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

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

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

timing and tectonic setting of grenvillian metamorphism constraints from a transect along georgian bay ontario systematic mapping of a transect along the well exposed shores of georgian bay ontario combined with the preliminary results of structural analysis geochronology and metamorphic petrology places some constraints on the geological setting of high grade metamorphism in this part of the central gneiss belt correlations within and between map units gneiss associations have allowed us to recognize five tectonic units that differ in various aspects of their lithology metamorphic and plutonic history and structural style the lowest unit which forms the footwall to a regional decollement locally preserves relic pre grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the grenvillian orogeny tectonic units above the decollement apparently lack the early granulite facies metamorphism out of sequence thrusting in the south produced a duplex like structure two distinct stages of grenvillian metamorphism are apparent the earlier stage c 1160 1120 ma produced granulite facies assemblages in the parry sound domain and upper amphibolite facies assemblages in the parry island thrust sheet the later stage c 1040 1020 ma involved widespread dominantly upper amphibolite facies metamorphism within and beneath the duplex deformation and metamorphism recently reported from south and east of the parry sound domain at c 1100 1040 ma have not yet been documented along the georgian bay transect the data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south east of the transect area with further advance to the north west during and after the waning stages of this deformation

pre grenvillian and grenvillian lithotectonic regions in eastern labrador correlations with the sveconorwegian orogenic belt in sweden the trans labrador batholith groswater bay terrane and lake melville terrane are three major crustal segments located adjacent to or within the grenville province in eastern labrador each crustal segment is a distinct lithotectonic entity displaying contrasts with each other in proportions of rock types structural style and metamorphic imprint together they indicate a unilateral polarity to the region partly reflecting grenvillian tectonism which sliced the region into thrust bound blocks in all three crustal segments an archean or aphebian gneissic basement is inferred onto or adjacent to which ca 1900 1700 ma supracrustal rocks were deposited deformation metamorphism and granitoid pluton emplacement were partly coeval with and partly postdated the supracrustal assemblages in the north tectonothermal effects can be assigned to hudsonian ketilidian orogenesis but their peak was 50 100 ma later farther south post tectonic granitoid plutons and layered mafic intrusions were emplaced at about

biomimetic properties of minerals and the search for life in the martian meteorite alh84001 the existence of extraterrestrial life was heralded by controversial claims made in 1996 that the martian meteorite alh84001 harbored relics of ancient microorganisms we review here the accumulated evidence for and against past extraterrestrial life in this martian meteorite the main pro life arguments the presence of polycyclic aromatic hydrocarbons magnetite crystals carbonate globules and structures resembling terrestrial life forms known as nanobacteria can be deemed ambiguous at best although these criteria are compatible with living processes each one of them can be explained by nonliving chemical processes by undergoing amorphous to crystalline transformations and binding to multiple substrates including other ions and simple organic compounds minerals especially those containing carbonate have been shown to display biomimetic properties producing forms that resemble bacteria this simple and down to earth explanation can account fully for the existence of mineral entities resembling put

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

calculated aqueous solution solid solution relations in the low temperature system cao mgo feo co2 h2o abstract a method of estimating an aqueous solution composition from that of solid solutions believed to be in equilibrium with it is derived the low temperature ternary ca fe mg phase relations of the rhombohedral carbonates are investigated using this method and their phase diagrams at 25 and 150 c are calculated algebraic manipulations of equilibrium constant equations representing dissolution of the carbonates yield equations for the phase boundaries separating calcite from an ankerite dolomite solid solution and an ankerite dolomite solid solution from a siderite magnesite solid solution a value for the free energy of formation of ankerite is estimated 1818 0 0 8 kj mol 25 c from compositions of natural coexisting carbonates necessary compositional information for the carbonates was derived from the relatively unmetamorphosed early proterozoic marra mamba banded iron formation of the hamersley basin of western australia the method yielded information on the a fe 2 a ca 2 and a mg 2 a ca 2 ratios of the solutions that deposited the carbonates of the marra mamba the method suggests a depositing solution for the carbonates of the lower marra mamba iron formation significantly richer in iron than was likely to have been the case for early proterozoic seawater

