meteorite accumulations on mars abstract we have modeled single body meteoroid atmospheric entry speeds at mars and the effect of drag and ablation and identify a narrow range of small masses 10 50 g that should impact mars at survivable speeds the rate of oxidative weathering is much lower than that on earth so this small flux of meteorites could give rise to significant accumulations ca 5 10 2 to 5 10 5 meteorites greater than 10 g in mass per square kilometer given that extremely large numbers of meteorites may be present on mars future sample return missions should consider the real possibility that they may recover meteoritic material due to the low weathering rate meteorites may survive on the surface of mars for more than 10 9 years preserving a record of the temporal variability of the meteoroid flux and the compositional evolution of the meteoroid complex intact carbonaceous chondrites may also preserve organic compounds from degradation by ultraviolet radiation terrestrial meteorites may be present but would probably be sterile

hydration state of the martian coarse grained hematite exposures implications for their origin and evolution 1 the discovery of areas of bulk gray hematite on the surface of mars is among the most interesting of the results from mars global surveyor mgs the individual outcrops are concentrated in equatorial low albedo regions from the valles marineris to the meridiani planum region to the east while the thermal emission spectrometer tes team favored an aqueous process for the formation of this deposit recent geologic studies have interpreted deposits in the region to have a volcanic origin competing ideas for the method of formation of bulk hematite range from aqueous alteration burial diagenesis and metamorphism to igneous processes here we show that some of the spectra collected over meridiani planum and aram chaos from the mariner 6 and 7 spacecraft have a high 3 m band depth this feature is associated with hydrated minerals in the surface layer and the hydrated spectra coincide with the mapped hematite regions this supports an aqueous mode of hematite formation and as hematite itself is not hydrated suggests the presence of other hydrous mineral phases in these areas we discuss the implications of this association for origin and evolution of these deposits

the new athena alpha particle x ray spectrometer for the mars exploration rovers 1 the new alpha particle x ray spectrometer apxs is part of the athena payload of the two mars exploration rovers mer the apxs sensor head is attached to the turret of the instrument deployment device idd of the rover the apxs is a very light weight instrument for determining the major and minor elemental composition of martian soils rocks and other geological materials at the mer landing sites the sensor head has simply to be docked by the idd on the surface of the selected sample x ray radiation excited by alpha particles and x rays of the radioactive sources is recorded by a high resolution x ray detector the x ray spectra show elements starting from sodium up to yttrium depending on their concentrations the backscattered alpha spectra measured by a ring of detectors provide additional data on carbon and oxygen by means of a proper calibration the elemental concentrations are derived together with data from the two other athena instruments mounted on the idd the samples under investigation can be fully characterized key apxs objectives are the determination of the chemistry of crustal rocks and soils and the examination of water related deposits sediments or evaporates using the rock abrasion tool attached to the idd issues of weathering can be addressed by measuring natural and abraded surfaces of rocks

rate of wind abrasion on mars estimation of the rate of aeolian abrasion of rocks on mars requires knowledge of 1 particle flux 2 susceptibilities to abrasion of various rocks and 3 wind frequencies on mars fluxes and susceptibilities for a wide range of conditions were obtained in the laboratory and combined with wind data from the viking meteorology experiment assuming an abundant supply of sand sized particles estimated rates range up to 2 1 10 2 cm of abrasion per year in the vicinity of viking lander 1 this rate is orders of magnitude too great to be in agreement with the inferred age of the surface based on models of impact crater flux the discrepancy in the estimated rate of abrasion and the presumed old age of the surface cannot be explained easily by changes in climate or exhumation of ancient surfaces we consider the primary reason to be related to the agents of abrasion either windblown grains are in very short supply or the grains are ineffective as agents of abrasion at least some sand sized 100 m grains appear to be present as inferred from both lander and orbiter observations high rates of abrasion occur for all experimental cases involving sands of quartz basalt or ash however previous studies have shown that sand is quickly comminuted to silt and clay sized grains in the martian aeolian regime experiments also show that these fine grains are electrostatically charged and bond together as sand sized aggregates laboratory simulations of wind abrasion involving aggregates show that at impact velocities capable of destroying sand aggregates form a protective veneer on the target surface and can give rise to extremely low abrasion rates

formation of recent martian gullies through melting of extensive water rich snow deposits the observation of gullies on mars indicates the presence of liquid water near the surface in recent times1 2 which is difficult to reconcile with the current cold climate gullies have been proposed to form through surface runoff from subsurface aquifers1 3 or through melting of near surface ice under warmer conditions4 but these gullies are observed to occur preferentially in cold mid latitudes2 where the presence of liquid water is less likely and on isolated surfaces where groundwater seepage would not be expected making both potential explanations unsatisfactory here i show that gullies can form by the melting of water rich snow that has been transported from the poles to mid latitudes during periods of high obliquity within the past 105 to 106 years refs 5 6 melting within this snow7 can generate sufficient water to erode gullies in about 5 000 years my proposed model for gully formation is consistent with the age and location of the gullies and it explains the occurrence of liquid water in the cold mid latitudes as well as on isolated surfaces remnants of the snowpacks are still present on mid latitude pole facing slopes and the recent or current occurrence of liquid water within them provides a potential abode for life

formation of jarosite bearing deposits through aqueous oxidation of pyrite at meridiani planum mars 1 the discovery of sulfate rich layered deposits with hematite spherules at the landing site of the opportunity rover is consistent with mineral deposition in an aqueous environment we evaluate conditions responsible for the formation of a jarosite goethite gypsum assemblage with speciation calculations the results show that the assemblage could have precipitated from acidic solutions formed through near surface aqueous oxidation of pyrite our hypothesis is that regional heating in the meridiani planum caused a release of sulfide rich hydrothermal waters leading to formation of pyrite rich regional deposits in a depression aqueous oxidation of these deposits by atmospheric o2 created an acidic environment that allowed formation of sulfates and goethite partial neutralization of the solution caused further goethite precipitation and conversion of jarosite to goethite leading to formation of goethite concretions subsequent dehydration of goethite to coarse grained hematite would also have been facilitated by regional heating

terrestrial weathering of antarctic stone meteorites formation of mg carbonates on ordinary chondrites abstract white efflorescences of weathering origin occur superposed on fusion crusts or along fractures in the interiors of approximately 5 of all meteorites in the us antarctic collection efflorescences from equilibrated ordinary chondrites consist of the hydrous mg carbonates nesquehonite hydromagnesite x ray diffraction and scanning electron microscope studies of efflorescences from lew 85320 h5 show abundant elongate prismatic crystals of nesquehonite idiomorphic not pseudomorphous after lansfordite with minor local encrustations of hydromagnesite abundances of na k ca and rb in efflorescences from lew 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea salts therefore cations in evaporite minerals on antarctic meteorites are most likely not products of contamination by terrestrial marine salts the mg in the efflorescences probably originated from weathering of meteoritic olivine other cations in the efflorescences are also of meteoritic provenance thermodynamic analysis of the reaction forsterite water carbon dioxide nesquehonite silica at antarctic temperatures and pco 2 indicates spontaneity for all water activities greater than 0 65 compatible with the presence of liquid water as brines and or thin films

visible and near infrared multispectral analysis of geochemically measured rock fragments at the opportunity landing site in meridiani planum 1 we have used visible and near infrared panoramic camera pancam spectral data acquired by the opportunity rover to analyze 15 rock fragments at the meridiani planum landing site these spectral results were then compared to geochemistry measurements made by the in situ instruments mossbauer mb and alpha particle x ray spectrometer apxs to determine the feasibility of mineralogic characterization from pancam data our results suggest that dust and alteration rinds coat many rock fragments which limits our ability to adequately measure the mineralogy of some rocks from pancam spectra relative to the different field of view and penetration depths of mb and apxs viewing and lighting geometry along with sampling size also complicate the spectral characterization of the rocks rock fragments with the same geochemistry of sulfate rich outcrops have similar spectra although the sulfate rich composition cannot be ascertained based upon pancam spectra alone feni meteorites have spectral characteristics particularly ferric oxide coatings that generally differentiate them from other rocks at the landing site stony meteorites and impact fragments with unknown compositions have a diverse range of spectral properties and are not well constrained nor diagnostic in pancam data bounce rock with its unique basalt composition is easily differentiated in the pancam data from all other rock types at meridiani planum our pancam analyses of small pebbles adjacent to these 15 rock fragments suggests that other rock types may exist at the landing site but have not yet been geochemically measured

sedimentary textures formed by aqueous processes erebus crater meridiani planum mars new observations at erebus crater olympia outcrop by the mars exploration rover opportunity between sols 671 and 735 a sol is a martian day indicate that a diverse suite of primary and penecontemporaneous sedimentary structures is preserved in sulfate rich bedrock centimeter scale trough festoon cross lamination is abundant and is better expressed and thicker than previously described examples postdepositional shrinkage cracks in the same outcrop are interpreted to have formed in response to desiccation considered collectively this suite of sedimentary structures provides strong support for the involvement of liquid water during accumulation of sedimentary rocks at meridiani planum

clay mineraloid weathering products in antarctic meteorites abstract clay mineraloids are associated with gypsum k fe sulfates possibly jarosite cryptocrystalline rust and possibly with zeolites and or secondary alkali feldspars as terrestrial weathering products in stony meteorites from the allan hills alh and elephant moraine eet collection areas in victoria land antarctica weathering formed fracture fillings in fusion crusts of eucrites eeta79004 and eeta79005 diogenites alha77256 and eeta79002 shergottite eeta79001 and h5 chondrite alh82102 and decomposition products of glass and plagioclase in interior samples of achondrites clay mineraloids displayed massive to incipient vermicular forms with elemental compositions that suggest smectite and mica like phases deposits were too small to yield pure material for phase identification although x ray diffractometry and differential scanning calorimetry of bulk samples suggested that the clay mineraloids were not well crystallized a typical structural formula calculated on a smectite basis follows k 0 26 ca 0 04 al 0 97 ti 0 02 fe 3 0 77 mg 0 03 si 4 00 o 10 oh 2 n 2 o net charge 0 28 excessions 0 58 si 0 02 cl 0 09 so 4 0 02 po 4 hydrocryogenic diagenesis can initiate clay mineral formation on timescales of 10 4 10 6 y even with only limited supplies of liquid water cosmochemists must take care not to confuse possible weathering induced elemental and isotopic fractionations in antarctic meteorite samples with signatures of pre terrestrial cosmic processes given proper sample selection and testing though occurrence of clay mineraloid and other weathering products should probably not impede progress in studies of antarctic meteorites

new insights into the mineralogy and weathering of the meridiani planum meteorite mars meridiani planum is the first officially recognized meteorite find on the surface of mars it was discovered at and named after the landing site of the mars exploration rover opportunity based on its composition it was classified as a iab complex iron meteorite mossbauer spectra obtained by opportunity are dominated by kamacite fe ni and exhibit a small contribution of ferric oxide several small features in the spectra have been neglected to date to shed more light on these features five iron meteorite specimens were investigated as analogs to meridiani planum with a laboratory mossbauer setup measurements were performed on 1 their metallic bulk 2 troilite fes inclusions 3 cohenite fe ni co 3c and schreibersite fe ni 3p and 4 corroded rims in addition to these room temperature measurements a specimen from the mundrabilla iab ungrouped meteorite was measured at mars equivalent temperatures based on these measurements the features in meridiani planum spectra can be explained with the presence of small amounts of schreibersite and or cohenite and iron oxides the iron oxides can be attributed to a previously reported coating on meridiani planum their presence indicates weathering through the interaction of the meteorite with small amounts of water

laboratory experiments on the weathering of iron meteorites and carbonaceous chondrites by iron oxidizing bacteria batch culture experiments were performed to investigate the weathering of meteoritic material by iron oxidizing bacteria the aerobic acidophilic iron oxidizer a ferrooxidans was capable of oxidizing iron from both carbonaceous chondrites murchison and cold bokkeveld and iron meteorites york and casas grandes preliminary iron isotope results clearly show contrasted iron pathways during oxidation with and without bacteria suggesting that a biological role in meteorite weathering could be distinguished isotopically anaerobic iron oxidizers growing under ph neutral conditions oxidized iron from iron meteorites these results show that rapid biologically mediated alteration of extraterrestrial materials can occur in both aerobic and anaerobic environments these results also demonstrate that iron can act as a source of energy for microorganisms from both iron and carbonaceous chondrites in aerobic and anaerobic conditions with implications for life on the early earth and the possible use of microorganisms to extract minerals from asteroidal material

the event that produced heat shield rock and its implications for the martian atmosphere 1 methods developed in previous work were used to estimate the mass trajectory and atmospheric conditions that produced heat shield rock the iron meteorite discovered on mars by the opportunity rover in january 2005 we find that heat shield rock encountered mars at high speed and shallow entry angle probably at a time when the planet possessed a thicker atmosphere it entered the atmosphere with a mass of more than 60 kg underwent significant ablation during atmospheric passage and ricocheted across the surface upon impact we conclude that heat shield rock probably represents physical evidence that mars once had a denser atmosphere

present day impact cratering rate and contemporary gully activity on mars the mars global surveyor mars orbiter camera has acquired data that establish the present day impact cratering rate and document new deposits formed by downslope movement of material in mid latitude gullies on mars twenty impacts created craters 2 to 150 meters in diameter within an area of 21 5 106 square kilometers between may 1999 and march 2006 the values predicted by models that scale the lunar cratering rate to mars are close to the observed rate implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating the new gully deposits formed since august 1999 are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water relatively long extended digitate distal and marginal branches diversion around obstacles and low relief the observations suggest that liquid water flowed on the surface of mars during the past decade

ventifacts on earth and mars analytical field and laboratory studies supporting sand abrasion and windward feature development abstract terrestrial ventifacts rocks that have been abraded by windblown particles are found in desert periglacial and coastal environments on mars their abundance suggests that aeolian abrasion is one of the most significant erosional processes on the planet there are several conflicting viewpoints concerning the efficacy of potential abrasive agents principally sand and dust and the relationships between wind direction and ventifact form our research supported by a review of the literature shows that sand rather than dust or other materials is the principle abrasive agent on earth and mars relative to dust sand delivers about 1000 the energy onto rock surfaces on a per particle basis even multiple dust collisions will do little or no damage because the stress field from the impact is much smaller than the spacing of microflaws in the rock the abrasion profiles of terrestrial ventifacts are consistent with a kinetic energy flux due to saltating sand not airborne dust furthermore scanning electron microscope images reveal surfaces that are fractured and cleaved by sand grain impact with respect to their distribution ventifacts are found in regions that contain sand or did so in the past but are not found where only dust activity occurs contrary to some published reports our evidence from field studies analytical models and wind tunnel and other experiments indicates that windward not leeward abrasion is responsible for facet development and feature formation pits flutes and grooves leeward abrasion is confined to fluvial conditions in which the high viscosity and density of water are able to entrain sand size material in vortices therefore ventifacts and abraded terrain provide an unambiguous proxy for the direction of the highest velocity winds and can be used to reconstruct palaeowind flow

the distribution and behavior of martian ground ice during past and present epochs mars undergoes significant oscillations in its orbit which will have a pronounced effect on its climate and in particular on the behavior of subsurface water ice we explore and map the behavior of ice in the martian near surface regolith over the past 1 m y using a diffusion and condensation model presented in an earlier paper with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance we find that the past behavior of ground ice differs significantly from that at the present epoch primarily the result of high amplitude oscillations in obliquity presently 25 in midlatitude and equatorial regions ground ice will condense from atmospheric water during times of higher obliquity filling the top few meters of the regolith with significant amounts of ice at an obliquity of 32 ground ice becomes stable globally during times of lower obliquity ground ice will sublime and diffuse back into the atmosphere dessicating the regolith to a depth of about 1 to 2 m equatorward of 60 to 70 latitude in the high latitude regions these oscillations are considerably subdued below this depth of cyclic saturation and dessication a long term stability of ice exists in some geographic regions we present a map of the distribution of ice expected at the present epoch cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits

climate and rock weathering a study of terrestrial age dated ordinary chondritic meteorites from hot desert regions abstract ordinary chondrites oc recovered from the desert areas of roosevelt county new mexico the nullarbor region of western australia and the algerian and libyan sahara for which 14c terrestrial ages have been determined were examined by 57fe mossbauer spectroscopy oc were chosen as a standard sample to investigate weathering processes as their well constrained trace and bulk element chemistry normative mineralogy and isotopic composition define a known pre weathering starting composition given that terrestrial ages are known it is possible to compare initially very similar samples that have been subsequently weathered in a range of climatic regimes from the present day to 44 ka bp in addition recently fallen equilibrated oc contain iron only as fe0 and fe2 thus the abundance of ferric iron is directly related to the level of terrestrial weathering mossbauer spectroscopy identifies two broad types of ferric alteration paramagnetic phases akaganeite lepidocrocite and goethite and magnetically ordered principally magnetite and maghemite oc finds show a range in the percentage of total fe existing as fe3 from zero to over 80 however oxidation is comparable between fragments of the same oc separated since their time of fall i e paired meteorites our results indicate several features of meteorite weathering that may result from climatic or geomorphologic conditions at the accumulation site 1 saharan samples are overall less weathered than non saharan samples which may be related to the relatively recent age ca 20 ka of the saharan accumulation surface 2 broad differences between sites in the rate of weathering arising from regional differences in climate 3 consistent differences in the weathering products between samples that fell during humid periods and those that fell during more arid periods those falling during humid periods contain a higher proportion of magnetically ordered ferric oxides 4 one region the nullarbor that shows a variation in the total amount of ferric species that closely matches the climatic record for this area of australia for the last 30 ka points 3 and 4 may be related to the identification of a rapid initial weathering phase the majority of weathering occurs in the first few hundred years after fall followed by passivation of weathering by porosity reduction porosity reduction and the associated restriction in the ability of water to penetrate the sample appears to be the mechanism whereby a weathering assemblage formed during the brief initial period of oxidation is preserved through subsequent climatic cycles over the terrestrial lifetime of the sample

mineralogical and compositional features of rock fulgurites a record of lightning effects on granite fulgurites are a naturally occurring glass formed when sand rock or soil is struck by atmospheric electrical discharges lightning the aim of this paper is to provide insights into the conditions occurring in rocks during the lightning strike rock fulgurites collected from mt mottarone baveno piedmont italy have been investigated to assess the mineralogical and compositional changes occurring in granite due to a lightning strike x ray powder diffraction showed that the samples represent the dominant granitic rock type of the baveno massif the so called pink baveno fulgurite coats the surface of the granite as a brown black glassy to very fine grained porous layer powder diffraction data for the fulgurite reveal the presence of cristobalite and quartz crystals in a glass matrix suggesting that temperature exceeded 1700 c at near atmospheric conditions assuming thermodynamic equilibrium electron probe microanalysis of the glass revealed that it is mainly composed of sio 2 and al 2 o 3 and that it has a porosity of 5 7 area in the studied zones the presence of the amorphous phase indicates that the abrupt electrical joule heating of the rock surface yielded high temperatures producing a thin melt layer on the surface which then cooled adiabatically idealized physical model was developed to simulate the effects of joule heating and subsequent thermal conduction close to the rock surface during and after a lightning strike the quantity of organic matter in the glass obtained via elemental analyzer suggests that rapid quenching of the melt trapped no x and co x gases produced during heating raman spectroscopy revealed the presence of polyaromatic hydrocarbon molecules which combined with the elemental analyzer data suggest that organic matter was pyrolyzed at around 300 350 c and then trapped in the glass matrix of the studied rock fulgurites

event and conditions that produced the iron meteorite block island on mars 1 the mars exploration rover opportunity has discovered four large iron nickel meteorites that range in size from 50 to 240 kg dispersed over 10 km of meridiani planum mars because these meteorites are covered with hollows that resemble regmaglypts their surfaces record their ablation through the atmosphere and they must have landed at speeds below hypervelocity 2 km s 1 to survive slowing massive iron meteorites requires a minimum atmospheric density which was quantified using a numerical model that integrates the equations of motion for incoming meteoroids through an atmosphere of a given surface density and scale height and records their outcomes as direct generally hypervelocity impacts that form craters longer over the horizon and fallback flight paths and skip outs the present atmosphere of mars is sufficient to slow iron meteoroids as large as block island the most massive meteorite via drag and significant ablation on long flight paths although for standard distributions of entering meteoroid masses velocities and entry angles such events are rare 0 007 of incoming iron meteoroids such events require entry angles of 10 13 entry velocities of 6 18 km s 1 and entry masses of 225 710 kg the absence of large stony meteorites is probably at least partially because they are much weaker and thus broken up into smaller fragments on impact although differential drag deceleration on long flight paths could disperse fragments of an entering meteoroid by tens of kilometers dynamic pressures are too low to break up an iron meteorite leaving the possibility that they are paired an open question

evidence for mechanical and chemical alteration of iron nickel meteorites on mars process insights for meridiani planum 1 the weathering of meteorites found on mars involves chemical and physical processes that can provide clues to climate conditions at the location of their discovery beginning on sol 1961 the opportunity rover encountered three large iron meteorites within a few hundred meters of each other in order of discovery these rocks have been assigned the unofficial names block island shelter island and mackinac island each rock presents a unique but complimentary set of features that increase our understanding of weathering processes at meridiani planum significant morphologic characteristics interpretable as weathering features include 1 a large pit in block island lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction 2 differentially eroded kamacite and taenite lamellae in block island and shelter island providing relative timing through crosscutting relationships with deposition of 3 an iron oxide rich dark coating 4 regmaglypted surfaces testifying to regions of minimal surface modification with other regions in the same meteorites exhibiting 5 large scale cavernous weathering in shelter island and mackinac island we conclude that the current size of the rocks is approximate to their original postfall contours their morphology thus likely results from a combination of atmospheric interaction and postfall weathering effects among our specific findings is evidence supporting 1 at least one possible episode of aqueous acidic exposure for block island 2 ripple migration over portions of the meteorites 3 a minimum of two separate episodes of wind abrasion alternating with 4 at least one episode of coating forming chemical alteration most likely at subzero temperatures

the presence and stability of ground ice in the southern hemisphere of mars abstract we calculate new estimates of ground ice stability and the depth distribution of the ice table the depth boundary between ice free soil above and ice cemented soil below and compare these theoretical estimates of the distribution of ground ice with the observed distribution of leakage neutrons measured by the neutron spectrometer instrument of the mars odyssey spacecraft s gamma ray spectrometer instrument suite our calculated ground ice distribution contains improvements over previous work in that we include the effects of the high thermal conductivity of ice cemented soil at and below the ice table we include the surface elevation dependence of the near surface atmospheric humidity and we utilize new high resolution maps of thermal inertia albedo and elevation from mars global surveyor observations results indicate that ground ice should be about 5 times shallower than in previous predictions while results are dependent on the atmospheric humidity depths are generally between a few millimeters and a few meters with typical values of a few centimeters results are also geographically similar to previous predictions with differences due to the higher resolution of thermal inertia and the inclusion of elevation effects on humidity comparison with the measured epithermal neutron count rates in the southern hemisphere indicate that the geographic distribution of the count rate is best correlated with ground ice in equilibrium with 10 to 20 pr m precipitable micrometers column abundance of atmospheric water assuming a uniform distribution with co2 however given the uncertainties 5 to 30 pr m also may be viable this water abundance represents a longer term average over 100 to 1000 yr there is a high degree of correlation between the depth of the ice table and the epithermal count rate that agrees remarkably well with predicted count rates as a function of ice table depth these results indicate that ground ice in the upper meter of the martian soil is in diffusive equilibrium with the atmosphere since ground ice in this depth zone is expected to undergo saturation desiccation cycles with orbital variations this ice should be younger than about 500 kyr and was emplaced under similar cold and dry climate conditions of today remaining differences between the predicted depths of the ice table and those inferred from the neutron data are likely to be due to subpixel heterogeneity in the martian surface including the presence of rocks slopes and patches of soil with varying thermophysical properties

ti al zoning of experimentally grown titanite in the system cao al2o3 tio2 sio2 nacl h2o f evidence for small scale fluid heterogeneity chemical zoning according to the substitution al oh f ti o is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions we synthesized titanite which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile visible in element distribution maps as a patchy irregular pattern run conditions in the system cao al 2 o 3 tio 2 sio 2 nacl h 2 o caf 2 were 600 c 0 4 gpa 1 to 107 days duration natural rutile crystals were placed in a perforated pt tube within an outer au capsule containing wollastonite al 2 o 3 optionally powdered caf 2 and a nacl brine to simulate conditions where rutile is transformed into titanite by a hydrous ca si al f fluid driven by a chemical potential gradient between inner and outer capsule spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 m large crystals in a reaction rim in the long time runs titanite is strongly zoned in al ti and shows up to 0 5 al per formula unit pfu in experiments containing f without f substitution is limited to 0 25 al pfu the range of the ti al exchange is already large in short run times and the same heterogeneity is observed in long time runs the al ti distribution with and without f in compositional domains up to several tens of micrometers is patchy and irregular no relation of the zoning with time core rim or preferred growth directions was observed in the case of fluid mediated growth mineral growth zoning represents a disequilibrium at supersaturated conditions the concentration of ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of al ca and si titanium is not transported in significant amounts into the outer capsule indicating low mobility in contrast to al ca and si which are transported into the inner capsule assuming buffered al concentration from dissolution of the al source we speculate that the ti al of the fluid at the precipitation site was controlled by the local ti concentration which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning

rapid weathering in holbrook an iron 57 mossbauer spectroscopy study samples of holbrook an l6 chondrite that fell in arizona in 1912 were analysed by 57fe mossbauer spectroscopy to determine the proportion of the total fe that had been converted to fe3 by weathering processes although samples recovered in 1912 showed negligible 40 ka a linear rate of weathering is unrealistic rapidly producing very high levels of oxidation in samples with low terrestrial ages however if weathering follows an approximate power law then we can model a rapid initial phase and the levels of oxidation that are more typical in samples with much longer terrestrial residence times recovered from roosevelt county our analysis together with work on terrestrial age dated samples indicates that hot desert weathering of ordinary chondrites is at least a two stage process with an initial rapid phase lasting 500 years before oxidation is passivated