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

pb isotopic evidence for an early enriched crust on mars abstract martian meteorite nwa 7533 is a regolith breccia that compositionally resembles the martian surface measured by orbiters and landers nwa 7533 contains monzonitic clasts that have zircon with u pb ages of 4 428 ga the pb isotopic compositions of plagioclase and alkali feldspars as well as u pb isotopic compositions of chlorapatite in the monzonitic clasts of nwa 7533 have been measured by secondary ion mass spectrometry sims the u pb isotopic compositions measured from the chlorapatite in nwa 7533 yield an age of 1 357 81 ga 2 the least radiogenic pb isotopic compositions measured in plagioclase and k feldspar lie within error of the 4 428 ga geochron these data indicate that the monzonitic clasts in nwa 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4 428 ga with a value 238 u 204 pb of at least 13 4 1 7 2 and a value 232 th 238 u of 4 3 this value is more than three times higher than any other documented martian reservoir these results indicate either the martian mantle is significantly more heterogeneous than previously thought value of 1 14 vs 1 5 and or the monzonitic clasts formed by the melting of martian crust with a value of at least 13 4 therefore nwa 7533 may contain the first isotopic evidence for an enriched differentiated crust on mars

constraints on martian differentiation processes from rbsr and smnd isotopic analyses of the basaltic shergottite que 94201 isotopic analyses of mineral leachate and whole rock fractions from the martian shergottite meteorite que 94201 yield rb sr and sm nd crystallization ages of 327 12 and 327 19 ma respectively these ages are concordant although the isochrons are defined by different fractions within the meteorite comparison of isotope dilution sm and nd data for the various que 94201 fractions with in situ ion microprobe data for que 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite this component is likely to have been added to que 94201 by secondary alteration processes on mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions initial 87sr 86sr of 0 701298 14 nd143 of 47 6 1 7 and whole rock nd142 of 0 92 0 11 indicate that que 94201 was derived from a source that was strongly depleted in 87rb 86sr and enriched in 147sm 144nd early in its history modeling demonstrates that the sm nd isotopic compositions of que 94201 can be produced by either four episodes of melting at 327 ma of cumulates crystallized from a magma ocean at 4 525 ga or five episodes of melting of an initially solid mars at 4 525 ga and 327 ma the neodymium isotopic systematics of que 94201 are not consistent with significant melting between 4 525 ga and 327 ma the estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the que 94201 parental magma rb sr based partial melting models are unable to reproduce the composition of que 94201 using the same model parameters employed in the sm nd based models implying a decoupling of rb sr and sm nd isotopic systems the initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate rb from sr compared to sm from nd the fact that all martian meteorites analyzed so far define a rb sr whole rock isochron age of 4 5 ga suggests that virtually all rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time thus the martian mantle cumulates and restites are not expected to evolve past 87sr86sr of 0 700 and could not have been significantly enriched in incompatible elements by crustal recycling processes all martian meteorites have initial 87sr86sr values that are higher than 0 700 and are therefore likely to be produced by mixing between evolved crustal like and depleted mantle reservoirs the absence of crustal recycling processes on mars may preserve the geochemical evidence for decoupling of the rb sr and sm nd isotopic systems underscoring one of the fundamental differences between geologic processes on mars and the earth

pb evolution in the martian mantle abstract the initial pb compositions of one enriched shergottite one intermediate shergottite two depleted shergottites and nakhla have been measured by secondary ion mass spectrometry sims these values in addition to data from previous studies using an identical analytical method performed on three enriched shergottites alh 84001 and chassigny are used to construct a unified and internally consistent model for the differentiation history of the martian mantle and crystallization ages for martian meteorites the differentiation history of the shergottites and nakhla chassigny are fundamentally different which is in agreement with short lived radiogenic isotope systematics the initial pb compositions of nakhla chassigny are best explained by the late addition of a pb enriched component with a primitive non radiogenic composition in contrast the pb isotopic compositions of the shergottite group indicate a relatively simple evolutionary history of the martian mantle that can be modeled based on recent results from the sm nd system the shergottites have been linked to a single mantle differentiation event at 4504 ma thus the shergottite pb isotopic model here reflects a two stage history 1 pre silicate differentiation 4504 ma and 2 post silicate differentiation to the age of eruption as determined by concordant radiogenic isochron ages the values 238u 204pb obtained for these two different stages of pb growth are 1 of 1 8 and a range of 2 from 1 4 4 7 respectively the 1 value of 1 8 is in broad agreement with enstatite and ordinary chondrites and that proposed for proto earth suggesting this is the initial value for inner solar system bodies when plotted against other source radiogenic isotopic variables sri 187 os e 143 nd and e 176 hf the second stage mantle evolution range in observed mantle values display excellent linear correlations r 2 0 85 and represent a spectrum of martian mantle mixing end members depleted intermediate enriched

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

the chlorine isotopic composition of martian meteorites 1 chlorine isotope composition of martian mantle and crustal reservoirs and their interactions the martian meteorites record a wide diversity of environments processes and ages much work has been done to decipher potential mantle sources for martian magmas and their interactions with crustal and surface environments chlorine isotopes provide a unique opportunity to assess interactions between martian mantle derived magmas and the crust we have measured the cl isotopic composition of 17 samples that span the range of known ages martian environments and mantle reservoirs the 37cl of the martian mantle as represented by the olivine phyric shergottites nwa 2737 chassignite and shergotty basaltic shergottite has a low value of approximately 3 8 this value is lower than that of all other planetary bodies measured thus far the martian crust as represented by regolith breccia nwa 7034 is variably enriched in the heavy isotope of cl this enrichment is reflective of preferential loss of 35cl to space most basaltic shergottites less shergotty nakhlites chassigny and allan hills 84001 lie on a continuum between the martian mantle and crust this intermediate range is explained by mechanical mixing through impact fluid interaction and assimilation fractional crystallization

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

detection of upper mantle flow associated with the african superplume a continental scale low seismic velocity anomaly in the mid to lower mantle beneath africa is a robust feature of global tomographic models assuming the low velocities are associated with warm less dense material the african seismic anomaly has been ascribed to a long lived thermal upwelling from the lower mantle such a large scale upwelling should also affect the regional horizontal flow field in the upper mantle to test this model we compare seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography we calculate splitting parameters at 13 ocean island stations surrounding africa splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy we tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions we find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations 500 km from a ridge axis they are a poor fit to the data at stations located farther off axis thus we conclude that far from a ridge axis the observed anisotropy is dominated by asthenospheric flow to test for an active component of mantle upwelling we considered several models with varying assumptions about the velocity at the base of the asthenosphere that it is 1 stationary below plates moving in the no net rotation nnr or hotspot reference frames 2 driven by plate motions at the earth s surface or 3 driven by a combination of plate motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction we find that the best fitting flow field is generated by plate motions and density heterogeneity associated with large scale upwelling originating in the lower mantle beneath southern africa and is manifest as a radial pattern of flow at the base of the asthenosphere this model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction the resulting sub asthenospheric flow field is estimated to have velocities of 0 3 cm year and an asthenospheric viscosity of 3 1019 pa s is found to be most consistent with the regional anisotropy geoid height and dynamic topography

martian soil component in impact glasses in a martian meteorite chemical compositions of impact melt glass veins called lithology c lith c in martian meteorite eet79001 were determined by electron microprobe analysis a large enrichment of s and significant enrichments of al ca and na were observed in lith c glass compared to lithology a lith a the s enrichment is due to mixing of plagioclase enriched lith a material with martian soil either prior to or during impact on mars a mixture of 87 lith a 7 plagioclase and 6 martian soil reproduces the average elemental abundances observed in lith c shock melting of such a mixture of plagioclase enriched fine grained lith a host rock and martian soil could yield large excesses of s observed in this study and martian atmospheric noble gases found by bogard et al 1983 in lith c these mixing proportions can be used to constrain the elemental abundance of phosphorus in martian soil

solving the martian meteorite age conundrum using micro baddeleyite and launch generated zircon the age of the representative martian meteorite nwa 5298 is determined using spatially correlated electron beam nanostructural and uranium lead isotopic measurements of microminerals resolving a paradox of different age interpretations for the evolution of martian crust a few of the many meteorites that fall to earth are of martian origin the true age of these rare samples of the martian surface has been the subject of a decades long debate with interpreted ages differing by up to 4 billion years desmond moser and colleagues resolve this problem using a new approach to dating meteorite launch events through nanoscale investigation of crystal growth zoning and structures their analysis of the resistant micromineral baddeleyite and host igneous minerals in the highly shocked martian meteorite northwest africa 5298 reveals it as a crystallization product of martian volcanism of the past 400 million years previous estimates of a 4 billion year age of formation have actually dated a remnant signature of the ancient mantle melting event from which the magma was derived these findings confirm the presence of an ancient non convecting mantle beneath a relatively young volcanic martian crust invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites1 however the interpreted ages of the ejected igneous crust of mars differ by up to four billion years1 2 3 4 5 6 a conundrum7 due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards earth here we solve the conundrum by combining in situ electron beam nanostructural analyses and u pb uranium lead isotopic measurements of the resistant micromineral baddeleyite zro2 and host igneous minerals in the highly shock metamorphosed shergottite northwest africa 5298 ref 8 which is a basaltic martian meteorite we establish that the micro baddeleyite grains pre date the launch event because they are shocked cogenetic with host igneous minerals and preserve primary igneous growth zoning the grains least affected by shock disturbance and which are rich in radiogenic pb date the basalt crystallization near the martian surface to 187 33 million years before present primitive non radiogenic pb isotope compositions of the host minerals common to most shergottites1 2 3 4 do not help us to date the meteorite instead indicating a magma source region that was fractionated more than four billion years ago9 10 11 12 to form a persistent reservoir so far unique to mars1 9 local impact melting during ejection from mars less than 22 2 million years ago caused the growth of unshocked launch generated zircon and the partial disturbance of baddeleyite dates we can thus confirm the presence of ancient non convecting mantle beneath young volcanic mars place an upper bound on the interplanetary travel time of the ejected martian crust and validate a new approach to the geochronology of the inner solar system

a petrogenetic model for the origin and compositional variation of the martian basaltic meteorites the major element trace element and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean the models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate mars from the literature these models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic cao al2o3 ratios and their source regions obtained their parent daughter 87 rb 86 sr 147 sm 144 nd and 176 lu 177 hf ratios calculated from the initial sr nd and hf isotopic compositions of the meteorites high pressure experiments suggest that majoritic garnet is the liquidus phase for mars relevant compositions at or above 12 gpa early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated cao al2o3 ratio and a high mg olivine pyroxene garnet dominated cumulates that crystallize subsequently will also be characterized by superchondritic cao al2o3 ratios melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites furthermore crystallization models demonstrate that some of these cumulates have parent daughter ratios that are similar to those calculated for the most incompatible element depleted source region i e that of the meteorite queen alexandra que 94201 the incompatible element abundances of the most depleted que 94201 like source region have also been calculated and provide an estimate of the composition of depleted martian mantle the incompatible element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted earth s mantle melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of que 94201 e g ba th k p hf zr and heavy rare earth elements fairly well but does not reproduce the abundances of rb u ta and light rare earth elements the source regions for meteorites such as shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile melting of this hybrid source yields liquids with major element abundances and incompatible element patterns that are very similar to the shergotty bulk rock