pre grenvillian evolution and grenvillian overprinting of the parautochthonous belt in key harbour ontario u pb and field constraints the parautochthonous belt in the region of key harbour ontario is composed of early proterozoic migmatitic para and orthogneiss and mid proterozoic granitoids which were reworked during the grenville orogeny grenvillian deformation is localized into anastomosing arrays of high strain shear zones enclosing elongate bands and lozenges of rock subjected to lower and near coaxial strain crosscutting relationships preserved in the low strain domains document two pre grenvillian plutonic and tectonometamorphic events which are bracketed in age by u pb zircon geochronology a 1694 ma leucogranite intrudes and provides a minimum age for high metamorphic grade gneisses formed during an earlier tectonometamorphic event d1 m1 the leucogranite was intruded by mafic dykes deformed and metamorphosed at uppermost amphibolite facies during d2 m2 before the emplacement of mid proterozoic granitoids at ca 1450 ma following the emplacement of gabbro dykes and pods at ca 1238 ma the area was overprinted by

alteration assemblages in the nakhlites variation with depth on mars secondary mineral assemblages in the nakhlite meteorites lafayette governador valadares gv nakhla yamato y 000593 y 000749 have been studied using scanning electron microscopy transmission electron microscopy and electron probe micro analysis the different nakhlites have distinctive secondary assemblages in their olivine grains and mesostases showing compositional fractionation correlated with their relative depths below the martian surface fracture filled veins in lafayette at the bottom of the pile consist of a siderite phyllosilicate fe oxide hydrated silicate gel assemblage corresponding veins in nakhla and gv further up the pile are predominantly a siderite gel assemblage with additional evaporites including gypsum y 000593 y 000749 veins are dominated by gel the gel s mg mg fe ratio decreases from lafayette 0 37 to gv 0 32 nakhla 0 24 and y 000593 0 15 we suggest that hydrothermal fluid flowed up this depth profile initiated by melting of buried h2o co2 ice our results show a complex mix of fe rich phyllosilicate within the veins and mesostasis of lafayette with d spacings of 0 7 1 1 nm suggesting a mixture of smectite and serpentine the phyllosilicate formed at close to neutral ph 150 c we also suggest that water rock ratios w r of 1 10 occurred in lafayette with smaller values for the other nakhlites this is reflected in the volume of alteration minerals 10 of olivine in lafayette to 3 in nakhla textural evidence of rapid cooling together with the w r and likely fluid velocities suggest that the secondary assemblages formed quickly e g within months a model is proposed in which the secondary assemblages formed in an impact induced hydrothermal system terminated by precipitation of the gel and evaporation of soluble salts

alteration minerals fluids and gases on early mars predictions from 1 d flow geochemical modeling of mineral assemblages in meteorite alh 84001 clay minerals although ubiquitous on the ancient terrains of mars have not been observed in martian meteorite allan hills alh 84001 which is an orthopyroxenite sample of the early martian crust with a secondary carbonate assemblage we used a low temperature 20 c one dimensional 1 d transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of alh 84001 while avoiding the coprecipitation of clay minerals we found that the carbonate in alh 84001 could have been produced in a process whereby a low temperature 20 c fluid initially equilibrated with the early martian atmosphere moved through surficial clay mineral and silica rich layers percolated through the parent rock of the meteorite and precipitated carbonates thereby decreasing the partial pressure of co 2 as it evaporated this finding requires that before encountering the unweathered orthopyroxenite host of alh 84001 the fluid permeated rock that became weathered during the process we were able to predict the composition of the clay minerals formed during weathering which included the dioctahedral smectite nontronite kaolinite and chlorite all of which have been previously detected on mars we also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc which is typically considered to be a higher temperature hydrothermal phase on earth but may have been a common constituent in the formation of martian soils through pervasive aqueous alteration finally goethite and magnetite were also found to precipitate in the secondary alteration assemblage the latter associated with the generation of h 2 apparently despite the limited water rock interaction that must have led to the formation of the carbonates 3 9 ga ago in the vicinity of the alh 84001 source rocks clay formation would have been widespread

