nitrogen isotope evidence for anoxic deep marine environments from the mesoarchean mosquito creek formation australia abstract current evidence for oxygenated environments in the mesoarchean is limited to the shallowest marine and fluvio lacustrine settings it is not until the neoarchean that signs of oxygenated surface waters above outer shelf and basinal depositional environments become evident in order to further explore the mesoarchean redox landscape for signs of basinward surface water oxygenation we present nitrogen and carbon isotope ratios from the turbiditic mosquito creek formation of the nullagine group 2 9 ga the 15n and 13corg values are invariant around 1 8 and 32 respectively throughout a 70 m section of drill core suggesting an ecosystem dominated by nitrogen fixers anaerobic nitrogen cycling and co2 fixation by the calvin cycle when compared with other archean isotopic data these results i provide further evidence that the mosquito creek formation was deposited in a marine basin and ii contain 15n values that highlight the prevalence of nitrogen fixation by mo nitrogenase and the dearth of aerobic nitrogen metabolisms in the mesoarchean

selenium isotopes support free o2 in the latest archean selenium se undergoes redox transformations and isotopic fractionations at relatively high redox potentials and could therefore provide insight into changes in oceanic and atmospheric o2 levels over earth s history we test this idea with se data from the 2 5 ga mount mcrae shale hamersley basin australia which records temporary enrichments in abundances and isotopes of other redox sensitive elements indicating a whiff of oxygen in earth s atmosphere before the great oxidation event se isotopic ratios expressed as 82 78se and abundances relative to crustal background show significant positive excursions of up to 1 1 and an enrichment 13 times above background respectively overlapping with excursions in mo and n isotopes and abundances because se has a relatively high redox potential and photosynthetic oxidation pathways are unknown our data thus suggest that se was mobilized by free o2 during this interval the isotopic fractionation likely occurred during transport of se oxyanions from the site of weathering to the outer shelf although o2 could have been produced locally on land and may not necessarily have increased in the global atmosphere our results strengthen the inference of an early origin of oxygenic photosynthesis long before the paleoproterozoic great oxidation event this is the first report of a se isotope excursion in the precambrian rock record and it confirms that se isotopes can serve as a useful redox proxy in deep time

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

visualisation and quantification of cv chondrite petrography using micro tomography abstract micro computed tomography is a non destructive technique that allows the study of 3d meteorite petrography the technique produces a unique and instructive visualisation of the meteorite for quantifying its components we studied the overall petrography of the two cv chondrites allende and mokoia to constrain their formation histories a set of movies and stereographic images detail the 3d petrography component modal abundances agree with previous reports and modal abundance differences between allende and mokoia support the chondrule matrix complementarity and that chondrules and matrix formed from the same chemical reservoir we identified two types of chondrules a normal type and one where a normal type i or ii chondrule is almost completely encapsulated by an opaque rich layer this layer was probably acquired during a late stage condensation process the appearance of opaques in chondrules and matrix is different not supporting a genetic relationships between these low abundances of compound chondrules 1 75 vol in allende and 2 50 vol in mokoia indicate low chondrule densities and or low relative component velocities in chondrule formation regions porosities on a scale

hydrocode simulation of the chicxulub impact event and the production of climatically active gases we constructed a numerical model of the chicxulub impact event using the chart d squared csq code coupled with the analytic equation of state aneos package in the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact related extinction hypothesis carbonates calcite and evaporites anhydrite simulations explored the effects of different projectile sizes 10 to 30 km in diameter and porosity 0 to 50 the effect of impact speed is addressed by doing simulations of asteroid impacts vi 20 km s and comet impacts vi 50 km s the masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event ranging from 350 to 3500 gt for co2 from 40 to 560 gt for s and from 200 to 1400 gt for water vapor while our results are in good agreement with those of ivanov et al 1996 our estimated co2 production is 1 to 2 orders of magnitude lower than the results of takata and ahrens 1994 indicating that the impact event enhanced the end cretaceous atmospheric co2 inventory by at most 40 consequently sulfur may have been the most important climatically active gas injected into the stratosphere the amount of s released by the impact is several orders of magnitude higher than any known volcanic eruption and with h2o is high enough to produce a sudden and significant perturbation of earth s climate

density porosity and magnetic susceptibility of carbonaceous chondrites abstract we report physical properties bulk and grain density magnetic susceptibility and porosity measured using nondestructive and noncontaminating methods for 195 stones from 63 carbonaceous chondrites grain densities over the whole population average 3 44 g cm 3 ranging from 2 42 g cm 3 ci1 orgueil to 5 66 g cm 3 cb bencubbin magnetic susceptibilities in log units of 10 9 m3 kg 1 averaged log 4 22 ranging from 3 23 cv3 axtell to 5 79 cb bencubbin porosities averaged 17 ranging from 0 for a number of meteorites to 41 for one stone of the co ornans notably we found significant differences in porosity between the oxidized and reduced cv subgroups with the porosities of cvo averaging approximately 20 and cvr porosities approximately 4 overall porosities of carbonaceous chondrite falls trend with petrographic type from type 1 ci near 35 type 2 cm cr averaging 23 type 3 cv co 21 to type 4 ck and some co averaging 15 there is also a significant decrease in porosity between meteorites of shock stage s1 and those of s2 indicative of shock compression

magnetic anisotropy and porosity of chondrites the measured magnetic anisotropy and porosity of various chondrites are found to be inversely correlated when they are considered by class of chondrites e h and l ll for chondrites with similar porosities the anisotropies for h and e chondrites are smaller than for l and ll chondrites which have lower metal contents the anisotropy and porosity are not dependent on the metamorphic grade of chondrites k ar ages of strongly anisotropic and or less porous chondrites are younger than those of less anisotropic and or more porous chondrites these observations suggest that impacts which reset the k ar ages produced the anisotropy and reduced the porosity of chondrites

high arsenic and boron concentrations in groundwaters related to mining activity in the bigadic borate deposits western turkey this study documents the environmental impacts of borate mines in bigadic district which are the largest colemanite and ulexite deposits in the world borate bearing formations have affected the concentrations of some contaminants in groundwater groundwater quality is directly related to the borate zones in the mines as a result of water rock interaction processes calcium is the dominant cation and waters are ca so4 and hco3 type in the mine tuluborate mine from which colemanite is produced however in the simav and acep borate mines ulexite and colemanite minerals are pro duced and waters from these open pit mines are na hco3 so4 types high so4 concentrations reaching 519 mg l might be explained by the existence of anhydrite gypsum and celestite minerals in the borate zone groundwater from tuff and borate strata showed relatively low ph values 7 8 compared to surface and mine waters 8 ec values ranged from 270 to 2850 ls cm boron and as were the two important contaminants determined in the groundwaters around the bigadic borate mines arsenic is the major pollutant and it ranged from 33 to 911 lg l in the groundwater samples the concentrations of b in the study area ranged from 0 05 to 391 mg l the highest b concentrations were detected at the mine areas the extension of the borate zones in the aquifer systems is the essential factor in the enrichment of b and as and some major and trace elements in groundwaters are directly related to the leaching of the host rock which are mainly composed of tuffs and limestones according to drinking water standards all of the samples exceed the tolerance limit for as copper mn zn and li values are enriched but do not exceed the drinking water standards sulfate al and fe concentrations are above the drinking water standard for the groundwater samples 2008 elsevier ltd all rights reserved

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

comparison of young volcanic associations of western and eastern anatolia formed under a compressional regime a review abstract turkey is characterised by widespread post oligocene volcanism in western anatolia volcanic activity began during the late oligocene early miocene in a compressional regime a widespread suite of andesitic and dacitic calc alkaline rocks was erupted in this period they were accompanied by locally developed high level granitic intrusions the abrupt change from n s compression to n s stretching in the middle miocene was accompanied by a gradual transition to alkali basaltic volcanism in eastern anatolia the collision related compressional tectonics and associated volcanic activity began in the late miocene to pliocene and continued almost without interruption into historical times one minor and two major volcanic phases may be distinguished an initial minor eruptive phase of basic and intermediate alkaline rocks was followed by widespread eruptions of andesitic and dacitic calc alkaline rocks during the pliocene partly penecontemporaneous with this a second more important major phase of volcanism involving alkaline and transitional lavas began and continued during the quaternary the calc alkaline lavas of both regions were erupted at a time when the compressional regime led to crustal thickening these lavas display many geochemical and isotopic signatures that may be interpreted in terms of contamination of magma involving continental crustal materials the alkaline lavas in both regions began during the final stage of the compressional regime and form two groups one group is geochemically and isotopically similar to the calc alkaline rocks and the other is more typically alkaline the volcanic evolution of these two regions of turkey followed a somewhat similar pattern producing highly contaminated calc alkaline lavas at the beginning and evolving towards uncontaminated alkaline lavas at the end

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

the role of regolith redistribution in influencing the evolution of the shapes of asteroids major surface fissures and relatively large scale angular surface irregularities are expected to have been present on many asteroids at early stages in their histories as a byproduct of at least two processes impact disruption and reassembly into rubble piles for all classes of asteroid and for carbonaceous chondrite parent bodies aqueous alteration which led to the low bulk densities currently being observed for asteroids however in all cases where high enough resolution images exist such abrupt deep irregularities are not observed we model the spatial redistribution of impact generated regolith on an asteroid with an idealized irregular shape to show how the complex gravitational field of such a body will lead to the systematic infilling of deep valleys in the surface our analysis emphasizes the high efficiency with which regolith redistribution can act to disguise the internal structures of asteroids with sizes in the 20 100 km range

size distributions in two porous chondritic micrometeorites quantitative size measurements of granular units gus and nanometer sized minerals in these units in two porous chondritic micrometeorites are investigated for the information that they carry on the processes in the early solar system the matrix of these micrometeorites consist of loosely packed 0 1 m sized gus these objects were a major component of the solar nebula dust that accreted into protoplanets the matrix in micrometeorite w7010 a2 has a fractal dimension with a small coefficient d 0 83 that supports efficient sticking of carbon rich gus during accretion the fractal nature of the matrix provides a way to calculate the density using the aggregate size the resulting very low density for porous chondritic micrometeorites is 0 08 0 14 g cm 3 which supports the view that they are the solid debris from unconsolidated solar system bodies e g comet nuclei or regolith deposits on carbon rich asteroids chondritic gus contain ultrafine olivines pyroxenes and sulfides embedded in hydrocarbons and amorphous carbons nanocrystals in the micrometeorites w7010 a2 and u2015 b show log normal size distributions the high incidence of disk shaped grains a changeover from disk shaped to euhedral grains the unevolved nature of the size distributions and multiple populations for grains 127 nm in size are consistent with continuous post accretion nucleation and growth in amorphous gus including coarsening via ostwald ripening

variation of mineralogy and organic material during the early stages of aqueous activity recorded in antarctic micrometeorites abstract micrometeorites mms recovered from surface snow near the dome fuji station antarctica are almost free from terrestrial weathering and contain very primitive materials and are suitable for investigation of the evolution and interaction of inorganic and organic materials in the early solar system we carried out a comprehensive study on seven porous and fluffy mms four chondritic porous cp mms and three fluffy fine grained fluffy fg mms and one fine grained type 1 fg c1 mm for comparison with scanning electron microscope transmission electron microscope x ray absorption near edge structure analysis and secondary ion mass spectrometer they show a variety of early aqueous activities four out of the seven cp mms contain glass with embedded metal and sulfide gems and enstatite whiskers platelets and do not have hydrated minerals despite the same mineralogy organic chemistry of the cp mms shows diversity two of them contain considerable amounts of organic materials with high carboxyl functionality and one of them contains nitrile c n and or nitrogen heterocyclic groups with d and 15 n enrichments suggesting formation in the molecular cloud or a very low temperature region of the outer solar system another two cp mms are poorer in organic materials than the above mentioned mms organic material in one of them is richer in aromatic c than the cp mms mentioned above being indistinguishable from those of hydrated carbonaceous chondrites in addition bulk chemical compositions of gems in the latter organic poor cp mms are more homogeneous and have higher fe si mg fe ratios than those of gems in the former organic rich cp mms functional group of the organic materials and amorphous silicate in gems in the organic poor cp mms may have transformed in the earliest stage of aqueous alteration which did not form hydrated minerals three fluffy fg mms contain abundant phyllosilicates showing a clear evidence of aqueous alteration phyllosilicates in thee mms are richer in fe than those in hydrated idps typical fine grained hydrated fg c1 mms and hydrated carbonaceous chondrites one of the fluffy fg mms contains amorphous silicate which is richer in fe than gems and contains little or no nanophase fe metal but contains fe sulfide because the chemical compositions of the amorphous silicate are within the compositional field of gems in cp idps the amorphous silicate may be alteration products of gems the entire compositional field of gems in the cp mms and the amorphous silicate in the fluffy fg mm matches that of the previously reported total compositional range of gems in idps one fluffy fg mm contains mg rich phyllosilicate along with fe rich phyllosilicate and mg fe carbonate mg rich phyllosilicate and mg fe carbonate may have been formed through the reaction of fe rich phyllosilicate mg rich olivine and pyroxene and water with c bearing chemical species these data indicate that cp mms and fluffy fg mms recovered from antarctic surface snow contain materials that throw a light on the earliest stages of aqueous alteration on very primitive solar system bodies because mineralogy and isotopic and structural features of organic materials in d10ib009 are comparable with isotopically primitive idps its parent body could be comets or icy asteroids showing mass ejection active asteroids by contrast organic poor cp mms may have experienced the earliest stage of aqueous alteration and fluffy fg mms experienced weak aqueous alteration the precursor materials of the parent bodies of fluffy fg mms probably contained abundant gems or gems like materials like cp idps which is common to fine grained matrices of very primitive carbonaceous chondrites such as cr3s however highly porous nature of organic poor cp mms and fluffy fg mms suggests that parent bodies of these mms must have been much more porous than the parent bodies of primitive carbonaceous chondrites given no phyllosilicate among the returned samples of 81p wild 2 comet the mms may have been derived from porous icy asteroids such as active asteroids as well as p and d type asteroids rather than comets

modal mineralogy of cm2 chondrites by x ray diffraction psd xrd part 1 total phyllosilicate abundance and the degree of aqueous alteration abstract cm carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids using position sensitive detector x ray diffraction psd xrd and a pattern stripping technique we quantify the modal mineralogy of cm2 chondrites mighei murray murchison nogoya and cold bokkeveld there is a narrow range in the combined modal volume vol of the most abundant phases mg serpentine 25 33 and fe cronstedtite 43 50 cold bokkeveld is anomalous in containing more mg serpentine 49 59 than fe cronstedtite 19 27 even including cold bokkeveld the range in modal total phyllosilicate is 73 79 average 75 total phyllosilicate abundance provides a non ambiguous measure of the degree of aqueous alteration and indicates that these meteorites have all experienced essentially the same degree of aqueous alteration this reflects pervasive hydration of matrix across cm2 samples apparent differences in the alteration of chondrules observed in petrographic studies represent various stages in the progression towards complete hydration of all components but are not manifest in significant differences in modal mineralogy for all samples there is a limited range in olivine 6 9 and pyroxene 5 abundances modal abundances of the remaining identified phases also show a limited range calcite 0 1 3 gypsum 0 1 6 magnetite 1 1 2 4 pentlandite 0 2 1 and pyrrhotite 1 3 8 as expected we observe a strong negative correlation in the modal abundance of anhydrous fe mg silicates olivine pyroxene and total phyllosilicate mg serpentine fe cronstedtite consistent with the idea that phyllosilicate is forming by aqueous alteration of the anhydrous components the negative correlation in the modal abundance between mg serpentine and fe cronstedtite indicates a mineralogic transformation of fe cronstedtite to mg serpentine by fluid driven recrystallisation or b that these meteorites had different initial abundances of olivine and pyroxene the observed positive correlation in the relative proportion of mg serpentine with increasing total phyllosilicate abundance reflects the evolution of increasingly mg rich phyllosilicate during aqueous alteration fe cronstedtite is the dominant phyllosilicate while cm chondrule olivines are forsteritic and will form mg serpentine during aqueous alteration this implies that matrix olivine was more fe rich than chondrule olivine prior to aqueous alteration

the halite bearing zag and monahans 1998 meteorite breccias shock metamorphism thermal metamorphism and aqueous alteration on the h chondrite parent body zag and monahans 1998 are h chondrite regolith breccias comprised mainly of light colored metamorphosed clasts dark clasts that exhibit extensive silicate darkening and a halite bearing clastic matrix these meteorites reflect a complex set of modification processes that occurred on the h chondrite parent body the light colored clasts are thermally metamorphosed h5 and h6 rocks that were fragmented and deposited in the regolith the dark clasts formed from light colored clasts during shock events that melted and mobilized a significant fraction of their metallic fe ni and troilite grains the clastic matrices of these meteorites are rich in solar wind gases parent body water was required to cause leaching of chondritic minerals and chondrule glass the fluids became enriched in na k cl br al ca mg and fe evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated grains of halite and in the case of monahans 1998 halite with sylvite inclusions precipitated at low temperatures 100 c in the porous regolith in both meteorites fluid inclusions were trapped inside the halite crystals primary fluid inclusions were trapped in the growing crystals secondary inclusions formed subsequently from fluid trapped within healed fractures

mineralogy of tagish lake an ungrouped type 2 carbonaceous chondrite in this paper we describe the recovery handling and preliminary mineralogical investigation of the tagish lake meteorite tagish lake is a type 2 carbonaceous chondrite which bears similarities to ci1 and cm chondrite groups but is distinct from both abundant phyllosilicates as well as chondrules however sparse and common olivine grains in the matrix preclude any other classification the bulk density of tagish lake 1 67 g cc which is far lower than ci or cm chondrites 2 2 2 3 and 2 6 2 9 g cc respectively or any other meteorite for that matter we have identified two lithologies a dominant carbonate poor lithology and a less abundant carbonate rich lithology the meteorite is a breccia at all scales we have noted similarities between tagish lake and some clasts within the enigmatic meteorite kaidun possibly there are genetic relationships here worth exploring in the paper we describe a clast of cm1 material within tagish lake which is very similar to a major lithology in kaidun

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

nitrogen isotope dynamics of the cariaco basin venezuela 1 we report 15n 14n measurements of water column nitrate and ammonium sinking particles and sediments from the cariaco basin an anoxic marine basin off the coast of venezuela water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic anoxic interface 275 m being free of a large denitrification signal the 15n of shallow thermocline nitrate is 3 5 significantly lower than the mean deep ocean nitrate 15n of 5 this may be due to the nitrification of newly fixed n whether it occurs within the basin or in open atlantic waters that flow into the cariaco over the sill the 15n 14n of the sinking flux in the deepest trap 1250 m is similar to that of thermocline nitrate as expected given the complete consumption of nitrate in the surface layer moreover the 15n 14n of the seafloor sediment is similar to that of the sinking flux as is common in environments of high export production low o2 and good organic matter preservation thus the modern cariaco basin records the 15n 14n of the thermocline nitrate which in turn may record the input of newly fixed n to the upper ocean be it local or more regional in origin

olivine basalt and trachyandesite peperites formed at the subsurface surface interface of a semi arid lake an example from the early miocene bigadic basin western turkey abstract miocene successions in western turkey are dominated by lacustrine fluvial and evaporitic sedimentary deposits these deposits include considerable amounts of volcaniclastic detritus derived from numerous ne trending volcanic centres in western turkey as well as in the bigadic region early miocene syn depositional ne trending olivine basalt and trachyandesite bodies that formed as intrusions and lava flows occur within the bigadic borate basin olivine basalts occur as partly emergent intrusions and trachyandesite dykes fed extensive lava flows emplaced in a semi arid lacustrine environment peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features although all the localities include a large variety of clast morphologies from blocky to fluidal fluidal clasts mainly globular ameboidal and pillow like varieties are widespread in the peperite domains associated with olivine basalts apparently due to large volume sediment fluidisation in contrast fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the intrusions and have commonly elongate and polyhedral shapes with digitate margins rather than globular and equant varieties blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration suggesting decreasing temperature and increasing viscosity during fragmentation abundance of blocky clasts with respect to fluidal clasts in the trachyandesite peperite indicates that the fluidal emplacement and low volume sediment fluidisation in the early stages were immediately followed by quench fragmentation due to the high viscosity of the magma size texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly controlled by sediment fluidisation pulsatory magma injection and magma properties such as composition viscosity vesicularity and size abundance and orientation of phenocrysts variously combining these contrasting features to varying degrees may form diverse juvenile clast shapes in peperitic domains

a multiple sulfur and organic carbon isotope record from non conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup south africa abstract multiple sulfur isotope ratios 36 s 34 s 33 s 32 s and organic carbon isotope ratios 13 c 12 c were measured from 198 non conglomeratic sedimentary samples that were collected from five deep diamond drill cores that cover the majority of the ca 2 96 2 82 ga witwatersrand supergroup 13 c org 34 s py and 33 s py values of the sample set range from 44 3 to 21 9 3 7 to 16 5 and 4 0 to 1 9 respectively these geochemical data vary relative to depositional facies proximal marine distal marine and fluvial dominated and tectonic setting trailing margin and foreland basin in the trailing margin setting of the 2 96 ga hospital hill subgroup the proximal marine depofacies is characterized by relatively high organic carbon contents up to 0 9 wt and 13 c org values around 28 in contrast to the distal marine depofacies that yields low organic carbon contents 0 01 wt and high 13 c org values up to 22 both depofacies yield low sulfur contents 0 02 wt a narrow range of 34 s values 3 2 and positive 33 s values up to 1 9 this data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production preservation in deeper water settings due to longer transport distances and effective biological degradation e g fe reducing microbes positive 33 s values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir in the foreland basin tectonic setting of the 2 94 ga government and 2 92 ga jeppestown subgroups shelf deposits associated with fluvial braidplain depofacies are characterized by 13 c depleted organic carbon 44 to 38 relatively high sulfur contents 0 2 1 3 wt variable 34 s values 3 7 to 16 5 and small negative 33 s values 0 4 these data suggest that the microbial community in the fluvial dominated depofacies may have consisted of photoautotrophs methanogens anaerobic methanotrophs and sulfate reducers sulfate was derived from a variety of sources that include photochemical crustal and marine sulfur reservoirs the occurrence of three regionally persistent diamictite deposits in the government subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly 13 c depleted organic carbon suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the mesoarchean glaciations drawdown of h 2 and ch 4 however the link between large 33 s anomalies 4 0 to 1 2 and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures

transient surface ocean oxygenation recorded in the 2 66 ga jeerinah formation australia many paleoredox proxies indicate low level and dynamic incipient oxygenation of earth s surface environments during the neoarchean 2 8 2 5 ga before the great oxidation event goe at 2 4 ga the mode tempo and scale of these redox changes are poorly understood because data from various locations and ages suggest both protracted and transient oxygenation here we present bulk rock and kerogen bound nitrogen isotope ratios as well as bulk rock selenium abundances and isotope ratios from drill cores sampled at high stratigraphic resolution through the jeerinah formation 2 66 ga fortescue group western australia to test for changes in the redox state of the surface environment we find that both shallow and deep depositional facies in the jeerinah formation display episodes of positive primary 15n values ranging from 4 to 6 recording aerobic nitrogen cycling that requires free o2 in the upper water column moderate selenium enrichments up to 5 4 ppm in the near shore core may indicate coincident oxidative weathering of sulfide minerals on land although not to the extent seen in the younger mt mcrae shale that records a well documented whiff of atmospheric oxygen at 2 5 ga unlike the mt mcrae shale jeerinah selenium isotopes do not show a significant excursion concurrent with the positive 15n values our data are thus most parsimoniously interpreted as evidence for transient surface ocean oxygenation lasting less than 50 my extending over hundreds of kilometers and occurring well before the goe the nitrogen isotope data clearly record nitrification and denitrification providing the oldest firm evidence for these microbial metabolisms

a test of the nitrogen limitation hypothesis for retarded eukaryote radiation nitrogen isotopes across a mesoproterozoic basinal profile abstract nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late mesoproterozoic however evidence for how the nitrogen cycle was operating during the middle precambrian is so far rare more specifically it is unknown which steps in the biogeochemical nitrogen cycle e g nitrogen fixation nitrification or ammonification were rate limiting and thus controlling microbial community structures the mesoproterozoic belt supergroup in western montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue bulk 15 n values show a clear trend from 1 in the deepest part of the basin to 5 along basin margins which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling the total fractionation of carbon isotopes between carbonate and organic carbon 13 c increases from 20 to 32 in the same direction possibly indicating an ecological response to redox stratification and nitrogen speciation evidence from the belt supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean

characterization of micron sized fe ni metal grains in fine grained rims in the y 791198 cm2 carbonaceous chondrite implications for asteroidal and preaccretionary models for aqueous alteration the presence of apparently unaltered micron sized fe ni metal grains juxtaposed against hydrated fine grained rim materials in the cm2 chondrite yamato y 791198 has been cited as unequivocal evidence of preaccretionary alteration we have examined the occurrence composition and textural characteristics of 60 fe ni metal grains located in fine grained rims in y 791198 using scanning electron microscopy sem and electron microprobe analysis in addition three metal grains prepared by focused ion beam fib sample preparation techniques were studied by transmission electron microscopy tem the metal grains are heterogeneously distributed within the rims electron microprobe analyses show that all the metal grains are kamacite with minor element contents p cr and co that lie either within or close to the range for other cm2 metal grains x ray maps obtained by electron microprobe show s p and or ca enrichments on the outermost parts of many of the metal grains z contrast stem imaging of fib prepared fe ni metal grains show the presence of a small amount of a lower z secondary phase on the surface of the grains and within indentations on the grain surfaces energy filtered tem eftem compositional mapping shows that these pits are enriched in oxygen and depleted in fe relative to the metal these observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of fe oh 2 a common oxidation product of fe metal the presence of such a layer could have inhibited further alteration of the metal grains these findings are consistent with alteration by an alkaline fluid as suggested by zolensky et al 1989 but the location of this alteration remains unconstrained because y 791198 was recovered from antarctica and therefore may have experienced incipient terrestrial alteration however we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in antarctica and that alteration in an extraterrestrial environment is more probable although the presence of unaltered or incipiently altered metal grains in these fine grained rims could be interpreted as evidence for preaccretionary alteration we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

isotopic and chemical signatures of sedimentary smectite and diagenetic clinoptilolite of a lacustrine neogene basin near bigadic western turkey abstract the major element chemistry and the rare earth element ree distribution of sedimentary smectite and diagenetic clinoptilolite from the lacustrine volcanic sedimentary succession near bigadic in northwestern anatolia turkey suggest a constant influx of volcanogenic material into the basin with especially a systematic negative eu anomaly this is supported by the sr isotope chemistry which shows slight differences in the 87 sr 86 sr of the sedimentary phases carbonates and smectites with values between 0 70835 7and0 70859 9 and the diagenetic clinoptilolite with values between0 70861 8and0 70893 17 this lacustrine volcanic sedimentary succession is early to middle miocene in age with k ar values for the biotite and hornblende of the basal volcanic rocks showing an average age of 22 ma the k ar data of the clinoptilolite are strongly dependent on the permeability of the tuffaceous host rocks they suggest an age of 14 ma for the end of the diagenetic recrystallization of the clinoptilolite in impermeable rocks and further chemical exchange with the interstitial fluids in the permeable rocks this latter process induced a loss of radiogenic 40 ar and ree out of the zeolites from the permeable tuffs and might have had an influence on their chemical composition

the cycling and redox state of nitrogen in the archaean ocean organisms that produce oxygen through photosynthesis existed during the late archaean eon about 2 500 million years ago but controversial evidence suggests that they may have evolved several hundred million years earlier oxygen is expected to react with oceanic nitrogen altering its redox state the reaction leaves a signature in the isotopic composition of the nitrogen bound in organic matter here we present a record of the nitrogen isotopic composition of kerogen extracted from minimally altered shales from the campbellrand malmani platform in south africa between the palaeo archaean and about 2 670 million years ago the 15n values of the kerogen rose by about 2 we interpret this increase as an indication of the onset of coupled nitrification and denitrification or anammox reactions in the surface oceans which require the presence of free oxygen a second increase in nitrogen isotopic composition around 2 520 million years ago implies instability of the n cycle with loss of fixed n this evidence for available oxygen in the oceans occurs at least 200 million years before geochemical indications of the presence of significant levels of atmospheric oxygen we suggest that coupled nitrification and denitrification drove the loss of fixed inorganic nitrogen leading to nitrogen limitation and conclude that the low levels of biologically available nitrogen limited the growth of oxygen producing plankton delaying the accumulation of oxygen in the atmosphere the initial production of oxygen in early earth s oceans altered the redox chemistry and cycling of nitrogen a record of nitrogen isotopes from preserved organic matter indicates nitrogen cycling in the presence of free oxygen 2 67 billion years ago about 200 million years before the first geochemical evidence for atmospheric free oxygen

physical properties of martian meteorites porosity and density measurements martian meteorites are fragments of the martian crust these samples represent igneous rocks much like basalt as such many laboratory techniques designed for the study of earth materials have been applied to these meteorites despite numerous studies of martian meteorites little data exists on their basic structural characteristics such as porosity or density information that is important in interpreting their origin shock modification and cosmic ray exposure history analysis of these meteorites provides both insight into the various lithologies present as well as the impact history of the planets surface we present new data relating to the physical characteristics of twelve martian meteorites porosity was determined via a combination of scanning electron microscope sem imagery image analysis and helium pycnometry coupled with a modified archimedean method for bulk density measurements our results show a range in porosity and density values and that porosity tends to increase toward the edge of the sample preliminary interpretation of the data demonstrates good agreement between porosity measured at 100 and 300 magnification for the shergottite group while others exhibit more variability in comparison with the limited existing data for martian meteorites we find fairly good agreement although our porosity values typically lie at the low end of published values surprisingly despite the increased data set there is little by way of correlation between either porosity or density with parameters such as shock effect or terrestrial residency further data collection on additional meteorite samples is required before more definitive statements can be made concerning the validity of these observations

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

pervasive oxygenation along late archaean ocean margins the photosynthetic production of oxygen in the ocean is thought to have begun at least 2 7 billion years ago the geochemistry of marine sediments deposited 2 6 billion years ago suggests that ocean margins were oxygenated at least 100 million years before the first significant increase in atmospheric oxygen concentrations

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

water extractable and exchangeable phosphate in martian and carbonaceous chondrite meteorites and in planetary soil analogs abstract aqueous extraction contributes to the formation and weathering of planetary materials and renders electrolytes such as phosphate available for biology in this context the solubility of phosphate is measured in planetary materials represented by the mars meteorites nakhla dar al gani 476 dag 476 elephant morraine 79001 eeta 79001 and terrestrial analogs and in the murchison cm2 and allende cv3 carbonaceous chondrites the mars meteorites contain high levels of phosphate that is readily extracted by water up to 15 mg kg 1 in nakhla and dag 476 and 38 mg kg 1 in eeta 79001 while the terrestrial analogs and the carbonaceous chondrites contain 0 5 to 6 mg kg 1 correspondingly high phosphate concentrations of 4 to 28 mg l 1 are obtained in extracts of the mars meteorites at high solid solution ratios exceeding the concentrations of 0 4 to 2 0 mg l 1 in the extracts of the terrestrial analogs a wide range of planetary conditions including n 2 and co 2 atmospheres solid solution ratios of 0 01 to 1 0 kg l 1 extraction times of 1 to 21 d and temperatures of 20 to 121 c affect the amounts of extractable phosphate by factors of only 2 to 5 in most materials phosphate fixing capacity and exchangeable phosphate are assessed by the isotopic exchange kinetics iek method which quantifies the amount of p isotopically exchangeable within 1 min e 1min and between 1 min and 3 months e 1min 3m and the amount of p that cannot be exchanged within 3 months e 3m the iek results show that the dag 476 mars meteorite and terrestrial analogs have low p fixing capacities while the carbonaceous chondrites have high p fixing capacities aqueous processing under early planetary co 2 atmospheres has large effects on the available phosphate for example the fraction of total p that is exchangeable in 3 months increases from 1 6 to 11 13 to 51 6 and 43 9 to 90 4 in the dag 476 mars meteorite allende and murchison respectively the results show that solutions with high phosphate concentrations can form in the pores of planetary lava ash and basalts and in carbonaceous asteroids and meteorites these solutions can help prebiotic synthesis and early microbial nutrition the martian and carbonaceous chondrite materials contain sufficient phosphate for space based agriculture

the waning stage of a greenstone belt the mesoarchaean mosquito creek basin of the east pilbara western australia abstract the mosquito creek basin stands apart in the archaean granite greenstone terrane of the east pilbara craton because of the rectangular outcrop but crescent shaped subcrop and its composition of low grade metamorphic mainly turbiditic clastic sedimentary rocks these sediments belong to the c 2 9 ga mosquito creek formation tectonic interpretations of the basin have varied from a simple synclinorium to an accretionary prism an extensional passive margin and more recently into a rift basin separating the kurrana terrane from the east pilbara terrane ept combined sedimentological and structural geological investigations show coarse clastic fan delta marginal facies with stacked unconformities rimming an e w basin centre with predominantly finer grained mass flow dominated sediments a n s traverse therefore contains a complete cross section of the mosquito creek basin shortened to about 50 of its original width by tight folding and thrusting the basin fill shows one major fu sequence from fan delta sandstone to distal turbidites unconformably overlying an older partially preserved clastic sequence of comparable facies a large slump unit occupies the basin axis the unconformities along the margins interfere with folding of the sediments which also influenced basin centre turbiditic palaeocurrents the basin is interpreted as a near symmetrical underfilled compressional intramontane clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low angle shear zones compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes this occurred after phases of low angle extensional and compressional deformation of the substrate not related to the present day geometry of the granitoid domes the geological setting of the mosquito creek basin does not differ fundamentally from that of other adjacent e woriented greenstone belts of the ept the basin is inferred to represent the last stratigraphically uppermost and because of subsequent erosion rarely preserved stage of greenstone belt evolution

spatial and temporal trends in precambrian nitrogen cycling a mesoproterozoic offshore nitrate minimum abstract fixed nitrogen is an essential nutrient for eukaryotes as n 2 fixation and assimilation of nitrate are catalyzed by metalloenzymes it has been hypothesized that in mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses restricting eukaryotes to near shore environments and limiting their evolutionary innovation to date this hypothesis has only been tested in the belt supergroup 1 4 ga with results that support an onshore offshore nitrate gradient as a potential control on eukaryote ecology here we present bulk nitrogen and organic carbon isotopic data from non isochronous cross basinal facies across the bangemall 1 5 ga and the roper 1 4 1 5 ga basins to better understand the extent and variability of onshore offshore nitrogen isotope gradients in the mesoproterozoic both basins show an average 1 2 enrichment in 15 n bulk from deep to shallow facies with a maximum range from 1 offshore to 7 5 onshore unlike the belt basin the bangemall and roper basins show some offshore 15 n bulk values that are enriched beyond the isotopic range associated with biological n 2 fixation alone this suggests a mixture of aerobic and anaerobic metabolisms offshore in shallow waters where 15 n bulk enrichment peaks an aerobic nitrogen cycle was evidently operating even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the bangemall and roper basins our data are consistent with a lateral gradient in nitrate availability within the photic zone with higher concentrations in near shore environments than offshore the variability in 15 n bulk values in each depositional environment and the consistently low 15 n bulk values from mesoproterozoic units compared to the paleoproterozoic and neoproterozoic suggest that nitrate concentrations in the global ocean were likely low this trend is now seen in all three mesoproterozoic basins so far examined and contrasts with the paleoproterozoic and neoproterozoic where nearly all 15 n bulk data plot above the n 2 fixation window thus we propose that the mesoproterozoic ocean was characterized by a nitrate minimum with the lowest concentrations in offshore environments this inference is consistent with a mesoproterozoic o 2 decline following a temporary paleoproterozoic o 2 peak and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near shore environments delaying their rise to ecological dominance until the neoproterozoic era

metamorphic grade and gradient from white k micas of na mica bearing sedimentary rocks in the mosquito creek basin east pilbara craton western australia abstract shales and phyllites from the turbidite sequences of the 2 9 ga mosquito creek formation of the east pilbara western australia contain varying amounts of paragonite and mixed na k micas mnkms the 0 0 l x ray diffraction reflections of which are unresolved from the 10 a reflections and only partly resolved from the 5 a reflections of white k mica wkm the kubler index crystallinity the full width at half maximum fwhm of the wkm obtained from these composite reflections by applying a three peak deconvolution procedure reveals a metamorphic zoning of the mosquito creek formation the highest epimetamorphic grade occurs in the largely na mica free southern part with lower medium to high anchimetamorphic grades in the central part notably in a wsw ene anticlinal zone extending from nullagine to the blue spec mine the na mica free metasediments of the glen herring shale of the fortescue group overlying the mosquito creek formation to the w show only a slightly lower metamorphic grade the low b0 lattice parameter of the wkms indicates a very low metamorphic p t gradient the na mica bearing metasediments of the mosquito creek formation correspond to a kaolinite bearing protolith strongly al enriched and k depleted with respect to the presumably granitic tonalitic source rock

effective radium 226 concentration in meteorites the analysis of noble gases in meteorites provides constraints on the early solar system and the pre solar nebula this requires a better characterization and understanding of the capture production and release of noble gases in meteorites the knowledge of transfer properties of noble gases for each individual meteorite could benefit from using radon 222 radioactive daughter of radium 226 the radon 222 emanating power is commonly quantified by the effective radium 226 concentration ecra the product of the bulk radium 226 concentration and of the emanation coefficient e which represents the probability of one decaying radium 226 to inject one radon 222 into the free porous network owing to a non destructive high sensitivity accumulation method based on long photomultiplier counting sessions we are now able to measure ecra of meteorite samples which usually have mass smaller than 15 g and ecra 0 5 bq kg 1 we report here the results obtained from 41 different meteorites based on 129 measurements on 70 samples using two variants of our method showing satisfactory repeatability and a detection limit below 10 2 bq kg 1 for a sample mass of 1 g while two meteorites remain below detection level we obtain for 39 meteorites heterogeneous ecra values with mean min max range of ca 0 1 0 018 1 30 bq kg 1 carbonaceous chondrites exhibit the largest ecra values and eucrites the smallest such values are smaller than typical values from most terrestrial rocks but comparable with those from archean rocks mean of ca 0 18 bq kg 1 an end member of terrestrial rocks using uranium concentration from the literature e is inferred from ecra for all the meteorite samples values of e for meteorites mean 40 4 are higher than e values for archean rocks and reported values for lunar and martian soils exceptionally large e values likely suggest that the 238u 226ra pair would not be at equilibrium in most meteorites and that uranium and or radium are most likely not uniformly distributed ecra of meteorites is correlated with e and seems to mainly reflect the gas permeability of the meteorite which could be one important property preserved in the meteorite of its parent body characterizing its history in space possibly modified by alteration shock metamorphism and eventually weathering on earth larger radon emanation values are associated with larger concentrations of the heaviest noble gases argon krypton xenon and larger 20ne 22ne and 36ar 38ar ratios suggesting earth s atmosphere contamination or solar wind implantation and probably a similar carrier phase such as q phase an unclear correlation is observed with 40ar which may rule out a purely radiogenic effect on radon emanation thus larger radon emanation suggests a larger capacity of collecting solar and terrestrial gases which should imply higher loss of gases generated in the meteorite and larger dispersion of pb u ratios for age determination this study provides the first quantification of natural radon 222 loss from meteorites and opens promising perspectives to quantify the relationship between pore space connectivity and the transfer properties for noble gases in meteorites and other extraterrestrial bodies

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

an aem tem study of weathering and diagenesis abert lake oregon i weathering reactions in the volcanics abstract abert lake in south central oregon provides a site suitable for the study of sequential weathering and diagenetic events in this first of two papers transmission electron microscopy was used to characterize the igneous mineralogy subsolidus alteration assemblage and the structural and chemical aspects of silicate weathering reactions that occur in the volcanic rocks basalts basaltic andesites and dacitic rhyolitic extrusive and pyroclastics that outcrop around the lake olivine and pyroxene replacement occurred topotactically whereas feldspar and glass alteration produced randomly oriented smectite in channels and cavities the tetrahedral octahedral and interlayer compositions of the weathering products largely dioctahedral smectites varied with primary mineral composition rock type and as the result of addition of elements released from adjacent reaction sites weathering of the highly evolved fe rich jug mountain complex at the north end of the lake produced a homogeneous smectite assemblage that contrasts with the heterogeneous smectite assemblage replacing the volcanics along the eastern margin of the lake the variability within and between the smectite assemblages highlights the microenvironmental diversity fluctuating redox conditions and variable solution chemistry associated with mineral weathering reactions in the surficial environment late stage exhalative and aqueous alteration of the volcanics redistributed many components and formed a variety of alkali and alkali earth carbonate chloride sulfate and fluoride minerals in vugs and cracks overall substantial mg si na ca and k are released by weathering reactions that include the almost complete destruction of the mg smectite that initially replaced olivine the leaching of these elements from the volcanics provides an important source of these constituents in the lake water the nature of subsequent diagenetic reactions resulting from the interaction between the materials transported to the lake and the solution will be described in part ii banfield et al 1991

chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in cv chondrites constraints on the evolution of nebular dust abstract a correlation of petrography mineral chemistry and in situ oxygen isotopic compositions of fine grained olivine from the matrix and of fine and coarse grained olivine from accretionary rims around ca al rich inclusions cais and chondrules in cv chondrites is used here to constrain the processes that occurred in the solar nebula and on the cv parent asteroid the accretionary rims around leoville vigarano and allende cais exhibit a layered structure the inner layer consists of coarse grained forsteritic and 16o rich olivine fa1 40 and 17o 24 to 5 the higher values are always found in the outer part of the layer and only in the most porous meteorites whereas the middle and the outer layers contain finer grained olivines that are more fayalitic and 16o depleted fa15 50 and 17o 18 to 1 the cv matrices and accretionary rims around chondrules have olivine grains of textures chemical and isotopic compositions similar to those in the outer layers of accretionary rims around cais there is a correlation between local sample porosity and olivine chemical and isotopic compositions the more compact regions the inner accretionary rim layer have the most mgo and 16o rich compositions whereas the more porous regions outer rim layers around cais accretionary rims around chondrules and matrices have the most mgo and 16o poor compositions in addition there is a negative correlation of olivine grain size with fayalite contents and 17o values however not all fine grained olivines are feo rich and 16o poor some small 20 olivine grains in the outer layers of the cai accretionary rims and in the matrix show significant enrichments in 16o 17o 20 to 10 we infer that the inner layer of the accretionary rims around cais and at least some olivine grains in the finer portions of accretionary rims and cv matrices formed in an 16o rich gaseous reservoir probably in the cai forming region grains in the outer layers of the cai accretionary rims and in the rims around chondrules as well as matrix may have also originated as 16o rich olivine however these olivines must have exchanged o isotopes to variable extents in the presence of an 16o poor reservoir possibly the nebular gas in the chondrule forming region s and or fluids in the parent body the observed trend in isotopic compositions may arise from mixtures of 16o rich forsterites with grain overgrowths or newly formed grains of 16o poor fayalitic olivines formed during parent body metamorphism however the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of 16o rich forsterite and subsequently exchanged fe mg and o isotopes in situ in the presence of aqueous solutions i e fluid assisted thermal metamorphism

the porosity and permeability of chondritic meteorites and interplanetary dust particles abstract we have investigated the porosity of a large number of chondritic interplanetary dust particles idps and meteorites by three techniques standard liquid gas flow techniques a new noninvasive ultrasonic technique and image processing of backscattered images the latter technique is obviously best suited to sub kilogram sized samples we have also measured the gas and liquid permeabilities of some chondrites by two techniques standard liquid gas flow techniques and a new nondestructive pressure release technique we find that chondritic idps have a somewhat bimodal porosity distribution peaks are present at 0 and 4 porosity a tail then extends to 53 type 1 3 chondrite matrix porosities range up to 30 with a peak at 2 the bulk porosities for type 1 3 chondrites have the same approximate range as exhibited by the matrix which indicates that other components of the bulk meteorites including chondrules and aggregates have the same average porosity as the matrix these results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar which implies that similar accretion dynamics operated through 12 orders of size magnitude permeabilities of the investigated chondrites vary by several orders of magnitude and there appears to be no simple dependence of permeability with degree of aqueous alteration chondrite type or porosity

interpretation of the nitrogen isotopic composition of precambrian sedimentary rocks assumptions and perspectives nitrogen isotope compositions in sedimentary rocks d 15 n sed are routinely used for reconstructing cenozoic n biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments here we review the existing knowledge and rationale behind the use of d 15 n sed as a proxy for the precambrian n biogeochemical cycle with the aims of i identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks ii developing a framework for interpreting the precambrian d 15 n sed record iii testing this framework against a database of precambrian d 15 n sed values compiled from the literature and iv identifying avenues of focused research that should increase confidence in interpreting precambrian d 15 n sed data this review highlights the intrinsic complexity of the d 15 n sed proxy and the significant effort that remains to realize its potential specifically it is crucial to gain a better understanding of how and when diagenesis and metamorphism affect the d 15 n of bulk and kerogen bound nitrogen ultimately more data are required to apply statistics to interpreting d 15 n sed variability within given geological time intervals finally numerical modeling of the d 15 n sed variability expected in different environments under varying redox scenarios is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record in spite of the challenges facing the application of this proxy to the precambrian the existing d 15 n sed record shows several features possibly related to the stepwise oxygenation of the surface environment underlining the potential for nitrogen isotopes to reveal clues about the evolution of early earth

strontium isotopes of tertiary phillipsites from the southern pacific timing of the geochemical evolution abstract the rb sr system of phillipsites from an indurated volcano sedimentary mass sampled in the southern pacific ocean became closed long after the deposition of the host rock middle miocene 14 7 3 3 ma and middle eocene respectively this isotopic age of about 15 ma probably represents the end of an induration phenomenon which stopped the chemical exchanges between the zeolites and their environment these minerals inherited sr primarily from volcanic material as demonstrated by their initial 87sr 86sr ratio at 0 70441 0 00036 2 which plots within the values of the known 87sr 86sr ratios of the nearby basalts the leaching technique with diluted acid was checked by recalculation of the rb and sr contents of the leachates and residues according to their percentage in the untreated sample and comparison with their amounts in the corresponding untreated sample any discrepancy is in most cases smaller than 3 5 percent for both rb and sr amounts this technique may therefore be used to obtain a correct amount of the distribution of these elements among the soluble and insoluble fractions the examination of the leachates also showed that the volcano sedimentary material forming the cores of nodules from the water sediment interface experienced a more complex history than the underlying analogous volcano sedimentary level

differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks abstract the last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks which have become an established tool for investigating the evolution of life and environmental conditions nitrogen is contained in sedimentary rocks in two different phases bound to kerogen or substituted in potassic minerals mostly k bearing phyllosilicates and feldspars isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases the community is split about which sample type more accurately captures the original composition of biomass to address this question we combined nitrogen isotopes and carbon to nitrogen ratios with carbon to hydrogen ratios which act as an independent proxy for metamorphic alteration our results reveal that metamorphism drives kerogen bound nitrogen isotopically lighter while silicate bound nitrogen becomes heavier for rocks up to greenschist facies the isotopic effect of this internal partitioning up to 3 4 is larger than the isotopic effect of metamorphic nitrogen loss from the system up to 1 2 the opposite may be true for higher metamorphic grades we conclude that for low grade sedimentary rocks with more than 60 of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by bulk rock measurements whereas for high grade rocks the kerogen extract may be the more accurate proxy the isotopic difference between nitrogen phases can serve as a rough indicator of the degree of metamorphic alteration

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 origins and evidence for diverse primitive nebular dust components abstract the mineralogical and chemical characteristics of dark matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 have been investigated in detail by scanning electron microscopy electron microprobe analysis and transmission electron microscopy matrix and fine grained rims on chondrules and other objects are compositionally homogeneous on the scale of 10 m and there are no compositional differences between matrix and rims mineralogically both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of si and fe rich amorphous material olivine pyroxene fe ni metal magnetite pentlandite pyrrhotite anhydrite and mixed layer phyllosilicate phases several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics which appear to represent basic fine grained units of nebular dust the microstructures of these different components show that they have experienced significantly different thermal histories unlike the ordinary chondrites an origin for any significant component of the matrix or rims in alha77307 from chondrules is improbable based on compositional and microstructural evidence the matrix and rim components in alha77307 formed by disequilibrium condensation processes as fine grained amorphous dust that is represented by the abundant amorphous component in the matrix such condensation could have occurred under a variety of conditions at different times and locations within the solar nebula or possibly earlier in a circumstellar environment subsequent thermal processing of this primitive condensate material in a variety of environments in the nebula resulted in partial or complete recrystallization of the fine grained dust the intimate association of fine grained components with disparate compositions and thermal histories shows that mixing of fine grained dust within the nebula must have been extremely thorough

fabric analysis of allende matrix using ebsd fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution electron backscatter diffraction ebsd has allowed fabrics in these fine grained materials to be visualized in detail for the first time our data reveal that allende a cv3 chondrite possesses a uniform planar short axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction interference between this matrix fabric and the larger more rigid components such as dark inclusions dis and calcium aluminium rich inclusions cais has lead to the development of locally oriented and intensified matrix fabrics in addition dis possess fabrics that are conformable with the broader matrix fabric these results suggest that dis were in situ prior to the deformational shortening event responsible for these fabrics thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid kojima and tomeoka 1996 fruland et al 1978 moreover both dis and allende matrix are highly porous 25 corrigan et al 1997 mobilizing a highly porous di during impact induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable we favor a model that involves allende dis cais and matrix accreting together and experiencing the same deformation events

magnetization of extraterrestrial allende material may relate to terrestrial descend abstract the origin of magnetization in allende may have significant implications for our understanding of core formation differentiation dynamo processes in chondrite parent bodies the magnetic allende data may contain information that could constrain the magnetic history of allende the measurements on allende chondrules reveal an existence of magnetization component that was likely acquired during the meteorite transit to terrestrial conditions both the pyrrhotite carrying magnetic remanence intensity and direction of the chondrules change erratically when subjecting the allende meteorite s chondrules to temperatures near 77 k and back to room temperature chondrules with more intense original magnetization are denser and contain larger inverse thermoremanent magnetization itrm temperature dependent monitoring of itrm revealed that magnetization was acquired at temperature near 270 k such temperature is consistent with the condition when in addition to temperature increase the atmospheric uniaxial pressure applied during the meteorite entry on the porous material was responsible for meteorite break up in the atmosphere during this process collapse of the pore space in the matrix and some chondrules would generate crystalline anisotropy energy accumulation within pyrrhotite grains in form of parasitic magnetic transition

uranium isotope fractionation suggests oxidative uranium mobilization at 2 50ga abstract geochemical data from late archean sedimentary rocks point to photosynthetic o 2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation oxygen oases prior to the early paleoproterozoic great oxidation event for example distinctive authigenic enrichments of mo and re in euxinic anoxic and sulfidic black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater in contrast it is not clear if low u enrichments in the same shales reflect negligible oxidation of u from the upper crust or a very small oceanic u inventory that was derived from oxidative u mobilization here we report u isotope data for the 2 50 ga mt mcrae shale hamersley basin western australia which provides a more sensitive test for the presence or absence of authigenic u compared to u concentrations and enrichment factors normalized to average shale or upper crustal compositions that may not be representative of the local detrital composition we find instances where the u isotope composition in the upper mt mcrae shale 238 u 0 2 to 0 0 relative to standard srm950a is isotopically heavier than average upper crust 238 u 0 31 0 14 2sd based on granitoids and basalts the high 238 u values point to u isotope fractionation in the late archean marine environment and hence indicate the presence of a small amount of dissolved u in seawater and authigenic u in the mt mcrae shale volume dependent equilibrium isotope fractionation during the reduction of dissolved u vi to insoluble u iv like that observed in the modern black sea may explain the high 238 u signatures if u removal from bottom waters was not quantitative alternatively quantitative u removal would require that late archean seawater had high 238 u which could have arisen from the preferential sequestration of 235 u to fe oxyhydr oxide minerals elsewhere in the hamersley basin the supracrustal 238 u signatures are associated with some of the highest mo and re enrichments in the mt mcrae shale as well as distinctive mo s and n isotope signatures that are indicative of mild environmental oxygenation hence our findings suggest that small amounts of u were oxidatively mobilized from the upper crust at 2 50 ga unlike mo and re however u oxidation may not have occurred on land instead we hypothesize that oxidative u mobilization occurred by submarine weathering in an oxygen oasis

new insights into the formation of fayalitic olivine from allende dark inclusions abstract although considerable progress has been made in unraveling the origin s of fayalitic olivines in dark inclusions dis many questions remain still unresolved and or controversial we combine a chemical and petrographic study of the allende dark inclusion 4884 2b amnh new york and atem studies of a fragment of the dark inclusion allende af nhm vienna and discuss an alternative way in which fayalitic olivines could have formed allende dark inclusion 4884 2b contains a few aggregates with variable proportions of transparent and feathery olivine two such objects aggregates a and b are the focus of this study as they preserve glasses that can help in deciphering the nature of the processes involved during olivine growth and subsequent olivine transformation the petrographic and chemical characteristics of aggregates a and b indicate that the forsteritic stack olivines may be pseudomorphs of clear olivine crystals the atem studies in all af suggest that fayalitic olivines may be the result of secondary processes e g metasomatic exchange reactions operating in the solar nebula transformation may have occurred through the mediation of a dry gas phase involving nonvolatile major elements such as mg and fe e g dohmen et al 1998 this mechanism could reveal olivine growth patterns e g stacked platelets due to a rapid growth regime and may have contributed to the development of their fibrous aspect while preserving the shape i e volume of the crystals this highly selective process did completely or partially transform ferromagnesian minerals but affected the fine grained mesostasis only slightly

interpretation of wild 2 dust fine structure comparison of stardust aluminum foil craters to the three dimensional shape of experimental impacts by artificial aggregate particles and meteorite powders new experimental results show that stardust crater morphology is consistent with interpretation of many larger wild 2 dust grains being aggregates albeit most of low porosity and therefore relatively high density the majority of large stardust grains i e those carrying most of the cometary dust mass probably had density of 2 4 g cm 3 similar to soda lime glass used in earlier calibration experiments or greater and porosity of 25 or less akin to consolidated carbonaceous chondrite meteorites and much lower than the 80 suggested for fractal dust aggregates although better size calibration is required for interpretation of the very smallest impacting grains we suggest that aggregates could have dense components dominated by m scale and smaller sub grains if porosity of the wild 2 nucleus is high with similar bulk density to other comets much of the pore space may be at a scale of tens of micrometers between coarser denser grains successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous type c tracks in aerogel which we have observed in recent shots we are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues impacted onto both foil and aerogel under stardust encounter conditions

why aqueous alteration in asteroids was isochemical high porosity high permeability carbonaceous chondrite meteorites are the most compositionally primitive rocks in the solar system but the most chemically pristine ci1 and cm2 chondrites have experienced pervasive aqueous alteration apparently within asteroid parent bodies unfractionated soluble elements suggest very limited flow of liquid water indicting a closed system at scales large than 100 s m consistent with data from oxygen isotopes and meteorite petrography however numerical studies persistently predict large scale 10 s km water transport in model asteroids either in convecting cells or via exhalation flow an open system at scales up to 10 s km these models have tended to use permeabilites in the range 10 13 to 10 11 m2 we show that the permeability of plausible chondritic starting materials lies in the range 10 19 to 10 17 m2 0 1 10 d around six orders of magnitude lower than previously assumed this low permeability is largely a result of the extreme fine grain size of primitive chondritic materials applying these permeability estimates in numerical models we predict very limited liquid water flow distances of 100 s m at most even in a high porosity water saturated asteroid with a high thermal gradient over millions of years isochemical alteration with flow over minimal lengthscales is not a special circumstance it is inevitable once we consider the fundamental material properties of these rocks to achieve large scale flow it would require average matrix grain sizes in primitive materials of 10 s 100 s m orders of magnitude larger than observed finally in addition to reconciling numerical modelling with meteorite data our work explains several other features of these enigmatic rocks most particularly why the most chemically primitive meteorites are also the most altered

formation of syngenetic and early diagenetic iron minerals in the late archean mt mcrae shale hamersley basin australia new insights on the patterns controls and paleoenvironmental implications of authigenic mineral formation a section through the late archean mt mcrae shale comprising in ascending order a lower shale interval lsi a banded iron formation bif an upper shale usi and a carbonate c1 has been analyzed for total fe and al contents and authigenic fe present as carbonate oxide sulfide and silicate phases the authigenic mineralogy is controlled by the episodic addition of fe from hydrothermal activity and removal of fe by sulfide relative to rates of clastic sedimentation the lsi and bif have mean fet al values of 2 and 5 respectively that record iron enrichment from hydrothermal sources iron was precipitated primarily as siderite accompanied by fe rich chlorite from anoxic bottom waters rich in dissolved fe pyrite formation was probably limited by the availability of sulfate which was present at low concentrations and became rapidly depleted the usi has generally lower fet al values 0 6 1 3 similar to those found in paleozoic shales with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle this interval contains authigenic fe predominantly as pyrite where high values for dop 0 8 indicate the existence of a water column that became rich in dissolved sulfide euxinic when sulfate concentrations increased due to a transient or secular increase in ocean atmosphere oxygenation high concentrations of dissolved sulfide maintained low concentrations of dissolved fe which allowed only minor amounts of fe to be precipitated as carbonates and silicates the usi also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin the carbonate c1 represents a basinal chemistry where sulfide has been removed and fet al values are 1 indicating that hydrothermal activity again produced dissolved fe rich bottom waters detailed iron speciation of the mt mcrae shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late archean hamersley basin just prior to the paleoproterozoic rise in atmospheric oxygenation and our refined methods have relevance to all fe rich deposits

mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates abstract petrographic mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from hot deserts and antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of fe ni metal producing fe oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins troilite weathers readily in hot desert environments but undergoes very little alteration under antarctic conditions most of the primary porosity of ordinary chondrites has been occluded by the time that 15 to 25 of the initial fe 0 and fe 2 has been oxidised to fe 3 in both environments results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products dilation of primary pores and brecciation which has been observed in parts of some meteorites will only occur if the meteorite is especially metal rich or has a low primary porosity these weathering products are absent from recent falls but have formed in a fall after 100 yr of museum storage cl bearing akaganeite and hibbingite are common weathering products in antarctic finds but occur in abundance in only one hot desert meteorite daraj 014 the majority of fe rich weathering products in meteorites from both environments contain low but variable concentrations of si mg and ca in most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a 1 0 nm lattice fringe spacing where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene in the heavily weathered antarctic finds alha 78045 and 77002 si is concentrated in cronstedtite a fe rich phyllosilicate an unidentified hydrous si fe ni mg mineral or gel has also partially replaced taenite in alha 78045 in addition to fe rich weathering products hot desert meteorites contain sulphates ca carbonate and silica whereas such minerals are largely absent from antarctic finds the abundance of silicate weathering products in antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments as preterrestrial cronstedtite is abundant in cm2 carbonaceous chondrites the antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites

molybdenum isotope evidence for mild environmental oxygenation before the great oxidation event abstract we use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of o2 in the environment 50 ma before the great oxidation event goe in a high resolution profile from the 2 5 ga mt mcrae shale the molybdenum isotope compositions 98 95mo from samples throughout the sequence span a range from 0 99 to 1 86 all samples have heavier 98 95mo compared to average upper continental crust in addition the upper s1 and lower s2 black shale units within the mt mcrae shale exhibit systematic differences in average isotopic compositions and distinct patterns of 98 95mo variation heavier 98 95mo values occur in the s1 unit where 98 95mo correlates with mo enrichments in the s2 unit 98 95mo is not as heavy and is relatively invariant based on sedimentary fe proxies we infer that s1 sediments were deposited under euxinic conditions so that mo removal from bottom waters was likely quantitative thus 98 95mo in this interval likely records coeval seawater the lighter 98 95mo values in the s2 unit may indicate a less fractionated ocean mo inventory relative to the s1 unit however sedimentary fe proxies suggest that s2 sediments accumulated under a water column that was ferruginous rather than euxinic raising the possibility of non quantitative mo scavenging and hence an expressed 98 95mo fractionation relative to coeval seawater because any associated fractionations during this process would have favored the light isotope in sediments the lighter 98 95mo values in the s2 unit represent a lower limit on the value in contemporaneous seawater after evaluating a range of hypotheses we conclude that the isotopically heavy 98 95mo values seen throughout the mt mcrae shale likely reflect the effects of oxidative weathering and adsorption of mo to oxide mineral surfaces on land or in surface oceans the extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global mo isotope budget because of the small ocean mo inventory in the late archean documented by low concentrations of mo and low mo toc the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans however when juxtaposed against the record of 98 95mo through time our findings provide further evidence of the onset of environmental oxygenation before the goe

evidence for impact induced pressure gradients on the allende cv3 parent body consequences for fluid and volatile transport carbonaceous chondrites such as those associated with the vigarano cv parent body exhibit a diverse range of oxidative reduced alteration mineralogy mcsween 1977 although fluids are often cited as the medium by which this occurs rubin 2012 a mechanism to explain how this fluid migrates and why some meteorite subtypes from the same planetary body are more oxidized than others remains elusive in our study we examined a slab of the well known allende cv3oxa meteorite using several petrological techniques e g fry s and flinn and computerized tomography ct we discover it exhibits a strong penetrative planar fabric resulting from strain partitioning among its major components calcium aluminum rich inclusions cais 64 5 ct matrix 21 5 fry chondrules 17 6 ct in addition to the planar fabric we found a strong lineation defined by the alignment of the maximum elongation of flattened particles interpreted to have developed by an impact event the existence of a lineation could either be non coaxial deformation or the result of a mechanically heterogeneous target material in the later case it could have formed due to discontinuous patches of sub surface ice and or fabrics developed through prior impact compaction macpherson and krot 2014 which would have encouraged preferential flow within the target material immediately following the impact compacting pore spaces we suggest that structurally controlled movement of alteration fluids in the asteroid parent body along pressure gradients contributed to the formation of secondary minerals which may have ultimately lead to the different oxidized subtypes

chemistry and origin of zeolites in sandstones at dsdp sites 445 and 446 daito ridge and basin province northwest pacific abstract andesitic volcaniclastic sandstones of middle eocene age recovered from dsdp sites 445 and 446 a back arc basin contain clinoptilolite heulandite and analcite as a pore filling cement clinoptilolite and heulandite at sites 445 and 446 contain different chemical composition from other deep sea clinoptilolites and heulandites the dominant cation of both clinoptilolite and heulandite is na ion activity in pore water was greater at the time of their formation volcanic glass has acted as a precursor for both clinoptilolite and heulandite at sites 445 and 446 biogenic silica is also suggested as a precursor high na ion activity in pore water also helped to transform clinoptilolite and heulandite to analcite downhole the necessary na ions for this clinoptilolite heulandite and analcite at sites 445 and 446 might have been provided by hydrothermal circulation of seawater through more permeable sandstones deposited during early stages of back arc basin rifting

heating effects of the matrix of experimentally shocked murchison cm chondrite comparison with micrometeorites micrometeorites have been significantly altered or melted by heating which has been mainly ascribed to aerodynamic drag during atmospheric entry however if a major fraction of micrometeorites are produced by impacts on porous asteroids they may have experienced shock heating before contact with the earth s atmosphere tomeoka et al 2003 a transmission electron microscope tem study of the matrix of murchison cm chondrite experimentally shocked at pressures of 10 49 gpa shows that its mineralogy and texture change dramatically mainly due to shock heating with the progressive shock pressures tochilinite is completely decomposed to an amorphous material at 10 gpa fe mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 gpa and is completely decomposed at 36 gpa at 49 gpa the matrix is extensively melted and consists mostly of aggregates of equigranular grains of fe rich olivine and less abundant low ca pyroxene embedded in si rich glass the mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites in particular the samples shocked at 10 and 21 gpa are similar to the phyllosilicate serpentine rich micrometeorites and the sample shocked at 49 gpa is similar to the olivine rich micrometeorites the shock heating effects also resemble the effects of pulse heating experiments on the ci and cm chondrite matrices that were conducted to simulate atmospheric entry heating we suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric entry heating processes

analcimic zones in the tertiary of anatolia and their geological positions abstract the anatolian tertiary and especially the neogene have some geological series with analcime as the principal rock forming mineral sedimentation basins receiving material in suspension and in solution permit the authigenesis of analcime dolomite calcite and opal c t analcimes formed by hydrothermal alteration on the ocean floor after the pillow lava and diabasic dyke complexes of ophiolites can form a source of detrital analcime when they are on the continental medium continental alteration of the pillow lavas and diabasic dykes permits the formation of solutions suitable in composition for analcime authigenesis analcimes formed in these environments are particularly related to the silica content of these media principal clay minerals formed with analcimic formations are smectites and 14 s 14 c mixed layers illite chlorite and corrensite are also present in smaller quantities

small is beautiful the analysis of nanogram sized astromaterials the capability of modern methods to characterize ultra small samples is well established from analysis of interplanetary dust particles idps interstellar grains recovered from meteorites and other materials requiring ultra sensitive analytical capabilities powerful analytical techniques are available that require under favorable circumstances single particles of only a few nanograms for entire suites of fairly comprehensive characterizations a returned sample of 1000 particles with total mass of just 1 g permits comprehensive quantitative geochemical measurements that are impractical to carry out in situ by flight instruments the main goal of this paper is to describe the state of the art in microanalysis of astromaterials given that we can analyze fantastically small quantities of asteroids and comets etc we have to ask ourselves how representative are microscopic samples of bodies that measure a few to many kilometers across with the galileo flybys of gaspra and ida it is now recognized that even very small airless bodies have indeed developed a particulate regolith acquiring a sample of the bulk regolith a simple sampling strategy provides two critical pieces of information about the body regolith samples are excellent bulk samples because they normally contain all the key components of the local environment albeit in particulate form furthermore because this fine fraction dominates remote measurements regolith samples also provide information about surface alteration processes and are a key link to remote sensing of other bodies studies indicate that a statistically significant number of nanogram sized particles should be able to characterize the regolith of a primitive asteroid although the presence of larger components e g chondrules calcium aluminum rich inclusions large crystal fragments etc within even primitive meteorites e g murchison points out the limitations of using data obtained from nanogram sized samples to characterize entire primitive asteroids however the most important asteroidal geological processes have left their mark on the matrix because this is the finest grained portion and therefore most sensitive to chemical and physical changes thus the following information can be learned from this fine grain size fraction alone 1 mineral paragenesis 2 regolith processes 3 bulk composition 4 conditions of thermal and aqueous alteration if any 5 relationships to planets comets meteorites via isotopic analyses including o 6 abundance of water and hydrated material 7 abundance of organics 8 history of volatile mobility 9 presence and origin of presolar and or interstellar material most of this information can be obtained even from dust samples from bodies for which nanogram sized samples are not truly representative future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano to microgram sized samples even further this highlights a key advantage of sample returns that the most advanced analysis techniques can always be applied in the laboratory and that well preserved samples are available for future investigations

iron and iron nickel sulfides in chondritic interplanetary dust particles abstract we report the results of a study of iron and iron nickel sulfides in chondritic anhydrous and hydrated interplanetary dust particles idps the idps were characterized using chemical and mineralogical techniques including energy dispersive x ray analyses and transmission electron microscopy it is widely believed that the first sulfur containing mineral to form in the early solar nebula should have been stoichiometric fes troilite and that pyrrhotite is of secondary origin all examined interplanetary dust particles both anhydrous and hydrous types contain more pyrrhotite than troilite suggesting that the former was a common possibly predominant nebular condensate phase pyrrhotite could form from sulfidation of troilite after the latter had exfoliated from substrate metal grains in contrast to anhydrous idps hydrated ones contain ni rich sulfides including pentlandite which are probably of secondary origin

dark inclusions in the mokoia cv3 chondrite evidence for aqueous alteration and subsequent thermal and shock metamorphism mokoia is a cv3 chondrite that contains abundant phyllosilicate mineralization we present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions dis that we found in mokoia the overall texture and constituent minerals of the dis resemble those in the host meteorite fe bearing saponite and na rich phlogopite the same phyllosilicates as in the host meteorite occur in the dis which strongly suggests that the dis have a similar alteration history to the host meteorite however the dis show several distinct differences from the host meteorite olivine grains in the di matrices are more homogeneous in fe fe mg ratio than those in the host meteorite matrix phyllosilicates in the dis are less abundant than in the host meteorite and they have been dehydrated to various extents these characteristics suggest that the dis have experienced higher degree of thermal metamorphism than the host meteorite in addition the matrices in the dis are more compacted than those in the host meteorite most olivine grains in the dis show undulatory extinction in transmitted crossed polarized light and some show planar fractures while such olivine grains are rare in the host meteorite two of the dis contain si mg fe and o rich melt veins these characteristics indicate that most dis have been shocked to shock stage s3 s4 while the host meteorite is shock stage s1 virtually unshocked thermal metamorphism of the dis was likely caused by shock heating these results are consistent with the contention previously proposed for the dis in cv3 chondrites i e the dis have experienced aqueous alteration and subsequent dehydration on the cv parent body we suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the mokoia parent body

isotope fractionation during nitrogen remineralization ammonification implications for nitrogen isotope biogeochemistry abstract the 15 n 14 n isotopic ratio of marine particulate organic matter usually increases as a consequence of preferential 14 n remineralization ammonification during decay in the oxic water column and at the oxic seafloor however a fractionation factor 15 e of sediment ammonification in natural environments has not been determined yet here the 15 e of sedimentary ammonification is calculated in a set of seven sapropel containing sediment cores two subrecent sediment cores and four sediment trap deployments from the eastern mediterranean sea data clearly reveal rayleigh type isotope fractionation and a fractionation factor 15 e between 1 43 and 2 3 hence sediments and probably sinking particulate organic matter as well can be important sources of isotopically depleted reactive nitrogen the knowledge of 15 e further enables to reconstruct paleo export fluxes from sediment cores if original and actual 15 n as well as n accumulation rates are known

experimental shock metamorphism of the murchison cm carbonaceous chondrite abstract a series of shock recovery experiments were carried out on the murchison cm carbonaceous chondrite by using a single stage propellant gun the murchison samples were shocked in nine experiments at peak pressures from 4 to 49 gpa the recovered samples were studied in detail by using an optical microscope a scanning electron microscope and an electron probe microanalyzer chondrules are flattened in the plane of the shock front at 4 to 30 gpa the mean aspect ratio of chondrules increases from 1 17 to 1 57 roughly in proportion to the intensity of shock pressure up to 25 gpa at 25 to 30 gpa the mean aspect ratio does not increase further and chondrules show increasingly more random orientations and degrade their preferred orientations and at 35 gpa they are extensively disrupted most coarse grains of olivine and pyroxene are irregularly fractured fracture density increases with increasing shock pressure and at 30 gpa almost all are thoroughly fractured with subgrains of local shock melting occurs as melt veins and pockets at 20 to 30 gpa fracture filling veins of fine grains of matrix are also produced at 25 to 30 gpa the melts and the fine grains seem to result mainly from frictional heating due to displacement along fractures at 35 gpa melting occurs pervasively throughout the matrix the melts are mainly produced from the matrix however they are consistently more enriched in fe s and ca which indicates that these elements are selectively incorporated into the melts the melts contain tiny spherules of fe ni metal fe sulfide and numerous vesicles at 49 gpa the matrix is totally melted and coarse grains of olivine are partially melted the melts contain much larger vesicles 50 300 m in diameter than those in the samples shocked at lower pressures which indicates that much more intense devolatilization and gas expansion took place for the purpose of comparing shock thermal effects between the experimentally shocked samples and naturally shocked targets surface materials in the murchison parent body we calculated internal energy increase for compression by multiple shock wave reflections experimental case and for compression by a single shock wave natural case the results suggest that postshock thermal effects observed at each experiment may be attained by impact on the natural targets at a considerably lower shock pressure than the peak shock pressure from the results of our experiments and calculations we conclude that if the murchison parent body were shocked on the surface at pressures higher than 25 gpa shocked material would probably undergo drastic increase in the degree of comminution and simultaneous generation of strong expansive forces on pressure release thus the results support the hypothesis of scott et al 1992 that volatile rich carbonaceous chondrites shocked above 20 to 30 gpa escaped from the parent body and formed particles that are too small to survive as meteorites

identification of an archean marine oxygen oasis abstract the early earth was essentially anoxic a number of indicators suggest the presence of oxygenic photosynthesis 2700 3000 million years ma ago but direct evidence for molecular oxygen o2 in seawater has remained elusive here we report rare earth element ree analyses of 2800 million year old shallow marine limestones and deep water iron rich sediments at steep rock lake canada these show that the seawater from which extensive shallow water limestones precipitated was oxygenated whereas the adjacent deeper waters where iron rich sediments formed were not we propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species fe ii to insoluble fe iii oxyhydroxide and estimate that at least 10 25 m oxygen concentration in seawater was required to accomplish this at steep rock this agrees with the hypothesis that an ample supply of dissolved fe ii in archean oceans would have hindered limestone formation there is no direct evidence for the oxygen source at steep rock but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria our findings support the view that during the archean significant oxygen levels first developed in protected nutrient rich shallow marine habitats they indicate that these environments were spatially restricted transient and promoted limestone precipitation if archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron rich seas then early oxygen oases are less elusive than has been assumed

a terrestrial origin for sulfate veins in ci1 chondrites white sulfate veins are a very well known petrological feature of the chemically primitive ci1 carbonaceous chondrites sulfate veins were first described in the orgueil meteorite in 1961 almost one century after its fall however we have observed such veins to form easily during typical sample storage we suggest that all ci1 sulfate veins formed during the terrestrial residence of these heavily brecciated porous stones reacting with atmospheric water sulfates originally present in the meteorites dissolved and remobilized and or sulfides oxidised filling the many open spaces offered to them by the very porous rock sulfate veins in ci1 chondrites can no longer be used as evidence of a late stage oxidation event in the ci1 parent body or of centimeter scale fluid transport on the parent asteroid

geological mineralogical and geochemical characteristics of zeolite deposits associated with borates in the bigadic emet and kirka neogene lacustrine basins western turkey the bigadic emet and kirka lacustrine basins of western turkey may be considered as tibet type graben structures that were developed during the miocene within the izmir ankara suture zone complex the volcanic sedimentary successions of these basins are made up of mudstone carbonate limestone and dolomite and detrital rocks and also of crystal or vitric tuffs about 135 to 200 m thick the degirmenli bigadic emirler bigadic kopenez emet and karaoren kirka tuffs constituting the zeolite deposits are situated beneath four borate deposits colemanite ulexite borax the most abundant diagenetic silicate minerals are k and ca clinoptilolites in the zeolite deposits and li rich trioctahedral smectites stevensite saponite and hectorite and k feldspar in the borate deposits in the degirmenli emirler kopenez and karaoren deposits the following diagenetic facies were developed from rhyolitic glasses rich in k and poor in na glass smectite k clinoptilolite opal ct caclinoptilolite k feldspar analcime quartz and k feldspar analcime quartz k feldspar which is also rarely associated with phillipsite karaoren and heulandite degirmenli and karaoren succeeds clinoptilolite and precedes analcime in these diagenetic facies where dioctahedral smectites opal ct and quartz are the latest minerals no diagenetic transformations exist between clinoptilolite k feldspar and analcime that were formed directly from glass the lateral facies distributions resulted from the differences in salinity and ph of pore water trapped during deposition of the tuffs but vertical distributions in vitric tuffs seem to have been controlled by the glass liquid ratio of the reacting system and the permeability or diffusion rate of alkali elements the bigadic emet and kirka zeolite deposits which were formed in saline basins rich in ca and mg ions show similar chemical changes i e loss of alkalis and gain in alkaline earth elements that have taken place during the diagenetic transformation of rhyolitic glasses to dioctahedral smectites or clinoptilolite the absence of sodic zeolites such as mordenite erionite chabazite and silica rich phillipsite is mainly due to the very high k na ratio of the starting materials rather than initial alkaline conditions or high na content in lake waters

the formation of ca fe rich silicates in reduced and oxidized cv chondrites the roles of impact modified porosity and permeability and heterogeneous distribution of water ices cv vigarano type carbonaceous chondrites comprising allende like cvoxa and bali like cvoxb oxidized and reduced cvred subgroups experienced differing degrees of fluid assisted thermal and shock metamorphism the abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups 1 ferroan olivine and diopside hedenbergite solid solution pyroxenes are common in all cvs 2 nepheline and sodalite are abundant in cvoxa rare in cvred and absent in cvoxb 3 phyllosilicates and nearly pure fayalite are common in cvoxb rare in cvred and virtually absent in cvoxa 4 andradite magnetite and fe ni sulfides are common in oxidized cvs but rare in reduced cvs the latter contain kirschsteinite instead thus a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the ca fe rich silicates pyroxenes andradite kirschsteinite among the cvox and cvred meteorites the extent of secondary mineralization was controlled by the distribution of water ices permeability and porosity which in turn were controlled by impacts on the asteroidal parent body more intense shock metamorphism in the region where the reduced cvs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids the mineralogy petrography and bulk chemical compositions of both the reduced and oxidized cv chondrites indicate that mobile elements were redistributed between ca al rich inclusions dark inclusions chondrules and matrices only locally there is no evidence for large scale several cm fluid transport published 53mn 53cr ages of secondary fayalite in cv co and unequilibrated ordinary chondrites and carbonates in ci cm and cr chondrites are consistent with aqueous alteration initiated by heating of water ice bearing asteroids by decay of 26al not shock metamorphism

stony meteorite porosities and densities a review of the data through 2001 in this review we summarize the data published up to december 2001 on the porosity and density of stony meteorites these data were taken from 925 samples of 454 different meteorites by a variety of techniques most meteorites have densities on the order of 3 to 4 g cm 3 with lower densities only for some volatile rich carbonaceous meteorites and higher densities for stony irons for the vast majority of stones porosity data alone cannot distinguish between different meteorite compositions average porosities for most meteorite classes are around 10 though individual samples can range as high as 30 porosity unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present the measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment a theoretical grain density based on composition can model pre weathered porosities the average model porosity for h and ll chondrites is 10 while l chondrite model porosities average only 6 a statistically significant difference

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis abstract the carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical processes in marine and lacustrine environments in order to reconstruct past environmental changes from sedimentary organic matter it is crucial to consider potential alteration of the primary isotopic signal by bacterial degradation in the water column and during early diagenesis in the sediments in a series of oxic and anoxic incubation experiments we examined the fate of organic matter and the alteration of its carbon and nitrogen isotopic composition during microbial degradation the decomposition rates determined with a double exponential decay model show that the more reactive fraction of organic matter degrades at similar rates under oxic and anoxic conditions however under oxic conditions the proportion of organic matter resistent to degradation is much lower than under anoxic conditions within three months of incubation the 13c of bulk organic matter decreased by 1 6 with respect to the initial value the depletion can be attributed to the selective preservation of 13c depleted organic compounds during anoxic decay the 15n values continuously decreased to about 3 below the initial value the decrease probably results from bacterial growth adding 15n depleted biomass to the residual material in the oxic experiment 15n values increased by more then 3 before decreasing to a value indistinguishable from the initial isotopic composition the dissimilarity between oxic and anoxic conditions may be attributed to differences in the type timing and degree of microbial activity and preferential degradation in agreement with the anoxic incubation experiments sediments from eutrophic lake lugano are on average depleted in 13c 1 5 and 15n 1 2 with respect to sinking particulate organic matter collected during a long term sediment trap study

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

modal mineralogy of carbonaceous chondrites by x ray diffraction and mossbauer spectroscopy carbonaceous chondrites are among the most analyzed geological materials on earth however despite this attention and unlike most terrestrial rocks little is known on the abundance of individual phases within them here we show how a combination of several novel x ray diffraction xrd techniques including a high brightness x ray microsource and mssbauer spectroscopy allows a complete modal mineralogy to be ascertained from even the most highly unequilibrated fine grained chondrites for all minerals of abundance 1 wt knowledge of the modal mineralogy of a sample also allows us to calculate grain density we analyzed allende murchison tagish lake and orgueil based on our modal data the grain density estimates for allende murchison and orgueil are close to literature values in the case of tagish lake there is no published grain density although a bulk density measurement does exist taking our estimate of grain density and the measured bulk density we calculate an exceptionally high porosity of 41 for this meteorite similar to some chondritic idps and in line with a porosity calculated from an entry model for the tagish lake fireball although it is an oxidized cv magnetite is present in allende at a level of 0 5 wt or 0 3 vol a result that is substantiated by several other instrumental studies this may be an oxidized meteorite but that oxidation is not manifested in abundant magnetite in addition we note appreciable fayalitic olivine in orgueil detected by both xrd and mossbauer we employed microsource xrd to look at heterogeneity in mineral abundance in orgueil and found substantial variation with phyllosilicates varying inversely with olivine the data suggest that orgueil was initially composed primarily of anhydrous materials which have been partially but not completely altered although the data are preliminary comparison between our xrd modal assessment bulk chemistry grain density and mossbauer data suggests that our estimates of mineral abundance are robust the advent of microsource xrd allows similar modal data to be acquired from samples as small as a few hundred micrograms

exposure history of the st robert h5 fall the compositionally typical h5 chondrite st robert has an exposure age 7 8 ma indistinguishable from that of the main cluster of h chondrites small values of the cosmogenic 22ne 21ne ratio in interior samples imply a pre atmospheric radius on the order of 40 cm sample depths based on tracks and the production rates of bhattacharya et al 1973 range from 6 to 40 cm and are generally larger than depths estimated from published 60co activities perhaps because the track production rates adopted are too high depth profiles of the production rates of 14c 36cl 26al 10be and 21ne in stony material show increases with depth and reach levels 5 to 15 higher than expected from modeling calculations the maximum concentrations in st robert are however generally comparable to those measured for the l5 chondrite knyahinya whose pre atmospheric radius of 45 cm is thought to lead to the maximum possible production rates in chondrites we infer that the pre atmospheric radius of st robert was within 5 cm of the value that supports maximum production rates i e 45 5 cm this radius corresponds to a pre atmospheric mass of 1 3 0 4 103 kg the agreement of exposure ages for st robert obtained in several different ways and the similarity of the depth profiles for 14c 26al 10be and 21ne argue against a lengthy pre exposure of st robert on the parent body and against a two stage exposure after launch from the parent body following morbidelli and gladman 1998 we suggest that st robert was chipped from deep in its parent body spent the next 7 8 ma without undergoing a major collision was nudged gradually into an orbital resonance with jupiter and then traveled quickly to earth

isotope geochemistry of zeolites two of the most important variables that can be determined in any geologic study are the timing and conditions of formation although the discovery of radioactivity and isotopes at the beginning of this century revolutionized our understanding of the age and formation conditions of rocks and minerals the isotope geochemistry of certain mineral groups such as the zeolites has remained elusive and poorly understood to date fewer than 40 papers have been published on the isotope geochemistry of zeolites this dearth of isotopic data is due to technical difficulties rather than lack of interest recently however great strides have been made in understanding the isotope geochemistry of zeolites as a result of a better understanding of their crystal structures coupled with the increased popularity of isotope geochemistry and a rapid growth in the number of isotope laboratories this chapter reviews the current status of our knowledge of isotopes in natural zeolites including the methods used to prepare samples and measure isotopic abundances and ratios some of the problems associated with these measurements and some of the implications of the results obtained because the reader may not be familiar with isotope geochemistry the fundamentals of the behavior of isotopes will also be reviewed briefly it should be pointed out that a vast literature exists on the behavior and characteristics of isotopes in synthetic zeolites and on isotopic studies related to industrial applications see e g chang et al 1995 this material is however outside the scope of this chapter and will only be mentioned when it has a direct bearing on the geochemistry of zeolites the earliest published report on isotopes in zeolites may have been that of taylor and urey 1938 who determined fractionation of stable li k and n isotopes by an unspecified zeolite species at columbia university taylor and

porosities of lunar meteorites strength porosity and petrologic screening during the meteorite delivery process porosity has been directly measured for eight lunar meteorite breccias and calculated for two more on the basis of literature density measurements lunar meteorite regolith breccias display systematically low porosity in comparison to otherwise analogous apollo regolith breccias among seven meteoritic regolith breccias porosity ranges from 1 to 11 and averages 7 5 1 3 2 whereas for 44 analogous apollo samples porosities mostly calculated from literature density data the average is 25 1 7 the origin of this disparity is enigmatic but the trend probably reflects mainly a bias in favor of strong compact breccias among fragments that manage to survive the violent process of launch to lunar escape velocity 2 38 km s in addition compaction during launch may play an important role the population of lunar meteorites is clearly not a random unmodified sample of lithic materials near the surface of the parent body