halogen and cl isotopic systematics in martian phosphates implications for the cl cycle and surface halogen reservoirs on mars abstract the cl isotopic compositions and halogen cl f br and i abundances in phosphates from eight martian meteorites spanning most rock types and ages currently available have been measured in situ by secondary ion mass spectrometry sims likewise the distribution of halogens has been documented by x ray mapping halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in martian samples or on the martian surface and the inter element ratios are highly variable similarly cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks phosphates in ancient 4 ga meteorites orthopyroxenite alh 84001 and breccia nwa 7533 have positive 37 cl anomalies 1 1 to 2 5 these samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity in contrast the phosphates in the younger basaltic shergottite meteorites 37 cl anomalies 0 2 to 5 6 phosphates with the largest negative 37 cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative 37 cl anomalies suggestive of interaction with the surface of mars during the latest stages of basalt crystallization the phosphates with no textural major element or halogen enrichment evidence for mixing with this surface reservoir have an average 37 cl of 0 6 supporting a similar initial cl isotope composition for mars the earth and the moon oxidation and reduction of chlorine are the only processes known to strongly fractionate cl isotopes both positively and negatively and perchlorate has been detected in weight percent concentrations on the martian surface the age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines which have been documented on the martian surface has been active throughout martian history

39ar 40ar dating of the zagami martian shergottite and implications for magma origin of excess 40ar the zagami shergottite experienced a complex petrogenetic formation history mccoy et al 1992 1999 like several shergottites zagami contains excess 40ar relative to its formation age to understand the origin of this excess 40ar we made 39ar 40ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution surprisingly all these separates show similar concentrations of excess 40ar 1 x 10 6 cm 3 g we present arguments against this excess 40ar having been introduced from the martian atmosphere as impact glass we also present evidence against excess 40ar being a partially degassed residue from a basalt that actually formed 4 gyr ago we utilize our experimental data on ar diffusion in zagami and evidence that it was shock heated to only 70 c and we assume this heating occurred during an ejection from mars 3 myr ago with these constraints thermal considerations necessitates either that its ejected mass was impossibly large or that its shock heating temperature was an order of magnitude higher than that measured we suggest that this excess 40ar was inherited from the zagami magma and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old k rich crustal material similar concentrations of excess 40ar in the analyzed separates imply that this magma maintained a relatively constant 40ar concentration throughout its crystallization this likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released these concepts have implications for excess 40ar in other shergottites

chronology and petrogenesis of young achondrites shergotty zagami and alha77005 late magmatism on a geologically active planet a study was undertaken to determine the chronology petrogenesis and relationships among the shergottites shergotty and zagami and the unique achondrite alha77005 these meteorites are the product of a variety of complex processes petrogenesis chondrite normalized abundance patterns of shergotty and zagami are very similar and show pronounced depletions of both the light ree la nd and heavy ree dy lu relative to sm gd these characteristic depletions are even more pronounced for alha77005 the light ree depletion is qualitatively consistent with the presence of cumulus pyroxene and or olivine in these meteorites but trace element models show that the parental magmas of all three meteorites were probably also light ree depleted both trace element model calculations and combined rb sr and sm nd isotopic systematics show that the meteorites could not have been co magmatic nor can alha77005 be representative of the source material of the shergottites light ree depletion of the parental magmas also implies light ree depletion of the source material the sm nd systematics of the shergottites require a time averaged sub chondritic light ree enriched sm nd ratio since 4 6 ae ago the sm nd systematics of alha77005 permit a time averaged super chondritic light ree depleted sm nd ratio if its crystallization age is less than tice 0 72 ae chronology rb sr internal isochrons for all three meteorites and a sm nd internal isochron for zagami are concordant at 180 myr 39ar 40ar plateau ages of shergotty and zagami maskelynite are 250 260 myr these ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite the concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity the time of this event is most precisely given by the rb sr age of 180 4 myr for zagami the crystallization ages of the meteorites were not precisely determined extreme upper limits are determined by sm nd model ages relative to an eucrite initial 143nd144nd 0 505835 at 4 6 ae ago these model ages for shergotty zagami and alha77005 are 3600 3500 and 2850 myr respectively the sm nd whole rock age of 1340 60 myr for the three meteorites gives the crystallization age if the sm nd ratios of the precursor materials were always the same we consider this 1340 myr age as a best estimate upper limit best estimate lower limits for shergotty and zagami are taken from the average 39ar 40ar ages of 1200 and 900 myr of pyroxene separates the average 39ar 40ar age of a whole rock sample of alha77005 was 1600 myr and can be partitioned between a low temperature feldspar phase and a high temperature olivine pyroxene inclusions phase the average apparent 39ar 40ar age of the low temperature phase is 1050 myr which is chosen as the best estimate lower limit to the age the crystallization ages of shergotty zagami and alha77005 probably lie within the ranges of 1200 1300 900 1300 and 1000 1300 myr respectively the rb sr whole rock age of 4400 400 myr and single stage babi model ages of 4800 5100 myr are interpreted as reflecting differentiation of the parent body at 4600 myr ago the complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

the unique dynamics of the pacific hemisphere mantle and its signature on seismic anisotropy abstract the portion of earth s mantle lying below the hemispherical region occupied by the pacific ocean basin is characterized by a unique style of convection the unique dynamics below the pacific hemisphere are revealed by analyzing the mantle flow predicted on the basis of high resolution seismic images of mantle structure in the pacific hemisphere we find a dome like upwelling originating in the lower mantle which actively transports material into the upper mantle a unique aspect of this flow found only in the pacific hemisphere is a demi tour or u turn of the circulation which occurs in the shallow mantle west of the east pacific ridge this demi tour connects the westward movement of the pacific plate to the dome like upwelling below the central pacific ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis in a global analysis of the correlation between mantle flow and seismic anisotropy we find the strongest agreement occurs in the pacific hemisphere mantle

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

the early differentiation history of mars from 182w 142nd isotope systematics in the snc meteorites we report here the results of an investigation of w and nd isotopes in the snc shergottite nakhlite chassignite martian meteorites we have determined that 182 w values in the nakhlites are uniform within analytical uncertainties and have an average value of 3 also while 182 w values in the shergottites have a limited range from 0 3 0 7 their 142 nd values vary considerably from 0 2 0 9 there appears to be no correlation between 182 w and 142 nd in the nakhlites and shergottites these results shed new light on early differentiation processes on mars particularly on the timing and nature of fraction ation in silicate reservoirs assuming a two stage model the metallic core is estimated to have formed at 12 myr after the beginning of the solar system major silicate differentiation established the nakhlite source reservoir before 4542 ma and the shergottite source reservoirs at 4525 19 ma these ages imply that within the uncertainties afforded by the 182 hf 182 w and 146 sm 142 nd chronometers the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contempo raneously possibly during crystallization of a global magma ocean the distinct 182 w 142 nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on mars two of which are depleted in incompatible lithophile elements relative to chondrites and the third is enriched the two depleted silicate reservoirs most likely reside in the martian mantle while the enriched reservoir could be either in the crust or the mantle therefore the 182 w 142 nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related a further implication is that the source reservoirs of the nakhlites and shergottites on mars have been isolated since their establishment before 4 5 ga therefore there has been no giant impact or efficient global mantle convection to thoroughly homogenize the martian mantle following the establish ment of the snc source reservoirs copyright 2005 elsevier ltd

a combined sm nd rb sr and u pb isotopic study of mg suite norite 78238 further evidence for early differentiation of the moon abstract lunar mg suite norite 78238 was dated using the sm nd rb sr and u pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of mg suite magmatism as well as to provide a direct comparison between the three isotopic systems the sm nd isotopic system yields a crystallization age for 78238 of 4334 37 ma and an initial e nd 143 value of 0 27 0 74 the age initial e nd 143 t i systematics of a variety of kreep rich samples including 78238 and other mg suite rocks kreep basalts and olivine cumulate nwa 773 suggest that lunar differentiation was completed by 4492 61 ma assuming a chondritic uniform reservoir bulk composition for the moon the rb sr isotopic systematics of 78238 were disturbed by post crystallization processes nevertheless selected data points yield two rb sr isochrons one is concordant with the sm nd crystallization age 4366 53 ma the other is 4003 95 ma and is concordant with an ar ar age for 78236 the 207pb 206pb age of 4333 59 ma is concordant with the sm nd age the u pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the pb pb system and may reflect fractionation of u and pb during sample handling despite the disturbed nature of the u pb systems a time averaged 238u 204pb value of the source can be estimated at 27 30 from the pb pb isotopic systematics because kreep rich samples are likely to be derived from source regions with the highest u pb ratios the relatively low value calculated for the 78238 source suggests the bulk moon does not have an exceedingly high value

constraints on the u pb isotopic systematics of mars inferred from a combined u pb rb sr and sm nd isotopic study of the martian meteorite zagami uranium lead rb sr and sm nd isotopic analyses have been performed on the same whole rock mineral and leachate fractions of the basaltic martian meteorite zagami to better constrain the u pb isotopic systematics of martian materials although the rb sr and sm nd systems define concordant crystallization ages of 166 6 ma and 166 12 ma respectively the u pb isotopic system is disturbed nevertheless an age of 156 6 ma is derived from the 238u 206pb isotopic system from the purest mineral fractions maskelynite and pyroxene the concordance of these three ages suggest that the 238u 206pb systematics of the purest zagami mineral fractions have been minimally disturbed by alteration and impact processes and can therefore be used to constrain the behavior of u and pb in the zagami source region the value of the zagami source region can be estimated with some confidence from the 238u 206pb isochron to be 3 96 0 02 disturbance of the u pb isotopic systems means that this represents a minimum value the value of the zagami source is significantly lower than the values estimated for most basaltic magma sources from earth and the moon this is surprising given the high initial 87sr 86sr ratio 0 721566 82 and low initial end value 7 23 0 17 determined for zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body this suggests that mars is characterized by a low bulk planet u pb ratio a feature that is consistent with its relatively volatile rich nature the leachates contain terrestrial common pb that was probably added to the meteorite during handling curation or sawing the mineral fractions particularly those with significant amounts of impact melt glass contain a second contaminant the presence of this contaminant results in pb pb ages that are older than the crystallization age of zagami indicating that the contaminant is characterized by a high 207pb 206pb ratio such a contaminant could be produced by removal of single stage pb from a relatively high martian reservoir before 1 8 ga and therefore could be an ancient manifestation of hydrous alteration of martian surface material

the isotopic record of lunar volcanism abstract the timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon a late heavy bombardment produced most of the observable lunar basins easily recognized stratigraphic markers and also may have influenced lunar volcanism most of the nearside basins seem to have been produced 3 8 4 0 ga ago during a period often referred to as the lunar cataclysm the pre cataclysm period was characterized by formation of the lunar crust plutonic rocks within the crust and pre mare volcanism isotopic analysis of breccia clasts pre dating the cataclysm has yielded an outline of this period the isotopic record of post cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking specifically the isotopic data show that some lunar anorthosites formed 4 45 ga ago consistent with early formation of an anorthositic lunar crust some mg suite norites troctolites and dunites also formed approximately contemporaneously with the oldest anorthosites but others are up to 200 300 ma younger more evolved rocks granites and quartz monzodiorites also formed between 4 4 and 4 0 ga ago the earliest known pre mare volcanism at 4 25 ga ago included aluminous mare like basalts and an olivine gabbronorite similar to apollo 12 basalts kreep basalt volcanism began at least 4 1 ga ago approximately contemporaneously with that of the ancient mare like basalts and continued during the cataclysm recorded instances of high ti mare basalt volcanism began during or immediately after the cataclysm low ti mare basalt volcanism is the most recent for which samples are available the isotopic data record an early primary lunar differentiation during which mare basalt sources formed probably by crystallization of cumulus minerals from a lunar magma ocean the isotopic record of post cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources but more complex petrogenesis also has been suggested more complex petrogenetic processes including assimilation and assimilation fractional crystallization afc seem to be required for pre mare volcanics the isotopic data contain a hint that petrogenesis of the oldest pre mare volcanics may be linked to formation of the so called procellarum basin possibly the oldest and largest lunar basin such a basin would provide a geologic context for the pre mare volcanics and its formation may have been a defining moment in lunar history

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

the loss of k ree th and u from a martian and a terrestrial basalt by acidic leaching the mobilization of k rare earth elements ree th and u from martian surface material upon contact with acidic solutions probably occurred extensively on mars about 4 to 3 5 ga ago and seems to have occurred locally in more recent times we have studied the dissolution of these elements by leaching the basaltic shergottite zagami and the terrestrial basalt be n at constant ph values ranging from 5 to 1 in the absence and presence of added salts potassium is nearly immobile in zagami and mobilized readily from be n the ree reside mostly in calcium phosphates and dissolve readily with those in whitlockite of zagami reacting slightly better than those in apatite of be n thorium and u also reside mostly in calcium phosphates both dissolve similarly for both basalts and less readily than the ree the experiments indicate the extent of the mobilization of k ree th and u when acidic water leached the surface of mars potassium was released slowly and in a small relative amount the ree and particularly the lree became mobile readily and were possibly distributed over large areas before immobilization thorium and u dissolved more slowly than the ree and were distributed less widely

the anisotropic and rheological structure of the oceanic upper mantle from a simple model of plate shear summary we have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology diffusion creep and dislocation creep to study the rheological and anisotropic structures of the oceanic upper mantle a semi analytic approach is used to solve for mantle stress and viscosity allowing fast calculations and exploration of a wide range of rheological parameters mantle stress in our model is due to shearing by a moving plate by comparing mantle stress with a transition stress for dislocation creep we identify regions where either diffusion creep or dislocation creep is active deformation by dislocation creep results in a mineral fabric that may be responsible for observed seismic anisotropy our study suggests that there is an important relation between plate motion seismic anisotropy mantle viscosity and transition stress using laboratory results for rheological parameters we find that dislocation creep exists only in a layer at certain depths in the upper mantle for a plate velocity of 10 cm yr 1 an asthenospheric viscosity of 10 19 pa s and an asthenospheric transition stress of 0 1 mpa our model predicts a 200 km thick dislocation creep layer which is broadly consistent with the observations of seismic anisotropy for a plate velocity of 10 cm yr 1 and an asthenospheric transition stress of 0 1 mpa the asthenospheric viscosity needs to be greater than 5 10 18 pa s to produce any dislocation creep deformation and the

petrogenesis of shergottite meteorites inferred from minor and trace element microdistributions abstract as an extension of our previous work on the lherzolitic shergottites alha77005 and lew88516 ion microprobe measurements of rees and other selected trace and minor elements were made in individual minerals of the basaltic shergottites shergotty zagami and eeta79001 whole rock ree abundances in these achondrites are dominated by whitlockite which is usually present only in trace amounts one of the most significant features is the extensive zoning of trace and minor elements in pyroxenes this zonation which provides information on the crystallization histories of these meteorites is of primary magmatic origin and not the result of secondary processes such as diffusive reequilibration and or metasomatic infiltration coherent trends in element element plots for low ca pyroxenes indicate that each shergottite formed essentially by progressive closed system fractional crystallization the calculated ree patterns of the parent melts of all shergottites are lree depleted and parallel to their whole rock ree patterns also indicative of closed system fractional crystallization on the basis of observations reported in this study as well as previously determined petrologic and geochemical including isotopic characteristics we present a model that suggests possible relationships among all five known shergottites

the age of the martian meteorite northwest africa 1195 and the differentiation history of the shergottites samarium neodymium isotopic analyses of unleached and acid leached mineral fractions from the recently identified olivine bearing shergottite northwest africa 1195 yield a crystallization age of 347 13 ma and an end143 value of 40 1 0 9 maskelynite fractions do not lie on the sm nd isochron and appear to contain a martian surface component with low 147sm 144nd and 143nd 144nd ratios that was added during shock the rb sr system is disturbed and does not yield an isochron terrestrial sr appears to have affected all of the mineral fractions although a maximum initial 87sr 86sr ratio of 0 7016 is estimated by passing a 347 ma reference line through the maskelynite fraction that is least affected by contamination the high initial end143 value and the low initial 87sr 86sr ratio combined with the geologically young crystallization age indicate that northwest africa 1195 is derived from a source region characterized by a long term incompatible element depletion the age and initial sr and nd isotopic compositions of northwest africa 1195 are very similar to those of queen alexandra range 94201 indicating these samples were derived from source regions with similar sr nd isotopic systematics these similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano the meteorites yamato 980459 dar al gani 476 sayh al uhaymir 005 008 and dhofar 019 also have relatively old ages between 474 and 575 ma and trace element and or isotopic systematics that are indicative of derivation from incompatible element depleted sources this suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible element depleted sources closed system fractional crystallization of this suite of meteorites is modeled with the melts algorithm using the bulk composition of yamato 980459 as a parent these models reproduce many of the major element and mineralogical variations observed in the suite in addition the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by melts other shergottites that demonstrate enrichments in incompatible elements and have evolved sr and nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group most notably although they exhibit a very limited range of incompatible trace element and isotopic compositions they have highly variable major element compositions this is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization if this scenario is correct it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element trace element and isotopic diversity of all shergottites

u pb isotopic systematics of shock loaded and annealed baddeleyite implications for crystallization ages of martian meteorite shergottites abstract shock recovery and annealing experiments on basalt baddeleyite mixtures were undertaken to evaluate shock effects on u pb isotopic systematics of baddeleyite shock pressures up to 57 gpa caused fracturing of constituent phases mosaicism of olivine maskelynitization of plagioclase and melting but the phase transition from monoclinic baddeleyite structure to high pressure temperature polymorphs of zro 2 was not confirmed the u pb isotopic systems of the shock loaded baddeleyite did not show a large scale isotopic disturbance the samples shock recovered from 47 gpa were then employed for annealing experiments at 1000 or 1300 c indicating that the basalt baddeleyite mixture was almost totally melted except olivine and baddeleyite fine grained euhedral zircon crystallized from the melt was observed around the relict baddeleyite in the sample annealed at 1300 c for 1 h the u pb isotopic systems of baddeleyite showed isotopic disturbances many data points for the samples annealed at 1000 c plotted above the concordia both radiogenic lead loss uranium gain and radiogenic lead gain uranium loss were observed in the baddeleyite annealed at 1300 c complete radiogenic lead loss due to shock metamorphism and subsequent annealing was not observed in the shock loaded annealed baddeleyites studied here these results confirm that the u pb isotopic systematics of baddeleyite are durable for shock metamorphism since shergottites still preserve fe mg and or ca zonings in major constituent phases i e pyroxene and olivine the shock effects observed in martian baddeleyites seem to be less intense compared to that under the present experimental conditions an implication is that the u pb systems of baddeleyite in shergottites will provide crystallization ages of martian magmatic rocks

stochastic analysis of shear wave splitting length scales abstract the coherence of azimuthal seismic anisotropy as inferred from shear wave splitting measurements decreases with the relative distance between stations stochastic models of a two dimensional vector field defined by a von karma n t von karma n progress in the statistical theory of turbulence j mar res 7 1948 252 264 autocorrelation function with horizontal correlation length l provide a useful means to evaluate this heterogeneity and coherence lengths we use the compilation of sks splitting measurements by fouch m fouch upper mantle anisotropy database accessed in 06 2006 http geophysics asu edu anisotropy upper and supplement it with additional studies including automated measurements by evans et al evans m s kendall j m willemann r j 2006 automated sks splitting and upper mantle anisotropy beneath canadian seismic stations geophys j int 165 931 942 evans m s kendall j m willemann r j automated splitting project database online at http www isc ac uk sks accessed 02 2006 the correlation lengths of this dataset depend on the geologic setting in the continental regions in young phanerozoic orogens and magmatic zones l 600 km smaller than the smooth l 1600 km patterns in tectonically more stable regions such as phanerozoic platforms our interpretation is that the relatively large coherence underneath older crust reflects large scale tectonic processes e g continent continent collisions that are frozen into the tectosphere in younger continental regions smaller scale flow e g slab anomaly induced may predominantly affect anisotropy in this view remnant anisotropy is dominant in the old continents and deformation induced anisotropy caused by recent asthenospheric flow is dominant in active continental regions and underneath oceanic plates auxiliary analysis of surface wave anisotropy and combined mantle flow and anisotropic texture modeling is consistent with this suggestion in continental regions the further exploration of a stochastic description of seismic anisotropy may form a useful counterpart to deterministic forward modeling particularly if we wish to understand the origin of discrepancies in heterogeneity estimates based on different seismological data sets

the origin of geochemical diversity of lunar mantle sources inferred from the combined u pb rb sr and sm nd isotope systematics of mare basalt 10017 the results of our combined u pb rb sr and sm nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources as well as the u pb isotope systematics of the moon the rb sr sm nd and 238 u 206 pb isotope systems yield concordant crystallization ages of 3 633 0 057 ga 3 678 0 069 ga and 3 616 0 098 ga respectively the 235 u 207 pb isochron yields an older though still concordant age of 3 80 0 12 ga neither the 206 pb 207 pb system nor u pb concordia system yields an age for 10017 that is concordant with the age determined from the sm nd rb sr and 238 u 206 pb systems the initial 87 sr 86 sr of 10017 is 0 69941 7 and the initial end is 3 2 0 4 initial pb isotopic compositions determined from the u pb isochrons are 206 pb 204 pbi 31 11 and 207 pb 204 pbi 34 15 together these initial pb compositions constrain the l value of the 10017 source to be 70 30 assuming a single stage pb growth model this is considerably lower than l values typically estimated for mare basalt sources 100 600 regardless the l values calculated for the sources of mare basalts as well as other lunar samples show a range that is larger than can be explained by fractionation of u from pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources the u pb isotope systematics may reflect late stage formation of a sulfide phase which strongly fractionates pb from u but has minimal effect on rb sr or sm nd compositions during crystallization of the lunar magma ocean 2007 elsevier ltd all rights reserved

39ar 40ar ages and origin of excess 40ar in martian shergottites we report new 39ar 40ar measurements on 15 plagioclase pyroxene and or whole rock samples of 8 martian shergottites all age spectra suggest ages older than the meteorite formation ages as defined by sm nd and rb sr isochrons employing isochron plots only los angeles plagioclase and possibly northwest africa nwa 3171 plagioclase give ages in agreement with their formation ages isochrons for all shergottite samples reveal the presence of trapped martian 40ar 40arxs which exists in variable amounts in different lattice locations some 40arxs is uniformly distributed throughout the lattice resulting in a positive isochron intercept and other 40arxs occurs in association with k bearing minerals and increases the isochron slope these samples demonstrate situations where linear ar isochrons give false ages that are too old after subtracting 40ar that would accumulate by 40k decay since meteorite formation and small amounts of terrestrial 40ar all young age samples give similar 40arxs concentrations of 1 2 x 10 6 cm 3 g but a variation in k content by a factor of 80 previously reported nasa johnson space center data for zagami shergotty yamato y 000097 y 793605 and queen alexandra range que 94201 shergottites show similar concentrations of 40arxs to the new meteorite data reported here similar 40arxs in different minerals and meteorites cannot be explained as arising from martian atmosphere carried in strongly shocked phases such as melt veins we invoke the explanation given by bogard and park 2008 for zagami that this 40arxs in shergottites was acquired from the magma similarity in 40arxs among shergottites may reveal common magma sources and or similar magma generation and emplacement processes

accretion timescale and impact history of mars deduced from the isotopic systematics of martian meteorites abstract high precision sm nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on mars associated with the formation of their mantle sources these data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic ages determined from the sm nd isotopic systematics of individual samples are strongly dependent on the assumed nd isotopic composition of the bulk planet this assumption is problematic given differences observed between the nd isotopic composition of earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary nd isotopic compositions ages determined from the slope of 146sm 142nd whole rock isochrons are not dependent on the assumed 142nd 144nd ratio of the planet but require the sample suite to be derived from complementary contemporaneously formed reservoirs in this work we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143nd 142nd whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet this expression is also applicable to mineral isochrons and has been used to successfully calculate 143nd 142nd model crystallization ages of early refractory solids as well as lunar samples this permits ages to be obtained using only nd isotopic measurements without the need for 147sm 144nd isotope dilution determinations when used in conjunction with high precision nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 6 ma because the sm nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the solar system recent thermal models however suggest that mars sized bodies cool rapidly in less than 5 ma after accretion ceases even in the presence of a thick atmosphere assuming these models are correct an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 ma late accretional heating of mars could either be associated with protracted accretion occurring at a quasi steady state or alternatively be associated with a late giant impact if this scenario is correct then accretion of mars sized bodies takes up to 60 ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation this further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed

a review of meteorite evidence for the timing of magmatism and of surface or near surface liquid water on mars 1 there is widespread photogeological evidence for ubiquitous water flowing on the surface of mars however the age of surface and near surface water cannot be deduced with high precision from photogeology while there is clear evidence for old and young fluvial features in the photogeologic record the uncertainty in the absolute calibration of the martian crater flux results in uncertainties of 1 5 gyr in the middle period of martian geologic history aqueous alteration of primary igneous minerals produces secondary minerals in martian meteorites here we use the ages of secondary alteration minerals in martian meteorites to obtain absolute ages when liquid water was at or near the surface of mars aqueous alteration events in martian meteorites occurred at 3929 37 ma carbonates in alh84001 633 23 ma iddingsite in nakhlites and 0 170 ma salts in shergottites furthermore these events appear to be of short duration suggesting episodic rather than continuous aqueous alteration of the meteorites the martian meteorites appear to be contaminated by martian surface pb characterized by a 207pb 206pb ratio near 1 lead of this composition could be produced by water based alteration on the martian surface the high 129xe 132xe ratio in the martian atmosphere compared to martian meteorites indicates fractionation of i from xe within 100 myr after nucleosynthesis of 129i such fractionation is difficult to achieve through magmatic processes however water very efficiently fractionates i from xe raising the intriguing possibility that mars had a liquid water ocean within its first 100 myr 1

nanoscale chemical characterisation of phase separation solid state transformation and recrystallization in feldspar and maskelynite using atom probe tomography the feldspar minerals occur in a wide variety of lithologies throughout the solar system often containing a variety of chemical and structural features indicative of the crystallization conditions cooling history and deformational state of the crystal such phenomena are often poorly resolved in micrometre scale analyses here atom probe tomography apt is conducted on ca rich bytownite and na rich albite plagioclase reference materials experimentally exsolved k feldspar sanidine shock induced plagioclase glass labradorite composition and shocked and recrystallized plagioclase to directly test the application of apt to feldspar and yield new insights into crystallographic features such as amorphisation and exsolution undeformed plagioclase reference materials amelia albite and stillwater bytownite appear chemically homogenous and yield compositions largely within uncertainty of published data within microstructurally complex materials apt can resolve chemical variations across a 20 nm wide exsolution lamella and define major element na k diffusion profiles across the lamella boundaries which appear gradational over a 10 nm length scale in experimentally exsolved k feldspar nnpp 04b the plagioclase glass within the zagami shergottite shows no heterogeneity in the distribution of major elements although the enrichment of fe mg and sr in the bulk microtip points to at least minor incorporation of surrounding phases pyroxene and with that supports a shock melt origin for the glass maskelynite the recrystallization of feldspar during post shock annealing such as in poikilitic shergottite nwa 6342 appears to induce a range of chemical nanostructures that locally effect the composition of the material these findings demonstrate the ability of apt to yield new insights into nanoscale composition and chemical structures of alumniosilicate phases highlighting an exciting new avenue with which to analyse these key rock forming minerals

various aspects of the petrogenesis of the martian shergottite meteorites several controversies are associated with the age and origin of the shergottite meteorites a suite of basaltic samples from mars here it will be argued that 1 the shergottites have a young igneous age 600 myr 2 their parent magmas were relatively dry 3 the range of initial isotopic compositions in shergottites is most likely due to assimilation of crustal materials by mantle derived basaltic magmas and 4 the intercumulus liquid compositions of shergottites such as shergotty and zagami are relatively well constrained

normal to inverse transition in martian spinel understanding the interplay between chromium vanadium and iron valence state partitioning through a crystal chemical lens figure 1 spinel is a very important rock forming mineral that is found in basalts from earth mars the earth s moon and basaltic meteorites spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock this paper highlights the role of increasing f o2 from iw 1 to fmq 2 in converting a 90 normal spinel to an 25 magnetite inverse spinel the trajectory of d vspinel melt as it relates to the ratio of v3 v4 in the melt and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both v3 and v4 this work examines the nuances of the v partitioning and provides a crystal chemical basis for understanding fe3 cr and v substitution into the octahedral sites of spinel understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets three potential examples for this use are provided in addition this work helps explain the ubiquitous miscibility gap between spinels with changing ulvospinel contents 1 pending yes

magmatic complexity on early mars as seen through a combination of orbital in situ and meteorite data abstract until recently mars was considered a basalt covered world but this vision is evolving thanks to new orbital in situ and meteorite observations in particular of rocks of the ancient noachian period in this contribution we summarise newly recognised compositional and mineralogical differences between older and more recent rocks and explore the geodynamic implications of these new findings for example the msl rover has discovered abundant felsic rocks close to the landing site coming from the wall of gale crater ranging from alkali basalt to trachyte in addition the recently discovered martian regolith breccia nwa 7034 and paired samples contain many coarse grained noritic monzonitic clasts demonstrably noachian in age and even some clasts that plot in the mugearite field olivine is also conspicuously lacking in these ancient samples in contrast to later hesperian rocks the alkali suite requires low degree melting of the martian mantle at low pressure whereas the later hesperian magmatism would appear to be produced by higher mantle temperatures various scenarios are proposed to explain these observations including different styles of magmatic activity i e passive upwelling vs hotspots a second petrological suite of increasing interest involves quartzo feldspathic materials that were first inferred from orbit in local patches in the southern highlands and in the lower units of valles marineris however identification of felsic rocks from orbit is limited by the low detectability of feldspar in the near infrared on the other hand the msl rover has described the texture mineralogy and composition of felsic rocks in gale crater that are granodiorite like samples akin to terrestrial ttg tonalite trondhjemite granodiorite suites these observations and the low average density of the highlands crust suggest the early formation of continental crust on mars although the details of the geodynamic scenario and the importance of volatiles in their generation are aspects that require further work

the case for old basaltic shergottites abstract the crystallization age of shergottites is currently not agreed upon although mineral 87 rb 87 sr 147 sm 143 nd 176 lu 176 hf and u pb isochrons all give very young ages typically in the range of 160 180 ma 207 pb 206 pb data support a much older crystallization age at 4 1 ga which is consistent with published whole rock 87 rb 87 sr data on basaltic shergottites different isotopic systems present different complexities but crater counting chronology which shows that a substantial fraction of the martian surface was resurfaced during the late heavy bombardment is in favor of an old martian lithosphere with ages in accordance with pb pb and rb sr isotopic data a 4 1 ga pb pb age of shergottites also agrees with the 142 nd and 182 w anomalies found in these rocks and concur with the presence of an actively convecting mantle during the first 500 myr of the planet s history we here present new sm nd lu hf and pb pb mineral isochrons for the basaltic shergottites shergotty and los angeles complementing our previous results on zagami bouvier a blichert toft j vervoort j d and albarede f 2005 the age of snc meteorites and the antiquity of the martian surface earth planet sci lett 240 221 233 the internal 147 sm 143 nd and 176 lu 176 hf isochrons give young ages of respectively 172 40 mswd 2 0 and 188 91 mswd 3 1 for shergotty and 181 13 mswd 0 14 and 159 42 mswd 0 01 for los angeles in contrast the pb isotope compositions of the leached whole rock fragments and maskelynite separates of shergotty and los angeles fall on the whole rock isochron previously established for zagami and other shergottite samples and collectively yield a pb pb age of 4050 70 ma for the crystallization of the basaltic shergottite suite the contrast between the 170 ma ages of internal isochrons and the 4 1 ga age supported by pb pb and 87 rb 87 sr on whole rocks simply reflects that the younger age dates the perturbation of a suite of rocks of noachian age the internal rb sr sm nd lu hf and u pb errorchrons are heavily biased by the presence of disseminated phosphate minerals and inclusions for which d h ratios d 4600 indicate strong interaction with martian subsurface waters in contrast baddeleyite occasionally present in sncs and also having extremely young u pb ages reflect resetting under the shock conditions that prevailed either during shergottite extraction from the planet or from impacts associated with a major break up event of planetesimals in the main asteroid belt we finally also re examine 39 ar 40 ar data on snc meteorites and suggest that they as well support old crystallization ages

the apparently isotropic australian upper mantle 1 we investigate shear wave splitting measurements performed on two years of data recorded at stations deployed in the tasmal experiment a network of 20 broadband seismological stations designed to record data on each side of the controversial tasman line in australia whereas a subset of measurements previously performed on one year of data exhibited a curvilinear pattern similar to that of the tasman line suggesting anisotropy frozen in the lithosphere considering the whole data set drastically changes the situation apparent isotropy in the australian upper mantle is observed at numerous stations this apparent isotropy together with the ew or ns orientations of the polarization plane of the fast s wave f observed at some stations is consistent with a two layer anisotropic system underneath the australian continent with a perpendicular orientation of f in each layer from the latest tomographic results the transition between the precambrian western and phanerozoic eastern australia appears to define blocks of various thickness unlike the situation across the tesz in europe these blocks do not seem to be correlated with a different behavior in terms of seismic anisotropy citation heintz m and b l n kennett 2006 the apparently isotropic australian upper mantle geophys res lett 33 l15319 doi 10 1029 2006gl026401

the pb isotopic evolution of the martian mantle constrained by initial pb in martian meteorites the pb isotopic compositions of maskelynite and pyroxene grains were measured in alh84001 and three enriched shergottites zagami roberts massif 04262 and larkman nunatuk 12011 by secondary ion mass spectrometry a maskelynite pyroxene isochron for alh84001 defines a crystallization age of 4089 73 ma 2 the initial pb isotopic composition of each meteorite was measured in multiple maskelynite grains alh84001 has the least radiogenic initial pb isotopic composition of any martian meteorite measured to date i e 206pb 204pb 10 07 0 17 2 assuming an age of reservoir formation for alh84001 and the enriched shergottites of 4513 ma a two stage pb isotopic model has been constructed this model links alh84001 and the enriched shergottites by their similar value 238u 204pb of 4 1 4 6 from 4 51 ga to 4 1 ga and 0 17 ga respectively the model employed here is dependent on a chondritic value 1 2 from 4567 to 4513 ma which implies that core segregation had little to no effect on the value s of the martian mantle the proposed pb isotopic model here can be used to calculate ages that are in agreement with rb sr lu hf and sm nd ages previously determined in the meteorites and confirm the young 170 ma ages of the enriched shergottites and ancient 4 ga age of alh84001

evolution of the martian mantle inferred from the 187re 187os isotope and highly siderophile element abundance systematics of shergottite meteorites abstract shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle in this study a suite of 23 shergottites spanning their known range in bulk compositions rb sr sm nd and lu hf isotopes were measured for 187re 187os isotopic systematics and highly siderophile element abundances hse including os ir ru pt pd re the chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long term hse budget of its derivative melts possible effects upon hses related to crustal contamination as well as terrestrial and or martian surface alteration are also examined some of the shergottites are hot arid desert finds their respective acetic acid leachates and residues show that both re and os display open system behavior during sample residence at or near the martian and or terrestrial surfaces in some meteorites the alteration effects can be circumvented by analysis of the leached residues for those shergottites believed to record robust re os isotopic systematics calculated initial 187os 188os are well correlated with the initial 143nd 144nd shergottites from mantle sources with long term melt depleted characteristics initial e143nd of 36 to 40 have chondritic initial 187os ranging from 0 5 to 2 5 shergottites with intermediate initial e143nd of 8 to 17 have a range in initial 187os of 0 6 to 2 3 which overlaps the range for depleted shergottites shergottites from long term enriched sources with initial e143nd of 7 are characterized by suprachondritic 187os values of 5 to 15 the initial 187os variations for the shergottites do not show a correlation with indices of magmatic differentiation such as mgo or any systematic differences between hot arid desert finds antarctic finds or observed falls the strong correlation between the initial e143nd and 187os in shergottites from approximately 40 and 0 to 7 and 15 respectively is assessed in models for mixing depleted mantle derived melts with ancient crust modeled to be similar to evolved shergottite in composition and with assimilation fractional crystallization these models show that the correlation is unlikely to result from participation of martian crust more likely this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca 4 5 ga these models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean the expanded database for the hse abundances in shergottites suggests that their martian mantle sources have similar hse abundances to the terrestrial mantle consistent with prior studies the relatively high hse abundances in both planetary mantles likely cannot be accounted for by high pressure temperature metal silicate partitioning at the bases of magma oceans as has been suggested for earth if the hse were instead supplied by late accretion this event must have occurred prior to the crystallization of the last martian magma ocean

martian meteorite chronology and the evolution of the interior of mars abstract the earth moon and mars are the only three planets for which satellite and ground observations can be confronted with extensive chronological and geochemical observations of rock samples here we use new pb isotope data on shergottites nakhlites chassigny and the alh 84001 orthopyroxenite to reassess chronological evidence and discuss its bearing on the evolution of the interior of mars we find that the depleted shergottites formed at 4 3 ga i e 170 ma after the last equilibration of pyroxene rich cumulates with the magma ocean while the younger enriched and intermediate shergottites as well as alh 84001 formed 200 ma later at 4 1 ga after the residual part of the mantle had been largely mixed back into these cumulates we also demonstrate that the th u ratios inferred from pb isotopes require shergottite ages older than 4 0 ga we finally reexamine the implications of this new chronological framework for the interpretation of 142 nd and 182 w anomalies in martian meteorites we conclude that shergottites represent samples that derived from a volcanic basement subsequently shattered by the 3 9 ga late heavy bombardment lhb although the magmatic activity of mars had strongly declined by the time of the lhb the new chronology of martian meteorites proposed here allows the martian mantle to be convective until today the 1 3 ga old emplacement of nakhlites and chassignites corresponds to decompression melting of ilmenite rich mantle domains and not to renewed global magmatic activity of the planet with the exception of this event the history of the interior of mars therefore somewhat resembles that of the moon

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

constraints on the petrogenesis of martian meteorites from the rb sr and sm nd isotopic systematics of the lherzolitic shergottites alh77005 and lew88516 abstract detailed rb sr and sm nd isotopic analyses have been completed on the lherzolitic shergottites alh77005 and lew88516 alh77005 yields a rb sr age of 185 11 ma and a sm nd age of 173 6 ma whereas the rb sr and sm nd ages of lew88516 are 183 10 and 166 16 ma respectively the initial sr isotopic composition of alh77005 is 0 71026 4 and the initial end value is 11 1 0 2 these values are distinct from those of lew88516 which has an initial sr isotopic composition of 0 71052 4 and an initial end value of 8 2 0 6 several of the mineral and whole rock leachates lie off the rb sr and sm nd isochrons indicating that the isotopic systematics of the meteorites have been disturbed the sm nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143nd 144nd ratio that was probably added to the meteorites on mars tie lines between leachate residue pairs from lew88516 mineral fractions and whole rocks have nearly identical slopes that correspond to rb sr ages of 90 1 ma this age may record a major shock event that fractionated rb sr from lattice sites located on mineral grain boundaries on the other hand the leachates could contain secondary alteration products and the parallel slopes of the tie lines could be coincidental nearly identical mineral modes compositions and ages suggest that these meteorites are very closely related nevertheless their initial sr and nd isotopic compositions differ outside analytical uncertainty requiring derivation from unique sources assimilation fractional crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma if the assimilant has a sr nd ratio near 1 and a radiogenic sr isotopic composition further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it a formed at 4 5 ga b has a high la yb ratio c is an oxidant and d is basaltic in composition or is strongly enriched in incompatible elements the composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved martian crustal component or hydrous metasomatism within the martian mantle

crystal chemistry of merrillite from martian meteorites mineralogical recorders of magmatic processes and planetary differentiation merrillite is a ubiquitous accessory phase in a variety of martian meteorite lithologies the martian merrillites exhibit a positive correlation between mg and na and a negative correlation between mg and both mn and vacancies in the octahedral na site their ree patterns are varied and range from lree depleted to lree enriched the dominant cation substitutions in the martian merrillites are fe2 vi mg sitemg2 vi mg site and ca2 vi na site vi na site2na vi na site the ree substitution into the 8 fold coordinated ca site is accommodated by the coupled substitution caviii ca site na vi na site y3 ree3 viii ca site vi na site the ree substitution is significantly more prevalent in lunar merrillite and can be used as a fingerprint to distinguish lunar from martian meteorites the substitution of oh whitlockite and or f bobdownsite for o2 on one of the phosphate tetrahedrons appears to be rather insignificant the correlations among na mg mn and na site vacancies are linked to the premerrillite crystallization history of the melt and the crystal chemical behavior of the mg and na sites the former reflects the sequence and extent of plagioclase and pyroxene crystallization the differences in ree pattern shapes among the merrillites reflect source regions for the martian basalts and the shapes are not greatly perturbed by the crystallization history the occurrence of merrillite does not imply low volatile component in the martian magmas however the low whitlockite and bobdownsite contents suggest that these samples were not altered by hydrothermal fluids and therefore not reset owing to aqueous fluid interactions consequently the young ages of the shergottites are probably true igneous crystallization ages

176lu 176hf and 147sm 143nd ages of the martian shergottites evaluation of the shock resetting hypothesis through diffusion kinetic experiments and modeling and petrological observations abstract we address the age controversy of the shergottite suite of martian meteorites 200 ma dates determined by 176lu 176hf 147sm 143nd and various other decay systems as opposed to a 4 ga whole rock pb pb date by calculating the timescales needed to reset hf and nd isotopes in clinopyroxene which is the primary host of rare earth elements amongst the minerals used to compose the 176lu 176hf and 147sm 143nd isochrons at the peak and post shock p t conditions likely to have been experienced by the shergottites it is concluded that impact heating is highly unlikely to have significantly reset the 176lu 176hf and or 147sm 143nd mineral isochrons of these meteorites this conclusion is bolstered by the nature of measured concentration profiles of cr and fe across olivine maskelynite boundaries and ti profiles across clinopyroxene maskelynite melt interfaces in the shergottite rbt 04262 the lack of any evidence of diffusion in the measured concentration profiles coupled with the qualitative incompatibility of the measured crystal melt fractionation trends at these interfaces with the nature of fractionation expected from equilibrium partitioning provides strong evidence that no substantial chemical exchange took place between the solid and melt phases during peak shock p t conditions furthermore we find that the compositional profiles of major elements in coexisting augite and pigeonite are homogenous and reflect a magmatic temperature of 1143 9 c we argue that the pyroxene compositions and consequently the 176lu 176hf and 147sm 143nd geochronometers were highly unlikely to have been reset by the thermal effect of a shock event x ray mapping of a selected section of rbt 04262 shows no evidence of enhanced grain boundary diffusion we thus conclude that the 200 ma dates recorded by the 176lu 176hf and 147sm 143nd geochronometers most likely record the crystallization age of the shergottites

modern u pb chronometry of meteorites advancing to higher time resolution reveals new problems in this paper we evaluate the factors that influence the accuracy of lead pb isotopic ages of meteorites and may possibly be responsible for inconsistencies between pb isotopic and extinct nuclide timescales of the early solar system instrumental mass fractionation and other possible analytical sources of error presence of more than one component of non radiogenic pb migration of ancient radiogenic pb by diffusion and other mechanisms possible heterogeneity of the isotopic composition of uranium u uncertainties in the decay constants of uranium isotopes possible presence of freshly synthesized actinides with short half life e g 234u in the early solar system possible initial disequilibrium in the uranium decay chains and potential fractionation of radiogenic pb isotopes and u isotopes caused by alpha recoil and subsequent laboratory treatment we review the use of 232th 238u values to assist in making accurate interpretations of the u pb ages of meteorite components we discuss recently published u pb dates of calcium aluminum rich inclusions cais and their apparent disagreement with the extinct nuclide dates in the context of capability and common pitfalls in modern meteorite chronology finally we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early solar system based on the combined use of the u pb system and extinct nuclide chronometers

recent deformations of the deep continental root beneath southern africa seismic anisotropy manifested by the splitting of shear waves into two orthogonal polarizations provides evidence for deformation in the earth s upper mantle where such deformation has been inferred beneath precambrian cratons it has been interpreted either as related to recent plate motion1 2 or inherited from the precambrian3 4 here we present data from a portable seismograph array in the kaapvaal craton of south africa which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern africa since the end of the jurassic period such an alignment has also been reported for the north american craton2 suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above the old continental roots that are likely to exist in this depth range5 6 must therefore be deformed by the plate motion but the deformations are not strong enough to be seen in the presently available seismic tomography data

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

defining the mechanisms that disturb the sm nd isotopic systematics of the martian meteorites examples from dar al gani 476 and allan hills 77005 microbeam studies of martian meteorites dar al gani dag 476 and allan hills alh 77005 have been conducted to identify potential causes of disequilibrium exhibited in their sm nd isotopic systematics olivine and maskelynite mineral fractions on the dag 476 isochron are displaced relative to their positions as dictated by measured mineral compositions the olivine mineral fractions from alh 77005 not only have a relatively low sm nd ratio but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions trace components such as melt inclusions impact melt high si mesostasis and altered olivine were analyzed using scanning electron microscopy quantitative electron microscopy and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions assuming that the mineral fractions were not completely pure mixing models indicate that the presence of melt inclusions in the dag 476 olivine mineral fraction lowered its sm nd ratio the maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the sm nd ratio of the fraction the position of two olivine mineral fractions below the alh 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non indigenous unradiogenic nd component furthermore the presence of rare earth elements in olivine and maskelynite from both igneous and non igneous components such as melt inclusions mesostasis and impact melt is observed on a fine 30 m scale despite the addition of this material the sm nd ages are not affected this study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples

a laser probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism spatially resolved argon isotope measurements have been performed on neutron irradiated samples of nw africa nwa 4797 shock heating of nwa 4797 completely melted and vesiculated precursor igneous plagioclase which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition ca k ratios using a focused ultraviolet laser beam is has been possible to distinguish between argon isotopic signatures from groundmass minerals igneous olivine pyroxene plagioclase and a shock vein this study focuses on the potential for this meteorite to shed light on shock ages of shergottites apparent 40 ar 39 ar ages of groundmass minerals show that there are large amounts of excess argon in this phase yielding a wide range of calculated ages from 690 30 ma to several apparent ages older than 4 5 ga a traverse of laser probe extractions across the 1 mm diameter shock vein in nwa 4797 yielded apparent 40 ar 39 ar ages younger than the groundmass a signature of the martian atmosphere identified by 40 ar 36 ar ratios of 1600 1900 was not found in the nwa 4797 shock vein this is distinct from other shergottites where the products of shock melting contain a nearly pure sample of martian atmosphere we attribute this to a distinct formation mechanism and hence gas trapping mechanism of the nwa 4797 shock vein we undertook 44 analyses of plagioclase areas identified by sem analysis ages ranged from 45 27 to 3771 109 ma and yield an average age of 375 77 ma considerably younger than ages obtained in this study from either the groundmass or the shock vein a plot of age v 37 ar 39 ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured older ages are correlated with higher ca k ratios of plagioclase indicating contamination from groundmass minerals rich in excess argon the youngest ages correlate to plagioclase extractions with the lowest ca k ratios interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a k rich mesostasis we see no evidence for multiple shock events in nwa 4797 rather we favour the interpretation that the cosmic ray exposure cre age of 3 0 0 5 ma obtained on nwa 4797 in this study using cosmogenic 38 ar approximates the timing of shock melting in this meteorite

experimental study of polybaric ree partitioning between olivine pyroxene and melt of the yamato 980459 composition insights into the petrogenesis of depleted shergottites abstract a synthetic composition representing the yamato 980459 martian basalt shergottite has been used to carry out phase relation and rare earth element ree olivine and pyroxene partitioning experiments yamato 980459 is a sample of primitive basalt derived from a reduced end member among martian mantle sources experiments carried out between 1 2 gpa and 1350 1650 c simulate the estimated pressure temperature conditions of basaltic melt generation in the martian mantle olivine melt and orthopyroxene melt partition coefficients for la nd sm eu gd and yb d ree values were determined by la icpms and are similar to the published values for terrestrial basaltic systems we have not detected significant variation in d values with pressure over the range investigated and by comparison with previous studies carried out at lower pressure we apply the experimentally obtained olivine melt and orthopyroxene melt d ree values to fractional crystallization and partial melting models to develop a three stage geochemical model for the evolution of martian meteorites in our model we propose two ancient 4 535 ga sources the nakhlite source located in the shallow mantle and the deep mantle source located close to the martian core mantle boundary these two sources evolved distinctly on the e 143 nd evolution curve due to their different sm nd ratios by partially melting the nakhlite source at 1 3 ga we are able to produce a slightly depleted residue nakhlite residue the nakhlite residue is left undisturbed until 500 ma at which point the depleted deep mantle source is brought up by a plume mechanism carrying with it high heat flow melts and isotopic signatures of the deep mantle e g e 182 w e 142 nd etc the plume derived deep mantle source combines with the nakhlite residue producing a mixture that becomes a mantle source herein referred to as the y98 source for yamato 980459 and the other depleted shergottites with the characteristic range of sm nd ratios of these meteorites the same hot plume provides a heat source for the formation of enriched and intermediate shergottites our model reproduces the ree patterns of nakhlites and depleted shergottites and can explain high e 143 nd in depleted shergottites furthermore the model results can be used to interpret whole rock rb sr and sm nd ages of shergottites

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

martian regolith in elephant moraine 79001 shock melts evidence from major element composition and sulfur speciation abstract shock veins and melt pockets in lithology a of martian meteorite elephant moraine eeta 79001 have been investigated using electron microprobe em analysis petrography and x ray absorption near edge structure xanes spectroscopy to determine elemental abundances and sulfur speciation s 2 versus s 6 the results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances the xanes spectra for eeta79001 glasses show a sharp peak at 2 471 kev characteristic of crystalline sulfides and a broad peak centered at 2 477 kev similar to that obtained for sulfide saturated glass standards analyzed in this study sulfate peaks at 2 482 kev were not observed bulk compositions of eeta79001 shock melts were estimated by averaging defocused em analyses vein and melt pocket glasses are enriched in al ca na and s and depleted in fe mg and cr compared to the whole rock petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins contributing to the enrichments estimates of shock melt bulk compositions obtained from glass analyses are biased towards fe and mg depletions because in general basaltic melts produced from groundmass minerals plagioclase and clinopyroxene will quench to a glass whereas ultramafic melts produced from olivine and low ca pyroxene megacrysts crystallize during the quench we also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt pyroxene en 72 75 fs 20 21 wo 5 7 and olivine fo 75 80 reconstruction of glass crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology a of the meteorite reflecting bulk melting of everything except xenolith chromite our results show that eeta79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts which can account for the observed compositional anomalies compared to the whole rock sample the observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites rocks from terrestrial impact structures and other martian meteorites the bulk composition of shock melts reflects the proportions of minerals melted large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy in the latter the modal abundance of sulfide globules may reach up to 15 vol we conclude the shock melt pockets in eeta79001 lithology a contain no significant proportion of martian regolith

isotopic and petrographic evidence for young martian basalts radiometric age data for shergottites yield ages of 4 0 ga and 180 575 ma the interpretation of these ages has been and remains a subject of debate here we present new 39 ar 40 ar laser probe data on lherzolitic shergottites allan hills alh 77005 and northwest africa nwa 1950 these two meteorites are genetically related but display very different degrees of shock damage on a plot of 40 ar 36 ar versus 39 ar 36 ar the more strongly shocked alh 77005 45 55 gpa does not yield an array of values indicating an isochron but the data are highly scattered with the shock melts yielding 40 ar 36 ar ratios of 1600 2026 apparent ages calculated from these extractions range from 374 8183 ma with 50 of the data particularly from the shock melts yielding impossibly old ages 4 567 ga on the same plot extractions from igneous minerals in the less shocked nwa 1950 30 44 gpa yield a fitted age of 382 36 ma argon extractions from the shock melts are well distinguished from minerals with the melts exhibiting the highest 40 ar 36 ar ratios 1260 1488 and the oldest apparent ages laser step heating was also performed on maskelynite separates from nwa 1950 yielding ages of 1000 ma at the lowest release temperatures and ages of 360 and 362 ma at higher temperature steps stepped heating data from previous studies have yielded ages of 500 and 700 ma to 1 7 ga for alh 77005 maskelynite separates if the ages obtained from igneous minerals represent undegassed argon from an ancient 4 0 ga rock then the ages are expected to anticorrelate with the degree of shock heating the data do not support this inference our data support young crystallization ages for minerals and martian atmosphere as the origin of excess 40 ar in the shock melts the shock features of shergottites are also reviewed in the context of what is known of the geologic history of the martian surface through remote observation the oldest most heavily cratered surfaces of mars are thought to be 4 0 ga we contend that ancient rocks from mars noachian 3 5 ga are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era young rocks late amazonian 0 6 ga should record a relatively simple history of emplacement and ejection from the near surface we show that although shergottites are strongly shocked they are relatively pristine crystalline igneous rocks and not pervasively altered breccias the petrography of shergottites is at odds with an ancient age interpretation a model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible

understanding the textures and origin of shock melt pockets in martian meteorites from petrographic studies comparisons with terrestrial mantle xenoliths and experimental studies we present a textural comparison of localized shock melt pockets in martian meteorites and glass pockets in terrestrial mantle derived peridotites specific textures such as the development of sieve texture on spinel and pyroxene and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets based on petrographic and compositional observations it is concluded that void collapse variable shock impedance is able to account for the occurrence of pre terrestrial sulfate bearing secondary minerals in the melts high gas emplacement efficiencies and s al ca and na enrichments and fe and mg depletion of shock melt compositions compared to the host rock previously used as arguments against such a formation mechanism recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in martian meteorites

geochronology of the martian meteorite zagami revealed by u pb ion probe dating of accessory minerals abstract the precise chronology of geological events on mars is hampered by the lack of absolute ages for the martian timescale and the significant uncertainties that result from the extrapolation of the lunar timescale to mars hartmann and neukum 2001 martian meteorites represent the only samples of mars currently available attempts to identify source craters for the meteorites have thus far proven inconclusive hamilton et al 2003 lang et al 2009 mouginis mark and boyce 2012 precluding their use in constraining the absolute martian timescale the majority of the known martian meteorites are basalts shergottites all dated shergottites have mineral separate rb sr or sm nd ages of borg et al 2005 here we report a 238u 206pb age of 182 7 6 9 ma by ion microprobe analysis of baddeleyite zro2 grains in the zagami shergottite there is no correlation between discordancy and baddeleyite grain location relative to shock metamorphism mineral petrography demonstrates that baddeleyite is the result of late stage igneous crystallization and raman spectroscopy shows that baddeleyite has not been transformed by shock into preservable high pressure polymorphs supported by an age of 153 81 ma for phosphate grains obtained using the same method we conclude that zagami crystallized at 180 ma in agreement with previous results from mineral separate geochronology therefore the shergottites represent igneous rocks preferentially ejected from young terrains on mars in a small number of ejection events

water undersaturated mantle plume volcanism on present day mars based on meteorite evidence the present day martian mantle has a combined abundance of up to a few hundred ppm of h2o cl and f which lowers the solidus and enhances the magma production rate adiabatic decompression melting in upwelling mantle plumes is the best explanation for young last 200 myr volcanism on mars we explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting relative to a dry mantle the reduction in solidus temperature due to water increases the magma production rate by a factor of 1 3 1 7 at 50 ppm water and by a factor of 1 9 3 2 at 200 ppm water mantle water also decreases the viscosity and increases the vigor of convection which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity at conditions relevant to mars these indirect effects can cause an order of magnitude increase in the magma production rate using geologic and geophysical observations of the late amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites present day mars is constrained to have a core mantle boundary temperature of 1750 to 1800 c and a volume averaged thermal rayleigh number of 2 106 to 107 indicating that moderately vigorous mantle convection has persisted to the present day melting occurs at depths of 2 5 6 gpa and is controlled by the rayleigh number at the low pressure end and by the mantle water concentration at high pressure

a pb isotopic resolution to the martian meteorite age paradox abstract determining the chronology and quantifying various geochemical reservoirs on planetary bodies is fundamental to understanding planetary accretion differentiation and global mass transfer the pb isotope compositions of individual minerals in the martian meteorite chassigny have been measured by secondary ion mass spectrometry sims these measurements indicate that chassigny has mixed with a martian reservoir that evolved with a long term 238u 204pb value two times higher than those inferred from studies of all other martian meteorites except 4 428 ga clasts in nwa7533 any significant mixing between this and an unradiogenic reservoir produces ambiguous trends in pb isotope variation diagrams the trend defined by our new chassigny data can be used to calculate a crystallization age for chassigny of 4 526 0 027 ga 2 that is clearly in error as it conflicts with all other isotope systems which yield a widely accepted age of 1 39 ga similar trends have also been observed in the shergottites and have been used to calculate a 4 ga age or alternatively attributed to terrestrial contamination our new chassigny data however argue that the radiogenic component is martian mixing occurred on the surface of mars and is therefore likely present in virtually every martian meteorite the presence of this radiogenic reservoir on mars resolves the paradox between pb isotope data and all other radiogenic isotope systems in martian meteorites importantly chassigny and the shergottites are likely derived from the northern hemisphere of mars while nwa 7533 originated from the southern hemisphere implying that the u rich reservoir which most likely represents some form of crust must be widespread the significant age difference between snc meteorites and nwa 7533 is also consistent with an absence of tectonic recycling throughout martian history

the age of dar al gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics samarium neodymium isotopic analysis of the martian meteorite dar al gani 476 yields a crystallization age of 474 11 ma and an initial end143 value of 36 6 0 8 although the rb sr isotopic system has been disturbed by terrestrial weathering and therefore yields no age information an initial 87sr 86sr ratio of 0 701249 33 has been estimated using the rb sr isotopic composition of the maskelynite mineral fraction and the sm nd age the sr and nd isotopic systematics of dar al gani 476 like those of the basaltic shergottite que94201 are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system nevertheless dar al gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the que94201 source region this is not consistent with the fact that the parental magma of dar al gani 476 is significantly more mafic than the parental magma of que94201 and underscores a decoupling between the major element and trace element isotopic systematics observed in the martian meteorite suite combining the end142 end143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4 513 0 033 0 027 ga using this age the parent daughter ratios of martian mantle sources are calculated assuming a two stage evolutionary history the calculated sources have very large ranges of parent daughter ratios 87rb 86sr 0 037 0 374 147sm 144nd 0 182 0 285 176lu 177hf 0 028 0 048 these ranges exceed the ranges estimated for terrestrial basalt source regions but are very similar to those estimated for the sources of lunar mare basalts in fact the range of parent daughter ratios calculated for the martian meteorite sources can be produced by mixing between end members with compositions similar to lunar mare basalt sources two of the sources have compositions that are similar to olivine and pyroxene rich mafic cumulates with variable proportions of a rb enriched phase such as amphibole whereas the third source has the composition of liquid trapped in the cumulate pile i e similar to kreep after 99 crystallization correlation between the proportion of trapped liquid in the meteorite source regions and estimates of fo2 suggest that the kreep like component may be hydrous the success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean and that assimilation of evolved crust by mantle derived magmas is not required furthermore the decoupling of major element and trace element isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions whereas the major element abundances are limited by eutectic melting processes at the time of magma formation differences in major element abundances of parental magma therefore result primarily from fractional crystallization after leaving their source regions

sr nd elements and isotopes as tracers of dust input in a tropical soil chronosequence abstract although hainan island is located in one of the relative least dusty regions in china asian dust has been identified in hainan soils we used sr and nd isotopes of soils saprolite bedrocks and asian dust to not only identify long range transported dust but also quantify the dust contribution to hainan soils sr and nd isotopic compositions of the soils are clearly affected by dust deposition the soils in near surface horizons have 87 sr 86 sr values as high as 0 7193 that are more radiogenic than the basalts indicating involvement of asian dust whereas deep horizons show a dominantly basaltic signature 87 sr 86 sr 0 7038 the e nd 0 values for hainan soils range from 6 46 to 1 85 the higher value closely approximates the values measured for the hainan lava and the lower value indicates the fingerprint of the asian dust the 87 sr 86 sr and e nd 0 values for soils also show a progressive trend with time which implies that dust accumulates with soil age we calculated the dust accretion rates for sr based and nd based to quantify the dust contributions to soils the estimate of dust accreted shows an approximate increasing trend with the ages of soils for sr based and nd based methods the soils in the old sites are likely to have undergone long term dust input and accumulated high dust levels we also find excellent agreement in the amount of dust present and dust accretion rates based on the two tracers at the 300 ka site and the 1120 ka site respectively besides lower dust deposition in the pliocene epoch than the late quaternary over eastern eurasian the loss of mineralogical and geochemical tracers over million year time scales also may lead to underestimates of time averaged dust deposition rates in the old sites therefore the young sites 180 ka and 300 ka should provide more realistic estimates of average dust accretion rates the importance of dust in the tropical hainan soil chronosequence highlights the significance of dust accretion to soils globally

zagami product of a two stage magmatic history abstract large specimens of the zagami shergottite show highly varied grain sizes and mineral abundances on a cm scale and preferred alignment of pyroxene laths we document the presence of whitlockite and of melt veins and pockets of shock origin pyroxene crystals have homogeneous mg rich pigeonite and augite cores overgrown by fe rich zoned pyroxene mostly pigeonite rims amphibole bearing magmatic melt inclusions occur exclusively in the cores we conclude that zagami experienced a twostage crystallization history the first stage occurred in a deep seated slowly cooling magma chamber there the homogeneous mg rich cores of the pyroxenes crystallized during relatively slow cooling the deep seated origin of the cores is also indicated by the presence of amphibole within them which requires pressures of formation equivalent to crystallization at depths 7 5 km on mars modest abundance of homogeneous mg rich cores 15 20 indicates that crystal settling did not play a significant role in this part of the magma chamber during the second stage the mg rich pyroxenes were entrained into a magma that either intruded to the near surface and cooled in a relatively thin dike or sill or extruded to the surface and crystallized in a lava flow 10 m thick again without indications for crystal settling this scenario is suggested by estimates of cooling rates of 0 1 0 5 c h based on sizes of the plagioclase maskelynite crystals and of 0 02 c h based on the width of pyroxene exsolution lamellae brearley 1991 irregular shapes of the mg rich cores result from resorption and possible solid state diffusion and the shapes and sizes of the pyroxenes after crystallization of fe rich pigeonite rims onto the cores were strongly controlled by the shapes and sizes of the cores the finer grained areas of the rock inherited smaller and more numerous mg rich pyroxene cores from the first stage than did the coarser grained areas crystallization of both augite and pigeonite cores at depth but mostly pigeonite rims in the nearsurface environment may be the result of a phase boundary shift and expansion of the pigeonite stability field at lower pressures the estimated depth of the magma chamber for zagami of 7 5 km and thickness of the putative lava flow of 10 m are consistent with calculations and observations of volcanic constructs and flows in the tharsis region of mars