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

insights into the formation of fe and mg rich aqueous solutions on early mars provided by the alh 84001 carbonates abstract the chemical and isotopic pattern of the zoned carbonate globules in the alh 84001 meteorite reveals a unique aqueous environment on early mars if the evolution of the fluid composition was dictated primarily by carbonate precipitation the zoning pattern of the carbonates can constrain the fluid to have had an mg ca mole ratio 5 3 and a fe ca mole ratio 1 prior to the formation of the carbonates chemical equilibrium modeling of water rock interactions indicates that low temperatures and low ph favor the formation of an aqueous solution with elevated mg and fe concentrations the modeling shows that a sufficiently fe and mg rich fluid could have formed through low temperature 2 fugacity 0 1 1 bar formation of alh 84001 carbonates could have been driven by degassing of co 2 and corresponding ph increase in near surface environments during an upwelling of subsurface co 2 rich solutions this scenario is consistent with the unaltered nature of the alh 84001 rock and with chemical and isotopic composition of its carbonates

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

northeast extension of proterozoic terranes of mid continental north america granite and rhyolite ranging in age from 1 8 1 34 m y are known to underlie much of mid continental north america west of the phanerozoic michigan basin u pb zircon ages of similar rocks exposed adjacent to the grenville province in ontario suggest that the mid continental terranes extend northeast beneath and beyond the michigan basin prevalence of similar u pb zircon ages obtained from metaplutonic rocks southeast of the grenville front and affected by the 1 2 1 0 ga grenvillian orogeny prompts the interpretation that deformed equivalents of the mid continental proterozoic rocks form the bulk of the crust in the central gneiss belt of the grenville province east of lake huron farther east pre 1 34 ga terranes appear to be tectonically occluded by 1 28 1 22 ga volcanic sedimentary and plutonic rocks of the central metasedimentary belt themselves deformed metamorphosed and intruded by younger grenvillian plutons

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

identification of carbonate rich outcrops on mars by the spirit rover decades of speculation about a warmer wetter mars climate in the planet s first billion years postulate a denser co2 rich atmosphere than at present such an atmosphere should have led to the formation of outcrops rich in carbonate minerals for which evidence has been sparse using the mars exploration rover spirit we have now identified outcrops rich in magnesium iron carbonate 16 to 34 weight percent in the columbia hills of gusev crater its composition approximates the average composition of the carbonate globules in martian meteorite alh 84001 the gusev carbonate probably precipitated from carbonate bearing solutions under hydrothermal conditions at near neutral ph in association with volcanic activity during the noachian era

the grenville front south of val d or quebec abstract the tectonometamorphic history of rock assemblages is unravelled throughout a 50 30 km strip across the grenville front the boundary between the superior and the grenville provinces in the canadian shield this was accomplished by applying geothermobarometric techniques to zoned garnets systematic analysis of garnetiferous assemblages revealed that 1 1 primary garnet grti cores grew during a high t low p metamorphic episode and 2 2 secondary garnet grtii cores as well as most of the grti rims grew during a low low t high p metamorphic episode the two sets of p t conditions can by comparison with available geochronological data be assigned to the kenoran late archaean and grenvillian mid proterozoic orogenies respectively a prominent ductile shear zone traverses the 50 30 km strip and divides the area southeast of the grenville front into two regions with contrasted tectonometamorphic behaviour the eastern region is composed of migmatitic and charnockitic gneisses representing intermediate to lower levels of the archaean crust the western region is composed of migmatitic metasediments which show no evidence of significant crustal thickening during the grenvillian orogeny along a transect across the grenville front eastern region significant offsets of grenvillian paleopressure levels indicate that the thickened crust is a stack of imbricated slices with vertical components of displacement in the order of several kilometres after compensating for the grenvillian movement along the appropriate shear zones offsets remain between some of the archaean paleopressure levels suggesting that delamination of the archaean crust was initiated prior to the peak of grenvillian metamorphism the thrust slices thus identified in the eastern region have thicknesses of a few kilometres but they could be much thinner in the western region here the grenville front is ill defined and lies in a zone of gradual transition between the pontiac group metasediments of the superior province and the lac lambert migmatites of the grenville province the overall deformations of the two regions southeast of the grenville front are incompatible and cause the development of an intervening shear zone although the two regions experienced different tectonometamorphic histories they both belong to the parautochthonous belt of the grenville orogen

formation of meteorite hydrocarbons from thermal decomposition of siderite feco3 thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds siderite decomposition in the presence of water vapor at 300 c generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds the results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite alh84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on mars the results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early earth

