studies indicate that much of the hydrogen in alh 84001 as texturally associated with concretions of nominally anhydrous carbonates glass and oxides boctor et al 1998 sugiura and hoshino 2000 we examined the physical and chemical properties of the host s of this hydrogen by stepped pyrolysis of variously pre treated subsamples a continuous flow method of water reduction and mass spectrometry eiler and kitchen 2001 was used to permit detailed study of the small amounts of this hydrogen poor sample available for study we find that the host s of d rich hydrogen released from alh 84001 at relatively low temperatures 500 c is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near complete isotopic exchange with water within hours at temperatures of 200 to 300 c these characteristics are most consistent with the carrier phase s being a hydrous salt e g carbonate sulfate or halide the thermal stability of this material is inconsistent with many examples of such minerals e g gypsum and instead suggests one or more relatively refractory hydrous carbonates e g hydromagnesite hydrous salts particularly hydrous carbonates are common on the earth only in evaporite sabkha and hydrocryogenic weathering environments we suggest that much if not all of the martian hydrogen in alh 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust

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

kinematics of the torcal shear zone transpressional tectonics in a salient recess transition at the northern gibraltar arc abstract complex strain patterns in the gibraltar arc derive from the interaction between the westward drift and concomitant back arc extension of the arc hinterland alboran domain and the europe africa convergence in order to explore strain partitioning modes within the arc and the role played by large scale oblique structures we have studied the kinematics of the torcal shear zone located at the northern branch of the gibraltar arc the torcal shear zone is a 70 km long e w brittle ductile shear zone that underwent overall dextral transpression during the late miocene to quaternary time within the torcal shear zone strain is highly partitioned at multiple scales into shortening oblique extensional and strike slip structures moreover strain partitioning is heterogeneous along strike giving rise to four distinct structural domains in the central sector the strain is pure shear dominated although narrow sectors parallel to the shear walls are simple shear dominated a single n99 e n109 e trending horizontal velocity vector v could explain the kinematics of the entire central sector of the torcal shear zone lateral domains have different strain patterns and are comparable to splay dominated and thrust dominated strike slip fault tips the torcal shear zone provokes the subvertical extrusion of the external betics units against the alboran domain and a dextral deflection of the structural trend moreover the estimated v points to the importance of the westward motion of the hinterland relative to the external wedge and fits well with the radial outward thrusting pattern identified in the arc

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

correlated chemical and isotopic zoning in carbonates in the martian meteorite alh84001 the meteorite alh84001 a sample of the ancient martian crust contains small quantities 1 of strongly chemically zoned carbonate high spatial resolution 10 m ion microprobe analyses show that the chemical zoning is strongly correlated with variations in oxygen isotope ratios early formed ca fe rich cores have 18o 7 increasing to 22 smow in the more mg rich outer cores and magnesite rims isolated areas of ankerite appear to be isotopically lighter with 18o 1 the large range in 18o requires a significant range in either fluid isotopic composition or temperature or both in the course of the deposition sequence our data are inconsistent with formation of the zoned carbonates by closed system rayleigh fractionation there is no unique interpretation of the oxygen data but the recent observation of existence of 17o excesses in the carbonate appears to rule out models which involve high temperature isotopic exchange with silicate comparison with terrestrial analogues suggests that alh84001 carbonates formed in a hydrothermal system with t 400 c and which at least in the early stages of formation may have involved water with 18o 0 smow the later stages of deposition probably occurred at temperatures below 150 c a conclusion which does not preclude the co existence of thermophilic bacteria temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria

carbonates in martian meteorite alh84001 a planetary perspective on formation temperature we have re examined the formation temperature of carbonates in alh84001 accounting for the prior evolution of the stable isotopes used in the analysis oxygen and carbon in the atmosphere likely have become isotopically fractionated due to various nonthermal loss mechanisms operating throughout geologic time taking this previous evolution into account we derive a formation temperature from oxygen isotope thermometry of 40 250 c this range of temperatures represents a lower limit an increase in groundwater salinity could further raise the 18o of the groundwater as seen in terrestrial hydrothermal systems allowing for an even higher temperature of formation derived from the isotopic data

two populations of carbonate in alh84001 geochemical evidence for discrimination and genesis we present major and trace element oxygen isotope textural and structural data for carbonates and related phases in the snc meteorite alh84001 these data document the existence of at least two distinct carbonate populations one composed of finely zoned chemically and isotopically heterogeneous concretions of magnesio siderite with distinct white magnesite rims and a second composed of relatively homogeneous isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine grained pyroxene we suggest on the basis of textural evidence and geochemical systematics that the first population consists of low temperature aqueous precipitates and the second is produced by shock melting of the first values of 18 o and sr ca ratios are correlated with one another in magnesio siderite concretions the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of 190 c for concretion cores to 20 c for magnesite rich concretion rims given the assumptions inherent in this temperature estimate the aqueous fluid parental to carbonate concretions is constrained to have a 18 o of 5 vsmow significantly mass fractionated compared with expected juvenile martian volatiles and minor element abundances broadly similar to terrestrial seawater

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

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

isotope geochemistry of caliche developed on basalt enormous variations in oxygen and carbon isotopes occur in caliche developed on 3 ma basalts in 3 volcanic fields in arizona significantly extending the range of 18o and 13c observed in terrestrial caliche within each volcanic field 18o is broadly co variant with 13c and increases as 13c increases the most 18o and 13c enriched samples are for subaerial calcite developed on pinnacles knobs and flow lobes that protrude above tephra and soil the most 18o and 13c depleted samples are for pedogenic carbonate developed in soil atmospheres the pedogenic caliche has 18o fixed by normal precipitation in local meteoric waters at ambient temperatures and has low 13c characteristic of microbial soil co2 subaerial caliche has formed from 18o rich evapoconcentrated meteoric waters that dried out on surfaces after local rains the associated 13c enrichment is due either to removal of 12c by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over layered during burial by calcite carrying the isotopic signature of the soil environment the large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if mars had a warm wet early period and photosynthesizing microbes were present in the early soils the approach can be similarly applied to terrestrial precambrian paleocaliche in the search for the earliest record of life on land large variations reported for 18o of carbonate in martian meteorite alh84001 do not necessarily require high temperatures playa lakes or flood runoff if the carbonate is an example of altered martian caliche

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