the age of the carbonates in martian meteorite alh84001 the age of secondary carbonate mineralization in the martian meteorite alh84001 was determined to be 3 90 0 04 billion years by rubidium strontium rb sr dating and 4 04 0 10 billion years by lead lead pb pb dating the rb sr and pb pb isochrons are defined by leachates of a mixture of high graded carbonate visually estimated as approximately 5 percent whitlockite trace and orthopyroxene approximately 95 percent the carbonate formation age is contemporaneous with a period in martian history when the surface is thought to have had flowing water but also was undergoing heavy bombardment by meteorites therefore this age does not distinguish between aqueous and impact origins for the carbonates

analysis of tide and offshore storm induced water table fluctuations for structural characterization of a coastal island aquifer analysis of water table fluctuations can provide important insight into the hydraulic properties and structure of a coastal aquifer system including the connectivity between the aquifer and ocean this study presents an improved approach for characterizing a permeable heterogeneous coastal aquifer system through analysis of the propagation of the tidal signal as well as offshore storm pulse signals through a coastal aquifer offshore storms produce high wave activity but are not necessarily linked to significant onshore precipitation in this study we focused on offshore storm events during which no onshore precipitation occurred extensive groundwater level data collected on a sand barrier island sable island ns canada show nonuniform discontinuous propagation of the tide and offshore storm pulse signals through the aquifer with isolated inland areas showing enhanced response to both oceanic forcing signals propagation analysis suggests that isolated inland water table fluctuations may be caused by localized leakage from a confined aquifer that is connected to the ocean offshore but within the wave setup zone two dimensional groundwater flow simulations were conducted to test the leaky confined unconfined aquifer conceptualization and to identify the effect of key parameters on tidal signal propagation in leaky confined unconfined coastal aquifers this study illustrates that analysis of offshore storm signal propagation in addition to tidal signal propagation provides a valuable and low resource approach for large scale characterization of permeable heterogeneous coastal aquifers such an approach is needed for the effective management of coastal environments where water resources are threatened by human activities and the changing climate

uranium lead isotope systematics of mars inferred from the basaltic shergottite que 94201 abstract uranium lead ratios commonly represented as 238u 204pb calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs to better define the range of values represented by the source regions of martian basalts we completed u pb elemental and isotopic analyses on whole rock mineral and leachate fractions from the martian meteorite queen alexandra range 94201 que 94201 the whole rock and silicate mineral fractions have unradiogenic pb isotopic compositions that define a narrow range 206pb 204pb 11 16 11 61 in contrast the pb isotopic compositions of weak hcl leachates are more variable and radiogenic the intersection of the que 94201 data array with terrestrial pb in 206pb 204pb 207pb 204pb 208pb 204pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions we calculate that only 1 7 contamination is present in the purified silicate mineral and whole rock fractions whereas the hcl leachates contain up to 86 terrestrial pb this terrestrial pb contamination generated a 206pb 207pb array in the que fractions that appears to represent an ancient age which contrasts with a much younger crystallization age of 327 10 ma derived from rb sr and sm nd isochrons borg l e nyquist l e taylor l a wiesmann h and shih c y 1997 constraints on martian differentiation processes from rb sr and sm nd isotopic analyses of the basaltic shergottite que 94201 geochim cosmochim acta 61 4915 4931 despite the contamination and accepting 327 10 ma as the crystallization age we use the u pb data to determine the initial 206pb 204pb of que 94201 to be 11 086 0 008 and to calculate the value of its mantle source to be 1 82 0 01 the value calculated for the que 94201 source is the lowest determined for any martian basalt source and when compared to the highest values determined for martian basalt sources indicates that values in martian source reservoirs vary by at least a factor of two additionally the range of source values indicates that the value of bulk silicate mars is approximately three the amount of variation in the values of the mantle sources 2 4 is greater than can be explained by igneous processes involving silicate phases alone we suggest the possibility that a small amount of sulfide crystallization may generate greater extents of u pb fractionation during formation of the mantle sources of martian basalts

seismic and mechanical anisotropy and the past and present deformation of the australian lithosphere abstract we interpret the three dimensional seismic wave speed structure of the australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation we infer the fossil strain field from the orientation of gravity anomalies relative to topography bypassing the need to extrapolate crustal measures and derive the current direction of mantle deformation from present day plate motion our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation the distribution of azimuthal seismic anisotropy is determined from multi mode surface wave propagation mechanical anisotropy or the directional variation of isostatic compensation is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands the joint interpretation of seismic and tectonic data resolves a rheological transition in the australian upper mantle at depths shallower than 150 200 km strong seismic anisotropy forms complex patterns in this regime the seismic fast axes are at large angles to the directions of principal shortening defining a mechanically coupled crust mantle lid deformed by orogenic processes dominated by transpression here seismic anisotropy may be considered frozen which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile the azimuthal seismic anisotropy below 200 km is weaker and preferentially aligned with the direction of the rapid motion of the indo australian plate the alignment of the fast axes with the direction of present day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle motion expressed in the hot spot reference frame matches the seismic observations better than the no net rotation reference frame thus seismic anisotropy supports the notion that the hot spot reference frame is the most physically reasonable independently from plate motion models seismic anisotropy can be used to derive a best fitting direction of overall mantle shear

disturbance of isotope systematics during experimental shock and thermal metamorphism of a lunar basalt with implications for martian meteorite chronology to better determine the effects of impact related processes on radiometric chronometers in meteorites we undertook an isotopic study of experimentally shocked and heated samples of lunar basalt 10017 shock experiments at 55 gpa were completed on one subsample and a second subsample was heated in an evacuated quartz tube at 1000 c for 170 h a third subsample was maintained as a control samarium neodymium rb sr 238 u 206 pb and 206 pb 207 pb isotopic analyses were completed on mineral fractions leached and unleached leached whole rocks and complementary acid leachates disturbance in the shocked and heated samples was evaluated through comparison of their isochron diagrams with those of the control sample the sm nd isotope system was the least disturbed the rb sr isotope system was more disturbed and the 238 u 206 pb and 206 pb 207 pb isotope systems were the most disturbed by shock and annealing samples that experienced extended heating demonstrated greater isotopic disturbances than shocked samples in some cases the true crystallization age was preserved and in others age information was degraded or destroyed in no case did the experiments generate isochrons that maintained linearity while being rotated or completely reset although our results show that neither experimental shock nor thermal metamorphism alone can account for the discordant ages represented by different isotope systems in some martian meteorites we postulate that shock metamorphism may render a meteorite more susceptible than its unshocked counterpart to subsequent disturbance during extended impact related heating or aqueous alteration the combination of these processes may result in the disparate chronometric information preserved in some meteorites

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

uranium lead age of baddeleyite in shergottite roberts massif 04261 implications for magmatic activity on mars 1 the issue of the duration of martian magmatic activity is one of the controversial debates recently whether the young radiometric ages of 180 ma for shergottites a class of martian meteorites reflect the timing of crystallization from a magma or rather record later events related to shock metamorphism in order to address the timescale over which mars was geologically active we have undertaken u pb isotopic studies on baddeleyite in the roberts massif rbt 04261 shergottite baddeleyite in rbt 04261 is usually associated with ilmenite and shows monoclinic structure suggesting that the baddeleyite could have formed by crystallization from a residual liquid and not by shock metamorphism in situ u th pb isotopic analyses of baddeleyite yield a young238u 206pb age of 200 ma since the u pb system of baddeleyite is considered to be more resistant to resetting during reheating events and olivine in rbt 04261 preserves an igneous calcium zoning the young age of baddeleyite could be interpreted as a crystallization age of rbt 04261 the present results imply that martian magma was still forming only 200 ma ago and that mars had been geologically active until the recent past

development of finite strain in the convecting lower mantle and its implications for seismic anisotropy 1 seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle a combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics specifically it is investigated whether lattice preferred orientation lpo can explain the anisotropy observed in regions of paleosubduction since lpo is caused by dislocation creep and is destroyed by diffusion creep we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric strain is then calculated in these regions and is related to mineral physics experiments combined with high pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range the effect of variations in transition stress activations parameters and strength of slabs on fabric development is investigated it is shown that lpo is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction

regolith breccia northwest africa 7533 mineralogy and petrology with implications for early mars northwest africa 7533 a polymict martian breccia consists of fine grained clast laden melt particles and microcrystalline matrix while both melt and matrix contain medium grained noritic monzonitic material and crystal clasts the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars microbasaltic clasts vitrophyric and microcrystalline spherules and shards the clast laden melt rocks contain clump like aggregates of orthopyroxene surrounded by aureoles of plagioclase some shards of vesicular melt rocks resemble the pyroxene plagioclase clump aureole structures submicron size matrix grains show some triple junctions but most are irregular with high intergranular porosity the noritic monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths and cooled more slowly than rocks with zoned pyroxene or fine grain size noritic material contains orthopyroxene or inverted pigeonite augite calcic to intermediate plagioclase and chromite to cr bearing magnetite monzonitic clasts contain augite sodic plagioclase k feldspar ti bearing magnetite ilmenite chlorapatite and zircon these feldspathic rocks show similarities to some rocks at gale crater like black trout mara and jake m the most magnesian orthopyroxene clasts are close to alh 84001 orthopyroxene in composition all these materials are enriched in siderophile elements indicating impact melting and incorporation of a projectile component except for ni poor pyroxene clasts which are from pristine rocks clast laden melt rocks spherules shards and siderophile element contents indicate formation of nwa 7533 as a regolith breccia the zircons mainly derived from monzonitic melt rocks crystallized at 4 43 0 03 ga humayun et al 2013 and a 147sm 143nd isochron for nwa 7034 yielding 4 42 0 07 ga nyquist et al 2016 defines the crystallization age of all its igneous portions the zircon from the monzonitic rocks has a higher 17o than other martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration nemchin et al 2014 this record of protolith interaction with atmosphere hydrosphere during regolith formation before melting demonstrates a thin atmosphere a wet early surface environment on mars and an evolved crust likely to have contaminated younger extrusive rocks the latest events recorded when the breccia was on mars are resetting of apatite much feldspar and some zircons at 1 35 1 4 ga bellucci et al 2015 and formation of ni bearing pyrite veins during or shortly after this disturbance lorand et al 2015