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

early proterozoic central plains orogen a major buried structure in the north central united states a major early proterozoic orogen central plains more than 1000 km long and at least 500 km wide has been delineated in the subsurface of nebraska kansas and missouri as a result of a new compilation of precambrian basement drill data in the northern midcontinent the orogen is composed of metamorphic and granitoid rocks in the ringe 1 63 1 80 ga and it is interpreted on the basis of isotopic ages rock types and structure as reflected by magnetic and gravity anomalies as being the eastward extension of the early proterozoic foldbelt exposed in basement uplifts in colorado and southeastern wyoming it truncates the southern margin of the superior archean craton and its marginal early proterozoic orogens the penokean 1 83 1 90 ga and the trans hudson 1 85 1 95 ga the pattern of intersecting proterozoic mobile belts marginal to archean cratons is comparable to that in other continents such as western australia and southern africa

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

the balinge conglomerate in northern sweden reinterpreted as a magmatic hydraulic breccia the balinge conglomerate has played a key role in the lithostratigraphy of northern sweden the conglomerate with pebbles dominated by granodiorites and tonalites in a generally biotite rich matrix borders volcanic rocks with unclear relative age relationships in nearby areas these volcanites have been intruded by plutonic rocks c 1 9 ga which are similar to the pebble material on these grounds the conglomerate has been regarded as younger than the volcanic rocks and an important marker in the geological evolution of the region in this preliminary study it is suggested that the features displayed by the balinge conglomerate are not compatible with an epiclastic origin on the contrary they indicate that this rock represents a hydraulic breccia where the roundness of the granitoid pebbles has resulted from erosion in a fluidised heterogeneous and magmatic environment in the actual area the conglomerate must thus be dismissed as a stratigraphic marker horizon it belongs to the early svecofennian evolution

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

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

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

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

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

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

combining meteorites and missions to explore mars laboratory studies of meteorites and robotic exploration of mars reveal scant atmosphere no evidence of plate tectonics past evidence for abundant water and a protracted igneous evolution despite indirect hints direct evidence of a martian origin came with the discovery of trapped atmospheric gases in one meteorite since then the study of martian meteorites and findings from missions have been linked although the meteorite source locations are unknown impact ejection modeling and spectral mapping of mars suggest derivation from small craters in terrains of amazonian to hesperian age whereas most martian meteorites are young 4 5 ga and formation of enriched and depleted reservoirs however the history inferred from martian meteorites conflicts with results from recent mars missions calling into doubt whether the igneous histor y inferred from the meteorites is applicable to mars as a whole allan hills 84001 dates to 4 09 ga and contains fluid deposited carbonates accompanying debate about the mechanism and temperature of origin of the carbonates came several features suggestive of past microbial life in the carbonates although highly disputed the suggestion spurred interest in habitable extreme environments on earth and throughout the solar system a flotilla of subsequent spacecraft has redefined mars from a volcanic planet to a hydrologically active planet that may have harbored life understanding the history and habitability of mars depends on understanding the coupling of the atmosphere surface and subsurface sample return that brings back direct evidence from these diverse reservoirs is essential

chemical divides and evaporite assemblages on mars assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments accordingly newly discovered regions of saline minerals on mars are high priority targets for exploration the chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on earth however basaltic weathering largely controls fluid compositions on mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret martian evaporites here we present a new chemical divide system that predicts evaporite assemblages identified in snc type meteorites ancient evaporites discovered on meridiani planum by the opportunity rover and mars express omega data we suggest that a common fluid type that has been buffered to different ph levels by basaltic weathering controls the variability among martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid b 2005 elsevier b v all rights reserved

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

age of deformation within the central metasedimentary belt boundary thrust zone southwest grenville orogen constraints on the collision of the mid proterozoic elzevir terrane u pb zircon and titanite geochronology on syntectonic intrusions from the ontario central metasedimentary belt boundary thrust zone has constrained the timing of two major periods of northwest directed ductile thrusting on this crustal scale shear zone initiation of deep seated uppermost amphibolite facies deformation is constrained to a period before 1190 ma over 100 ma earlier than previous estimates major reimbrication of the shear zone took place ca 1080 1060 ma and was more pronounced in the southwestern segment the initiation of deformation within the boundary thrust zone coincides with the emplacement of the hanging wall elzevir terrane of the central metasedimentary belt onto the mid proterozoic laurentian margin the structural nature and temporal and kinematic consistency of the reimbrication phase along the 200 km exposed length of the shear zone is thought to be due to within plate reactivation of the existing rheologically weak structure a result of continent continent collision ca

north west propagation of the grenville orogen grenvillian structure and metamorphism near key harbour georgian bay ontario canada abstract we report structural and metamorphic data from a c 25 km transect across the eastern grenville front tectonic zone gftz to the britt domain at the northern end of georgian bay near key harbour ontario constrasting grenvillian structural and fabric elements characterize the eastern gftz northern britt domain and a narrow transition zone between them moderately to steeply dipping foliations with strong down dip lineations in all three divisions appear to be associated with nw directed thrusting in the transition zone and northern britt domain early s l fabrics with steep lineations are overprinted by younger structures s l with shallow se sse plunging lineations in which sparse dominantly but not exclusively normal sense kinematic indicators are recognized pressure and temperature estimates from grenvillian metamorphic assemblages in metadi abase indicate that conditions of p 12 kbar and t c 800 c were achieved before or during the thrust related deformation with p t t paths that indicate near isothermal decompression to p c 4 kbar and t c 700 c correlation of fabric elements with points on the p t t paths suggests that exhumation occurred during two stages the first associated with thrusting 1035 ma and the second with extension and thrusting pre 1003 ma the gftz contains steeply to moderately dipping thrust related fabrics and lacks shallow extensional structures the latest episode of thrusting in the gftz is inferred to have taken place at 990 980 ma the data are interpreted in terms of a tectonic model involving two stages of propagation of the grenville orogen towards its foreland 1035 ma and 980 ma with an intervening period of extension although the tectonic regime probably remained compressional on the scale of the orogen

last gasp of the grenville orogeny thermochronology of the grenville front tectonic zone near killarney ontario we present u pb titanite zircon and 40 ar 39 ar hornblende mica k feldspar data from a transect across the western part of the grenville front tectonic zone gftz near killarney ontario high grade metamorphic assemblages 1450 ma in this part of the gftz pre date the grenvillian orogeny and were primarily exhumed with little or no metamorphic overprinting by grenvillian deformation the titanite and zircon data form a discordant array with an upper intercept of 1454 pm 8 ma and a lower intercept of 978 pm 13 ma these data are interpreted in terms of partial lead loss during a short lived thermal event that increased in intensity from west to east across the transect 40 ar 39 ar data from hornblende indicate cooling through 450 c at 993 979 ma multiple diffusion domain models for the interpretation of discordant k feldspar spectra indicate cooling through 365 340 c at 990 960 ma and muscovite data indicate cooling through 320 c at 930 ma biotite data are not easily

experimental epithermal alteration of synthetic los angeles meteorite implications for the origin of martian soils and identification of hydrothermal sites on mars 1 the dissolution behavior and secondary minerals generated by experimental alteration of a synthetic los angeles basaltic shergottite la were evaluated at epithermal 75 c conditions starting ph values of 1 1 and 3 6 and water rock ratios ranging from 100 to 2000 were studied using flow through and batch reactor experimental apparatus dissolution of plagioclase dominates effluent solution chemistry under all experimental conditions little mg is released to solution because nearly 100 of the mg in unaltered la is contained in pyroxene m1 sites which are relatively resistant to dissolution secondary minerals generated include siliceous residue amorphous sio2 lepidocrocite and ferrihydrite anhydrite alunogen rhomboclase ferrinatrite halite and unidentified ca and na sulfates the dominance of alteration products of plagioclase suggests that la type basalts have not contributed significantly to the martian soils the search for hydrothermal sites by remote sensing may be hampered by the general similarity of the alteration products found in the near surface epithermal environment to those thought to exist in martian soils sulfates iron oxyhydroxides amorphous silica however the unique chemical composition of secondary products formed by la alteration may make detection of recent hydrothermal alteration of evolved basaltic rocks more straightforward in addition alteration in terrestrial epithermal environments associated with mafic to intermediate volcanic rocks generally produces abundant sulfides and kaolinite two potentially unique mineral phases which might also indicate the presence of active or extinct hydrothermal systems

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

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

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

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

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

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

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

a40ar 39ar study of post tectonic cooling in the britt domain of the grenville province ontario abstract we report the results of a 40 ar 39 ar study of rocks from the britt domain northwestern grenville province most of the age spectra obtained for micas and amphiboles have well defined age plateaus typically k feldspars yielded spectra with age gradients and arrhenius plots with kinks or offsets hence some of the data were interpreted in terms of a multi diffusion domain size model this 40 ar 39 ar data base 5 amphiboles 9 micas 4 k feldspar along with some recently acquired uzpb data provide reasonable constraints on the post peak metamorphic cooling history which began at depth in the upper amphibolite facies comparison of the data with exhumation paths generated by thermal models suggest that cooling taking place over 400 ma was for the most part due to erosionally controlled exhumation although the decompression may have initiated with a short period of rapid extensional uplift the entire cooling history may have been erosionally controlled there is an apparent difference in cooling ages between north and south in the 40 ar 39 ar mica and k feldspar data for the britt domain which may be due to somewhat higher exhumation rates in the north and or an elevated geotherm in the south

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

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

interplay between folding and ductile shearing in the proterozoic crust of the muskoka parry sound region central ontario grenville gneiss of the central georgian bay region was subjected to ductile deformation that produced narrow mylonite zones as well as three sets of superimposed folds differing greatly in structural signature size and orientation some mylonite zones are concordant to gneissosity and are repeatedly folded others cut gneissosity and postdate the folding gneissosity was generated as a regionally subhorizontal feature either by crustal thinning or like the early mylonite zones by low angle reverse shearing an attempt is made to account for the initially subhorizontal gneissosity the mylonite zones and the folds in a regime of large scale reverse shearing that strikes parallel to the grenville front upright northwest southwest to north south buckle folds dominate the map pattern and are subperpendicular to the reverse grenville front boundary fault these set 2 folds cannot be attributed to reverse simple shearing but require a large component of east west compression such stress could have been

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

regional cross section of the southern province adjacent to lake huron ontario implications for the tectonic significance of the murray fault zone the southern province in the lake huron area of ontario and the penokean fold belt of minnesota wisconsin and michigan have the earmarks of a collisional orogen the huronian supergroup a southerly facing passive margin sequence was deposited during early aphebian early proterozoic crustal stretching and ocean basin formation along the southern margin of the superior province archean craton it accumulated as four unconformity bounded northerly tapering onlapping clastic wedges megacycles the thickness and facies variations reflect syndepositional down to the basin south normal faulting that controlled the accumulation and preservation of the lower three huronian megacycles these are overlapped northward by the youngest megacycle an extensive sheet of clastic sediments recording post stretching regional subsidence of the cratonic margin due to cooling and thermal contraction soft sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slu

migmatite structures in the central gneiss complex boca de quadra alaska migmatite structures in the coast plutonic metamorphic complex are well exposed in the inlet of boca de quadra southeast alaska two types of anatectic migmatites are present patch migmatites formed by in situ melting and subsequent crystallization of melt diktyonitic migmatites comprise a discontinuous veined network of leucocratic material in which leucosomes enclose boudins of host rock the margins of these boudins show the development of both melanosomes and shear band fabrics strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20 27 this volume decrease is attributed to melt extraction into the adjacent fracture filling leucosomes thus diktyonitic migmatites formed by shear induced segregation of partial melt whereas in patch migmatites the lack of shear stresses inhibited melt segregation the variable structural style of anatectic migmatites in boca de quadra is not related to host rock composition but may be due to differences in the amount of differential stress during migmatization these in turn may be controlled by host rock strength and or diachroneity of migmatization and deformation determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels