imaging near surface heterogeneities by natural migration of backscattered surface waves s u m m a r y we present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface this migration method denoted as natural migration uses recorded green s functions along the surface instead of simulated green s functions the key assumptions are that the scattering bodies are within the depth interrogated by the surface waves and the green s functions are recorded with dense receiver sampling along the free surface this natural migration takes into account all orders of multiples mode conversions and non linear effects of surface waves in the data the natural imaging formulae are derived for both active source and ambient noise data and computer simulations show that natural migration can effectively image near surface heterogeneities with typical ambient noise sources and geophone distributions

alteration geochemistry of the nopal i uranium deposit sierra pena blanca mexico a natural analogue for a radioactive waste repository in volcanic tuffs natural analogues provide an approach to characterize and test the long term modelling of a repository performance this article presents geochemical information about the alteration conditions of the nopal i uranium deposit mexico an analogue for the proposed yucca mountain radioactive waste repository mineralization and hydrothermal alteration of volcanic tuffs are contemporaneous according to petrographic observations trace element geochemistry u th ree provides evidence for local mobilization of uranium under oxidizing conditions and further precipitation under reducing conditions o and hisotope geochemistry of kaolinite smectite opal and calcite suggests that argillic alteration proceeded at shallow depth with meteoric water at 25 75 c a low temperature context unusual for volcanic hosted uranium deposits this temperature range is compatible with some post closure evolution models of the proposed yucca mountain repository

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

migration of surface seismic data with vsp green s functions seismic migration is the process by which wavefields recorded on or near the surface are mapped back into the subsurface to form an image of the subsurface structure common to all current migration methods is that they rely on a model of the subsurface for computing green s functions these computed green s functions are approximations of the true green s function in the subsurface the more accurate the method used for computing the green s functions and to some extent also the choice of imaging condition the better image we will get most state of the art migration methods will use green s functions that include multiple arrivals and some finite frequency effects allowing them to image fairly complex geologic structures however the computational model in most cases uses only scalar velocity that is smooth on a wavelength scale thus ignoring scattering effects from small scale structure anisotropy and other intrinsic rock effects further whatever the model is it has to be derived from the dat

imaging near surface heterogeneities by natural migration of backscattered surface waves field data test abstractwe have developed a methodology for detecting the presence of near surface heterogeneities by naturally migrating backscattered surface waves in controlled source data the near surface heterogeneities must be located within a depth of approximately one third the dominant wavelength of the strong surface wave arrivals this natural migration method does not require knowledge of the near surface phase velocity distribution because it uses the recorded data to approximate the green s functions for migration prior to migration the backscattered data are separated from the original records and the band passed filtered data are migrated to give an estimate of the migration image at a depth of approximately one third each band passed data set gives a migration image at a different depth results with synthetic data and field data recorded over known faults validate the effectiveness of this method migrating the surface waves in recorded 2d and 3d data sets accurately reveals the locations of kn

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

oligo miocene rift of sardinia and the early history of the western mediterranean basin the geodynamic evolution of the western mediterranean basin in spite of many studies is still uncertain there is some consensus for interpreting this basin as a kind of small oceanic marginal basin its opening has generally been related to a subduction process which was active during the oligocene miocene somewhere east of sardinia corsica1 7 as the margins of the basin are deeply buried below miocene to present sediments direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8 10 or missing altogether to obtain such data detailed field studies have been undertaken in sardinia fig 1 and the first results are presented here this approach is justified by the fact that in that island oligocene and miocene sediments were deposited in a rift fossa tettonica sarda of verdabasso11 which is the easternmost arm of the complex rift system that affected the european plate during oligocene and miocene times one of these arms evolved towards a small oceanic basin the western mediterranean or algero provencal basin while others such as the gulf of valencia and the sardinia rift aborted and remained at the rift stage its exceptional exposures make it possible to examine the sardinia rift to clarify the sequence of events which created it and to establish a sedimentological model which we believe is directly applicable to the western mediterranean basin

shock induced melting recrystallization and exsolution in plagioclase from the martian lherzolitic shergottite grv 99027 plagioclase in the martian lherzolitic shergottite grove mountains grv 99027 was shocked melted and recrystallized the recrystallized plagioclase contains lamellae of pyroxene olivine and minor ilmenite 1 m wide both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools the formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt distinct from other martian meteorites grv 99027 contains no maskelynite but does contain recrystallized plagioclase this shows that the meteorite experienced a slower cooling than maskelynite bearing meteorites we suggest that the parent rock of grv 99027 could have been embedded in hot rocks which facilitated a more protracted cooling history

decline of giant impacts on mars by 4 48 billion years ago and an early opportunity for habitability the timing of the wane in heavy meteorite bombardment of the inner planets is debated its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival and so bounds the occurrence of conditions that allow planets to be habitable here we determine this timing for mars by examining the metamorphic histories of the oldest known martian minerals 4 476 4 429 gyr old zircon and baddeleyite grains in meteorites derived from the southern highlands we use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life limiting shock pressure of 78 gpa 97 of the grains exhibit weak to no shock metamorphic features and no thermal overprints from shock induced melting by contrast about 80 of the studied grains from bombarded crust on earth and the moon show such features the giant impact proposed to have created mars hemispheric dichotomy must therefore have taken place more than 4 48 gyr ago with no later cataclysmic bombardments considering thermal habitability models we conclude that portions of mars crust reached habitable pressures and temperatures by 4 2 gyr ago the onset of the martian wet period about 0 5 gyr earlier than the earliest known record of life on earth early abiogenesis by 4 2 gyr ago is now tenable for both planets the oldest known minerals from mars have no strong shock features indicating early cessation of giant impacts there according to microanalysis of zircon and baddeleyite grains in meteorites

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

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

strong sh to love wave scattering off the southern california continental borderland seismic scattering is commonly observed and results from wave propagation in heterogeneous medium yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging in this study we analyze broadband waveforms recorded by the southern california seismic network and observe strongly scattered love waves following the arrival of teleseismic sh wave these scattered love waves travel approximately in the same azimuthal direction as the incident sh wave at a dominant period of 10 s but at an apparent velocity of 3 6 km s as compared to the 11 km s for the sh wave back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the southern california continental borderland in particular the patton escarpment finite difference simulations using a simplified 2 d bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers the modeling suggests a relatively low shear wave velocity in the continental borderland

mineralogical and fluid inclusion studies of low sulfidation epithermal veins at osilo sardinia italy several precious metal bearing low sulfidation epithermal veins occur in the rolling topography of the osilo area northern sardinia the sa pala de sa fae and the sa pedra bianca veins were subject to intense diamond drilling exploration in the mid 1990 s the veins extend for 1 3 km dip steeply and range from 1 to 10 m in width high k calc alkaline volcanic deposits containing plagioclase phenocrysts along with lesser pyroxene amphibole magnetite olivine and sanidine form the main host rocks gold grades in drill intersections range from 50 m from the vein the assemblage albite fe chlorite illite pyrite montmorillonite calcite k feldspar prevails regionally and its formation is attributed to minor metasomatism of the country rock involving the addition of water carbon dioxide and hydrogen sulfide at distances 10 m from the vein the assemblage quartz k feldspar pyrite illite dominates forming an alteration envelope that cross cuts regional alteration quartz and k feldspar increase in abundance towards the vein quartz is the main vein mineral and it displays a range of morphologies and textures including crustiform colloform banding quartz pseudomorphs of platy calcite breccias and coarse euhedral crystals electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb shaped crystals of k feldspar adularia pyrite plus lesser marcasite arsenopyrite stibnite and sphalerite are the other sulfide phases in veins kaolinite halloysite jarosite form a late assemblage overprinting earlier hydrothermal alteration it is mostly restricted to shallow depths of a few meters except near veins most of this assemblage likely formed from weathering and oxidation of sulfides microthermometric measurements were made on quartz hosted two phase liquid vapor inclusions containing 75 liquid mean homogenization temperatures 750 measurements range from 220 to 250 c and ice melting temperatures 550 measurements range from 0 0 to 2 3 c the presence of co existing vapor rich and liquid rich inclusions with quartz pseudomorphs of platy calcite indicate that boiling conditions existed slight vapor bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas this study shows that gold silver mineralization formed in subvertical channels from ascending solutions at 250 c at around 300 to 450 m below the paleo water table in a typical low sulfidation epithermal environment hydrothermal solutions that produced vein mineralization and related alteration were dilute 4 1 equivalent wt nacl and 4 wt co2 near neutral ph reduced and at times boiling

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

signatures of the martian atmosphere in glass of the zagami meteorite isotopic signatures of nitrogen argon and xenon have been determined in separated millimeter sized pockets of shock melted glass in a recently identified lithology of the meteorite zagami a shergottite the ratio of nitrogen 15 to nitrogen 14 which is at least 282 per mil larger than the terrestrial value the ratio of xenon 129 to xenon 132 2 40 and the argon isotopic abundances match the signatures previously observed in the glassy lithology of the antarctic shergottite eeta 79001 these results show that the signatures in eeta 79001 are not unique but characterize the trapped gas component in shock melted glass of shergottites the isotopic and elemental ratios of nitrogen argon and xenon closely resemble the viking spacecraft data for the martian atmosphere and provide compelling evidence for a martian origin of the two shergottites and by extension of the meteorites in the shergottites nakhlites chassignites snc group

concordant rb sr sm nd and ar ar ages for northwest africa 1460 a 346 ma old basaltic shergottite related to lherzolitic shergottites abstract multiple lines of evidence show that the rb sr sm nd and ar ar isotopic systems individually give robust crystallization ages for basaltic or diabasic shergottite northwest africa nwa 1460 in contrast to other shergottites nwa 1460 exhibits minimal evidence of excess 40ar thus allowing an unambiguous determination of its ar ar age the concordant rb sr sm nd and ar ar results for nwa 1460 define its crystallization age to be 346 17 ma 2 in combination with petrographic and trace element data for this specimen and paired meteorite nwa 480 these results strongly refute the suggestion by others that the shergottites are 4 1 ga old current crystallization and cosmic ray exposure cre age data permit identification of a maximum of nine ejection events for martian meteorites numbering more than 50 unpaired specimens as of 2008 and plausibly as few as five such events although recent high resolution imaging of the martian surface has identified limited areas of sparsely cratered terrains the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come or that the cratering chronology needs recalibration time averaged 87rb 86sr 0 16 for the mantle source of the parent magma of nwa 1460 480 over the 4 56 ga age of the planet is consistent with previously estimated values for bulk silicate mars in the range 0 13 0 16 and similar to values of 0 18 for the lherzolitic shergottites initial end for nwa 1460 480 at 350 16 ma ago was 10 6 0 5 which implies a time averaged 147sm 144nd of 0 217 in the martian mantle prior to mafic melt extraction similar to values of 0 211 0 216 for the lherzolitic shergottites these time averaged values do not imply a simple two stage mantle melt evolution but must result from multiple episodes of melt extractions from the source regions much higher late stage end values for the depleted shergottites imply similar processes carried to a greater degree thus nwa 1460 480 the lherzolitic shergottites and perhaps eet 79001 give the best albeit imperfect estimate of the sr and nd isotopic characteristics of bulk silicate mars

mineral chemistry and in situ upb geochronology of the mare basalt northwest africa 10597 implications for low ti mare volcanism around 3 0 ga abstract unbrecciated mare basalts are rare in the lunar meteorite collection found in 2015 northwest africa nwa 10597 is a medium grained low ti mare basalt with a subophitic texture the meteorite consists mostly of mm sized pyroxene and plagioclase with minor olivine spinel ilmenite phosphates silica and trace zr rich minerals such as baddeleyite zirconolite and tranquillityite a portion of plagioclase and silica has been transformed to their high pressure polymorphs due to shock metamorphism nwa 10597 has a low tio2 content 2 9 wt but is relatively enriched in rare earth elements ree laaverage 65 ci with an overall unfractionated pattern except for a negative eu anomaly calculated ree concentrations of parent melts in equilibrium with mg rich pyroxene and ca rich plagioclase suggest no significant assimilation of ree rich melts after the onset of pyroxene crystallization in situ u pb analyses of baddeleyite and apatite reveal a mutually consistent age of 3 0 ga which is also in excellent agreement with that of low ti mare basalts nwa 4734 and lapaz icefield lap 02205 dated with other independent techniques the concordance suggests no significant thermal disturbance in the u pb isotopic system of nwa 10597 although it was heavily shocked nwa 10597 closely resembles nwa 4734 in terms of petrographic texture mineral chemistry and geochronology indicating a pairing relationship

variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events abstract the accurate dating of igneous and impact events is vital for the understanding of solar system evolution but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole rock isotopic systems used for geochronology baddeleyite monoclinic zro2 is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust u pb isotopic systematics but there is little understanding of shock effects on this phase here we present new nano structural measurements of baddeleyite grains in a thin section of the highly shocked basaltic shergottite northwest africa nwa 5298 using high resolution electron backscattered diffraction ebsd and scanning transmission electron microscopy stem techniques to investigate shock effects and their linkage with u pb isotopic disturbance that has previously been documented by in situ u pb isotopic analyses the shock altered state of originally igneous baddeleyite grains is highly variable across the thin section and often within single grains analyzed grains range from those that preserve primary magmatic twinning and trace element zonation baddeleyite shock group 1 to quasi amorphous zro2 group 2 and to recrystallized micro granular domains of baddeleyite group 3 these groups correlate closely with measured u pb isotope compositions primary igneous features in group 1 baddeleyites n 5 are retained in high shock impedance grain environments and an average of these grains yields a revised late amazonian magmatic crystallization age of 175 30 ma for this shergottite the youngest u pb dates occur from group 3 recrystallized nano to micro granular baddeleyite grains indicating that it is post shock heating and new mineral growth that drives much of the isotopic disturbance rather than just shock deformation and phase transitions our data demonstrate that a systematic multi stage microstructural evolution in baddeleyite results from a single cycle of shock loading heating and cooling during transit to space and that this leads to variable disturbance of the u pb isotope system furthermore by linking in situ u pb isotopic measurements with detailed micro to nano structural analyses it is possible to resolve the timing of both endogenic crustal processes and impact events in highly shocked planetary materials using baddeleyite this opens up new opportunities to refine the timing of major events across the solar system

mineralogical and stable isotope studies of kaolin deposits shallow epithermal systems of western sardinia italy large kaolin deposits hosted by miocene silicic pyroclastic rocks in northwestern sardinia represent hydrothermal alteration formed within 200 m of the miocene paleosurface boiling hydrothermal fluids ascended steeply dipping faults that are enveloped by altered rock the broadly stratiform kaolin deposits constitute advanced argillic alteration that was produced in a steam heated zone near the paleosurface overlying the deeper hydrothermal systems the deeper zones represent two distinct types of epithermal systems weakly acidic inferred low sulfidation systems at tresnuraghes and acidic high sulfidation systems at romana tresnuraghes is characterized at depth by chalcedony quartz barite veins within a 50 m wide zone of k feldspar quartz illite alteration and overlying local occurrences of chalcedony sinter which define the paleosurface kaolin deposits near the paleosurface are characterized by zonation outward and downward from an inner shallow zone of kaolinite 1t opal dickite alunite 20 m diam grains to an outer deeper kaolinite 1m montmorillonite cristobalite this zonation indicates formation by descending acidic fluids the system evolved from ascending weakly acidic or neutral fluids that boiled to produce h2s rich vapor which condensed and oxidized within the near surface vadose zone to form steam heated acid sulfate waters and kaolin alteration at romana veins at depth contain chalcedony or quartz and minor pyrite and are enclosed in up to 20 m wide zones of kaolinite 1t quartz alteration near hydrothermal vents along the paleosurface chalcedonic silica is enclosed within a zone of kaolinite 1t alunite 50 m diam grains quartz opal dickite cristobalite kaolin quarries near the paleosurface display outward and downward zoning to kaolinite 1t opal cristobalite and then to montmorillonite kaolinite 1t opal consistent with formation by descending low ph fluid the siliceous and advanced argillic alteration along steep conduits formed from acidic ascending magmatic hydrothermal fluids whereas the near surface kaolin formed from steam heated meteoric waters alteration mineral assemblages and stable isotope data provide evidence of the temperature and source of hydrothermal fluids barite from tresnuraghes average 18o 17 1 34s 18 8 one alunite sample from romana 18o 12 0 d 3 34s 16 7 and quartz from both localities 18o 15 9 22 0 formed in hydrothermal feeders source fluids were likely mixtures of meteoric water and minor magmatic fluid similar to other epithermal systems kaolinite dickite minerals from the kaolin deposits 18o 16 6 21 4 d 43 to 53 formed from steam heated meteoric water having d 20 per mil consistent with the presence of anomalous hg and fine grained na and fe poor alunite the laterally extensive kaolin deposits in sardinia and possibly similar deposits elsewhere in the world appear to represent the uppermost parts of large hydrothermal systems that may be prospects for gold at depth

shock induced deformation of shergottites shock pressures and perturbations of magmatic ages on mars shergottites and chassignites practiced major deformation effects whose nature magnitude and relevance were controversially evaluated and disputatively debated our studies of many shocked shergottites present contrary to numerous previous reports ample evidence for pervasive shock induced melting amounting of at least 23 vol of the shergottite consisting of maskelynite and pyrrhotite partial melting of pyroxene titanomagnetite ilmenite and finding of several high pressure polymorphs and pressure induced dissociation reactions our results cast considerable doubt on using the refractive index ri or cathodoluminescence cl spectra of maskelynite in estimating the magnitudes of peak shock pressure in both shergottites and ordinary chondrites ri of maskelynite was set after quenching of the feldspar liquid before decompression to maskelynite glass followed by glass relaxation after decompression at the closure temperature of relaxation the ri procedure widely practiced in the past 38 years revealed unrealistic very high pressure estimates discrepant with the high pressure mineral inventory in shocked shergottites and ordinary chondrites and with results obtained by robust laboratory static experiments shergottites contain the silica high pressure polymorphs the scrutinyite structured polymorph seifertite a monoclinic ultra dense polymorph of silica with zro2 structure stishovite a dense liquidus assemblage consisting of stishovite na hexa aluminosilicate na cas and both k lingunite and ca lingunite applying individual high pressure silica polymorphs alone like stishovite to estimate the equilibrium shock pressure is inadequate due to the considerable shift of their nominal upper pressure bounds intrinsically induced by spatially variable absorptions of minor oxides like al2o3 na2o feo mgo and tio2 this practice revealed variable pressure estimates even within the same shergottite subjected to the same peak shock pressure occurrence of na cas stishovite lack of the naalsio4 ca ferrite structured polymorph or jadeite indicates that the peak shock pressures barely exceeded 22 gpa we present convincing and ample evidence refuting the claim that the shock induced high pressure inventory in shergottites and ordinary chondrites are disequilibrium assemblages resulted from local pressure spikes in excess of 80 gpa and during the decompression stage such scenario calls for a series of incomplete and quenched retrograde reactions starting with the crystallization of mg silicate perovskite magnesiowustite if the claimed peak shock pressure was in excess of 80 gpa this would be followed by replacement of this pair by majorite pyrope ss magnesiowustite or akimotoite magnesiowustite below 23 gpa and 2000 degrees c polycrystalline ringwoodite above 16 gpa respectively and finally replacement by polycrystalline olivine below 16 gpa such incomplete retrograde reactions were never encountered in any shergottite chassignite or shocked ordinary chondrite so far olivine ringwoodite phase transformation in the l6 y 791384 commences with the coherent mechanism producing ringwoodite lamellae with their 111 planes parallel to the 100 of olivine followed by the incoherent mechanism due to build up of strain in the parental olivine this is in accord with the olivine ringwoodite settings produced in static laboratory experiments in a multi anvil device olivine ringwoodite phase transitions were also encountered in comparable settings in the shergottite nwa 1068 application of experimentally obtained kinetic parameters of the olivine ringwoodite phase transitions reveals possible duration of the natural dynamic events up to few seconds thus unambiguously refuting the claimed disequilibrium decompression mechanism the shock induced pervasive melting of labradorite pyrrhotite titanomagnetite ilmenite and partial melting of clinopyroxene strongly suggests shock induced partial to complete resetting of the ar ar rb sr sm nd re os u pb and lu hf radiometric systems this also casts considerable doubt on the radiometric ages shorter than 575 ma reported in the past 38 years to allegedly be the igneous crystallization ages these short ages probably resulted from partial or total shock induced age resetting c 2012 elsevier ltd all rights reserved

mineralogic and stable isotope zonation at the surface over the el salvador porphyry copper deposit chile we examined in detail the mineralogic and stable isotope characteristics of alteration minerals exposed at the surface of the el salvador porphyry copper deposit a total of 276 samples was collected from 203 localities over an area of 5 km 2 at elevations between 2 900 and 3 300 m the alteration assemblages can be separated into two groups broadly corresponding to the stages previously identified from underground studies although the early stage k silicate alteration identified underground is not evident at the surface a transitional to late stage is characterized at the surface by assemblages of 1 muscovite andalusite with trace diaspore including some areas rich in andalusite and 2 muscovite with trace diaspore there is a sharp contact outward with a marginal propylitic assemblage chlorite calcite epidote pyrophyllite occurs as a retrograde overprint of muscovite in both of the transitional to late stage assemblages these assemblages are concentrically zoned and centered on a core of least altered granodiorite porphyry a late intramineral intrusion the muscovite andalusite assemblage occurs up to 500 m away from the granodiorite porphyry and andalusite rich zones are located near the brecciated margin of the porphyry further outward is the muscovite zone from 500 to 1 500 m wide the pyrophyllite overprint overlaps the contact of the muscovite andalusite and muscovite zones and occurs mainly above 3 000 m elevation such that its present distribution is controlled largely by topography very late alteration minerals occur in radial pebble dikes of hydrothermal breccia and their extensions and they are dominated by an advanced argillic assemblage of alunite aluminum phosphate sulfate minerals diaspore zunyite pyrophyllite dickite with residual muscovite in one case a breccia extends upward and appears to be the root of a 300 500 m outcrop 80 m thick of massive quartz alunite replacement of rhyolite pure mineral separates were prepared from these samples for o h and s isotope analysis including 48 samples of silicate minerals muscovite pyrophyllite dickite and supergene kaolinite and 20 samples of alunite some supergene in origin information on mineral assemblage was used to estimate temperatures of formation which then allowed calculation of the isotopic composition of associated hydrothermal fluids the fluid responsible for forming the muscovite had a relatively narrow range of isotopic composition 6 5 1 5 per mil 18 o and 40 10 per mil d the heaviest 18 o and d values about 8 and 35 per mil respectively were obtained from the samples with andalusite muscovite diaspore assemblage this composition is close to that expected for a magmatic fluid with 10 to 20 percent meteoric water component responsible for forming the other muscovite samples the fluid compositions calculated for the very late stage alunite range from 5 and 30 per mil to 1 and 40 per mil 18 o and d respectively this range in composition agrees with the trend predicted for magmatic vapor absorbed by local groundwater the latter similar in composition to that responsible for forming the supergene kaolinite there is no systematic spatial zonation of the alunite samples in terms of isotopic composition or paleotemperature the temperature estimates from s isotope geothermometry are higher than those based on mineral assemblage possibly caused by partial supergene replacement of some hypogene alunites there is no strong isotopic zonation of hydrothermal minerals from surface alteration assemblages at el salvador that can be applied to generate clear exploration guidelines by contrast mapping of alteration mineralogy in the field backed up by laboratory checking of mineralogy and examination of relict minerals and replacement textures in thin section clearly indicates the center of the late intramineral intrusion

genesis of acid sulfate alteration and au cu ag mineralization at summitville colorado the summitville au cu ag deposit occurs within a porphyritic quartz latite primarily in a zone of vuggy silica alteration characterized by leaching of all major elements except silica and iron vuggy silica is best developed between an elevation of 3 700 and 3 500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz alunite alteration the vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3 400 m these deep vuggy silica zones are enclosed by quartz kaolinite alteration with little or no alunite sulfide mineralization occurs primarily within the vuggy silica the sulfide assemblage changes with decreasing elevation from covellite luzonite or native gold to chalcopyrite tennantite sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite gold also occurs in a near surface barite goethite jarosite assemblage that crosscuts the vuggy silica consideration of alunite kaolinitc and pyrite stability relations and of aluminum speciation at 250 degrees c indicates that a ph of 2 or below and a log f sub o 2 of 31 or 1 at 250 degrees c were necessary to produce vuggy silica alteration the chalcopyrite tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions the covellite luzonite or gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization barite gold mineralization is interpreted as a near surface assemblage produced under highly acidic oxidizing conditions these textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid leaching event the change from extreme acid leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic vapor dominated to a liquid dominated system leading to more reducing and less acidic conditions during sulfide deposition

preservation of primordial signatures of water in highly shocked ancient lunar rocks abstract spurred by the discovery of water in lunar volcanic glasses about a decade ago the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water this is due to its ability to contain significant amounts of oh in its structure along with the widespread presence of apatite in lunar rocks there is a general understanding that crustal cumulate rocks of the lunar magnesian mg suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks such as mare basalts consequently water bearing minerals in mg suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior mg suite rocks and most other apollo samples that were collected at the lunar surface display variable degrees of shock deformation in this study we have investigated seven apollo 17 mg suite samples that include troctolite gabbro and norite lithologies in order to understand if shock processes affected the water abundances and or h isotopic composition of apatite the measured water contents in apatite grains range from 31 to 964 ppm with associated d values varying between 535 134 and 147 194 2 considering the full dataset there appears to be no correlation between h2o and d of apatite and the level of shock each apatite grain has experienced however the lowest d was recorded by individual water poor 100 ppm h2o regardless of the complexity of the shock induced nanostructures there appears to be no evidence of water loss or alteration in their d the weighted average d value of 24 such water rich apatites is 192 71 and of all 36 analyzed spots is 209 47 indistinguishable from that of other kreepy lunar lithologies or the earth s deep mantle despite experiencing variable degrees of shock deformation at a later stage in lunar history water rich apatite in some of the earliest formed lunar crustal material appears to retain the original isotopic signature of h in the moon

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

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

geochemistry of the high pco2 waters in logudoro sardinia italy abstract a geochemical study of the high pco2 waters in logudoro northern sardinia was carried out starting from regional hydrogeochemical prospecting for geothermal energy based on the major dissolved components and some minor elements this preliminary investigation led to the identification of five different lithologies marking the different aquifers the high pco2 waters can be divided into the less saline tds

in situ u pb sims in sims micro baddeleyite dating of mafic rocks method with examples abstract an in situ u pb sims in sims method to date micro baddeleyite crystals as small as 3 m is presented with results from three samples that span a variety of ages and geologic settings the method complements id tims geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation x ray mapping and bse imaging are used to locate target grains in thin section followed by sims analysis on a cameca ims 1270 using the field aperture in the transfer column to screen out ions from host phases internal age precisions for the method are anticipated to range from 0 1 for precambrian rocks to 3 7 for phanerozoic rocks results establish a 2689 5 ma age for mafic dikes in the wyoming craton usa a 1540 30 ma age for a subaerial lava flow from the thelon basin of northern canada and a 457 34 ma age for mafic dikes in the platform sequence of southeastern siberia the method is ideal for relatively non destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples

a discussion of isotopic systematics and mineral zoning in the shergottites evidence for a 180 m y igneous crystallization age abstract proposed igneous and metamorphic ages of the shergottites a suite of achondritic basaltic meteorites are examined in terms of the shergottites petrography and mineral chemistry no metamorphic or hydrothermal event appears capable of resetting the refractory sr and nd isotopic systems of the shergottites since the time of their igneous crystallization the approximately concordant rb sr 180 20 m y and u th pb 190 30 m y internal isochrons are thus interpreted as representing the time of igneous crystallization the sm nd internal isochrons which yield older apparent ages also show large scatter and are regarded as artifacts of the shergottites complex igneous history rubidium sr sm nd and pb pb wholerock isochrons are interpreted as mixing lines and are reasonably attributed to igneous processes such as wall rock assimilation and magma mixing if the shergottites are indeed less than 200 m y old they represent the youngest known extraterrestrial basalts this conclusion is supportive of the hypothesis that the snc meteorites are samples of the planet mars

solfataric alteration in the san juan mountains colorado oxygen isotope variations in a boiling hydrothermal environment an oxygen isotope study was made on hydrothermal quartz and altered volcanic rocks from six areas of advanced argillic alteration in the san juan volcanic field southwest colorado these include the historically productive mining districts at summitville delta 18 o quartz 13 0 15 5ppm and the national belle pipe at red mountain pass delta 18 o quartz 10 0 12 3ppm other areas studied are engineer pass delta 18 o quartz 6 4ppm delta 18 o sub whole rock 1 9 3 4ppm carson camp delta 18 o quartz 11 9 13 6ppm and calico peak near rico delta 18 o quartz 6 9 9 0ppm delta 18 o sub whole rock 4 7 6 9ppm samples were also analyzed from surface outcrops delta 18 o quartz 4 9 11 2ppm and from an 837 m cored drill hole in the red mountain quartz latite lava dome on the eastern edge of the 23 m y old lake city caldera at this locality the altered volcanic rocks delta 18 o sub whole rock 1 7 8 2ppm and the hydrothermal quartz delta 18 o quartz 1 4 11 2ppm exhibit a distinct vertical 18 o 16 o gradient of about 0 8 per mil 100 m with the highest delta 18 o values at the highest elevations these data are incorporated into a general model for solfataric alteration in these mid to late tertiary hot spring systems this involves a convecting meteoric hydrothermal fluid which at shallow depths rises along the hydrostatic boiling curve the delta 18 o variation in solfataric quartz in the uppermost 300 m of the hot spring system from 5 11ppm is mainly a function of a temperature variation from about 225 degrees to 125 degrees c along the boiling curve and to a lesser extent a result of the delta 18 o effects that accompany vapor phase separating during boiling

kaolinite transformation into dickite during burial diagenesis the mechanism of kaolinite transformation into dickite has been investigating using 13 samples from the froy and rind oil fields broad fourteens basin north sea 3 kaolinite specimens with different crystal order kga 2 kaolinite api 17 keokuk kaolinite and 2 dickite rich samples natural history museum collection detailed analysis of xrd thermal analysis and sem data show that 1 as dickite content increases there is also an increase of the crystal order of kaolinite 2 in dickite rich specimens kaolinite and dickite have crystals or xrd coherent domains of the same size 3 there is no specific dehydroxylation temperature for each polytype rather particle size and crystal order control dehydroxylation temperature independently of polytype 4 with progressive dickite content the development of both particle size and the size of the coherent crystal domains within particles is greater in the c direction than in the a b plane 5 the growth of defect free segments in the c direction is not connected with the growth in the a and b directions as would be expected in crystallization from solution 6 textural features indicate coalescence of kaolin plates with burial 7 there is a very weak positive correlation between particle dimensions and relative kaolinite dickite content these results are interpreted as resulting from a double reaction taking place in the solid state with burial some kaolinite domains grow in size and crystal order while other domains are transformed into dickite presumably also the dickite domains formed early in the transformation grow in crystal order the transformation into dickite stops at 90 95 dickite because the remaining kaolinite domains are so large and stable that the stability increase produced by the polytype transformation would be negligible

tectonic and litho stratigraphic controls on kaolin deposits within volcanic successions insights from the kaoliniferous district of north western sardinia italy abstract in northwestern sardinia a tertiary calc alkaline volcanic succession linked to the subduction of insubric oceanic crust and subsequent post collisional slab break off crops out and hosts several kaolin deposits we have undertaken field and laboratory studies to establish how the tertiary geodynamic setting of the western mediterranean and the composition and texture especially porosity of the volcanic rocks control the geometry and quality of the deposits massive andesites dacites and mildly welded rhyodacitic ignimbrites are the precursors of the kaolin kaolin deposits from the andesites contain pyrite and jarosite and the deposits which formed from the dacites contain up to 5 alunite the amounts of pyrite and alunite are negligible in deposits which formed from ignimbrite fracture controlled fluid dominated systems izawa 1986 responsible for the formation of the deposits are linked to nnw sse trending normal faults which formed during burdigalian to serravallian extension following on from collision between adria and europe corsica sardinia crust four types of deposit morphology bedform mushroom fault parallel and funnel are recognised based on the attitude fracturing and porosity of the precursor rock the remarkable funnel morphology was generated by repeated hydrothermal eruptions

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

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

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

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

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

shocked titanite records chicxulub hydrothermal alteration and impact age abstract hydrothermal activity is a common phenomenon in the wake of impact events yet identifying and dating impact hydrothermal systems can be challenging this study provides the first detailed assessment of the effects of shock microstructures and impact related alteration on the u pb systematics and trace elements of titanite catisio5 focusing on shocked granite target rocks from the peak ring of the chicxulub impact structure mexico a 1 mm long shock twinned titanite grain preserves a dense network of irregular microcracks some of which exploit shock twin interfaces secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks in situ laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms analysis reveals a mixture of three end member pb components the pb components are 1 common pb consistent with the pb isotopic signature of adjacent alkali feldspar 2 radiogenic pb accumulated since magmatic crystallization and 3 a secondary younger pb signature due to impact related complete radiogenic pb loss the youngest derived ages define a regression from common pb that intersects concordia at 67 4 ma in agreement with the established age of 66 04 0 05 ma for the chicxulub impact event contour maps of la mc icpms data reveal that the young ages are spatially restricted to microstructurally complex domains that correlate with significant depletion in trace elements ree y zr nb mo sn th and reduction in magnitude of the eu eu anomaly mapping by time of flight secondary ion mass spectrometry tof sims show that patterns of localised element depletion in titanite are spatially related to microcracks which are enriched in al the spatial correlation of ages and trace element abundance is consistent with localised removal of pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid mediated element transport process associated with the impact event here we interpret the 67 4 ma u pb age to represent hydrothermal pb loss in the chicxulub peak ring in the wake of the impact event these results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post impact hydrothermal activity in impact structures elsewhere

the age of snc meteorites and the antiquity of the martian surface we report new sm nd lu hf and pb pb mineral and whole rock isotope data for the basaltic shergottite zagami as well as pb pb whole rock isotope data for the basaltic shergottite los angeles the lherzolitic shergottite dar al gani 476 dag 476 and the clinopyroxenite nakhla in agreement with previous findings our new sm nd and lu hf mineral ages on the martian meteorite zagami are young 155 and 185 ma respectively the 207pb 206pb 204pb 206pb compositions of the insoluble fractions of shergottites zagami los angeles and literature data for shergotty and eeta79001 form an excellent alignment indicative of a 4 0 ga crystallization age the range of pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial pb and argues against mixing relationships the age of 4 048 0 017 ga mswd 1 5 provided by the pb isotope compositions of the zagami whole rock and residues is therefore taken to date the crystallization of this rock which so far was believed to be only not vert similar 180 ma old based on this result we argue that the lithosphere of mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the martian surface this interpretation is consistent with photographic interpretations of erosional features on mars it also relieves the constraint imposed by the presence of anomalies of 142nd and 182w both products of extinct radioactive nuclides that the martian mantle should have preserved primordial isotopic heterogeneities thus allowing for the planet interior to be actively convecting

sr and nd isotopic systematics of shergotty meteorite abstract strontium and neodymium isotopic measurements are reported on mineral separates whole rock samples and their leachates and residues of shergotty meteorite a very large variation of the sm nd ratio in the phosphates of shergotty is observed two sources of nd are observed which are not mixed completely within shergotty resulting in two ages 1 an apparent age of 147 20 m y from the whole rock leachateresidue nd trend and 2 360 16 m y from a well defined nd isochron of minerals and their leachates and residues about 60 of the sr in the rock resides in the plagioclase and mesostasis and this was reset at 167 m y for the pyroxenes this resetting event had a more complex but also a more localized effect from the general pattern of this resetting a shock event at this time seems possible the 360 m y whole rock leachate residue age in the rb sr isotopic system coincides with the sm nd isochron age we think that the 360 m y age represents the crystallization of shergotty the apparently younger nd age of the phosphates which are indicated by the leaching experiments might be caused by late or post magmatic contamination of the whitlockites with radiogenic nd this same contamination process must also have affected the sr isotopic system the 360 m y crystallization age implies that the plagioclase and phosphates behaved as an open system to the contaminant after the crystallization of the pyroxenes the sr of the contaminant affected both the plagioclase and whitlockite but nd influenced only the whitlockites reflecting the partitioning of sr and nd between these phases the general chemical pattern of this contamination process is different from contamination patterns observed in terrestrial rocks this pattern may represent the finger print of localized fluid rock interaction on mars since a similar trend is observed in other snc meteorites it could indicate that a potassium phase could be a mantle mineral on mars this would have important effects on the chemical evolution of a planet especially in respect to isotopic tracers like ar pb sr

geology mineralogy and fluid inclusion data of the furtei high sulfidation gold deposit sardinia italy the epithermal high sulfidation gold deposit at furtei sardinia italy contains at least 2 15 million metric tons mt at 2 82 g t au 725 000 t oxidized ore 1 375 000 t sulfide ore and 50 000 t mixed ore potential by products are silver and copper the geology of the area features an oligocene miocene volcano sedimentary sequence including andesite porphyry domes and associated block and ash flows pyroclastic flows fall and surge deposits and interlayered epiclastic and sedimentary sequences a phreatic and or phreatomagmatic diatreme breccia also crops out in the mineralized area the volcanic rocks belong to the calc alkaline sardinian magmatism their k ar ages in the area are 23 6 to 25 5 ma hydrothermal circulation affected most of the volcano sedimentary sequence over an area of about 5 km 2 during or immediately after volcanic activity early hydrothermal activity was responsible for the acid alteration typical of high sulfidation deposits with formation of four hypogene alteration types of decreasing intensity silicification advanced argillic alteration argillic alteration and propylitization these alteration styles show a rough spatial zonation silicification consists of massive and vuggy silica veins and bodies these are controlled by preferential structural directions n 140 degrees n 40 degrees and n s and by the contacts between the andesite porphyry domes and the diatreme breccias or between domes and epiclastic pyroclastic sequences the advanced argillic alteration borders vuggy and massive silica alteration zones and is characterized by the widespread presence of kaolinite dickite minerals this alteration pervasively affects the diatreme breccia and epiclastic pyroclastic units advanced argillic alteration grades outward into argillic alteration marked by the presence of montmorillonite which passes to a districtwide propylitic alteration halo the ore stage clearly postdates hydrothermal alteration as indicated by the occurrence of ore minerals in vuggy cavities and fractures in silica bodies the furtei mineralization comprises a surficial oxidized zone of supergene nature and a deeper primary sulfide zone and is largely hosted by the diatreme breccia the sulfide zone up to 300 m of vertical extension is characterized by a vertical zoning of the mineral assemblage at higher levels pyrite enargite luzonite gold are present whereas in the deep zone the above minerals are accompanied by tennantite and tellurides native gold with 0 7 7 8 wt ag occurs as blebs in enargite and luzonite fluid inclusions indicate that the fluid present at the time of alteration and epithermal mineralization had temperatures generally between 190 degrees and 280 degrees c and relatively low salinity max 5 5 wt nacl equiv the local coexistence of liquid and vapor rich inclusions suggests boiling conditions the inclusions were trapped at depths of between 60 and 680 m below the water table the occurrence of high salinity 31 5 44 8 wt nacl equiv high temperature 390 degrees 500 degrees c inclusions in cataclastic hydrothermal quartz suggests that a dense brine of probable magmatic derivation circulated in the furtei system this is consistent with geophysical data suggesting the presence of an intrusion at approximately a 1 to 1 5 km depth a few relatively high salinity around 22 wt nacl equiv low temperature 255 degrees 275 degrees c inclusions may be the result of episodic mixing of deep saline brines with low salinity meteoric fluids late stage barite contains low salinity

los angeles a tale of two stones we compare and contrast the mineralogy and petrology of the 2 stones of the extremely ferroan and cr poor martian meteorite los angeles the 2 stones are similar in many characteristics strongly suggesting that they originated from a single flow or shallow intrusion however stone 2 is more ferroan and enriched in late stage materials than its larger and more widely studied sibling stone 2 has a far higher abundance 25 vol than stone 1 10 vol of combined opaques meaning not only conventional opaque minerals but also and more abundantly fine grained symplectitic intergrowths of fayalite ferroan augite silica interpreted as pyroxferroite breakdown material pbm the bulk composition of the pbm is close to that of stoichiometric pyroxferroite with roughly 45 wt feo extensive zonation within the pyroxenes of both stones is consistent with origin by closed system fractional crystallization of the parent basaltic melt s however the compositional and modal disparity between the two stones suggests that they formed in an environment where at least mild multi cm scale differentiation occurred probably in both stones crystallization began from similar melts with mg 27 28 mol but during crystallization significant migration of the melt component occurred perhaps by crystal settling and or filter pressing stone 2 acquired an enhanced proportion of residual melt and thus higher proportions of late stage materials such as pbm oxides and phosphates within the pbm clinopyroxene poikiloblastically encloses fayalite and silica at least some of the pbm had already formed by decomposition of pyroxferroite before the major shock that caused the very scarce brecciation within los angeles however the low abundance of fractures within pbm in comparison to pyroxene and some other minerals may be an indication that the textures of pbm regions typically did not assume their final detailed configuration until after the last major shock the steep slope of a pyroxene mg cr correlation suggests that igneous crystallization occurred at higher fo2 in los angeles than in otherwise similar shergottites such as que 94201 shergotty and zagami

local migration with extrapolated vsp green s functions we have developed a novel vertical seismic profile vsp imaging method that requires only a local velocity model around the well this method is denoted as a local migration where the model based forward propagation and backward propagation operators are computed using a local velocity model between the well and the target body the velocity model for the complex overburden and salt body is not needed and the source side statics are automatically accounted for in addition kinematic and dynamic effects including anisotropy absorption and other unknown undefined rock effects outside of this local velocity model are mostly accounted for numerical tests on an acoustic 2d sigsbee vsp data set an elastic salt model and offset 2d vsp data in the gulf of mexico partly validate the effectiveness of this method by accurately imaging the sediments next to the vertical well

a boom in boomless seismology taking advantage of cheap long lasting sensors developed by the oil and gas industry seismologists are increasingly exploring the top 10 kilometers of the crust with dense surveys of hundreds or even thousands of seismometers what s more the surveys are being operated passively which means that ambient noise from earth is used to illuminate the subsurface passive surveys are cheaper and easier to permit than active surveys which rely on explosives or vibrations from special thumper trucks seismologists predict that the dense surveys will yield precision imaging of magma chambers underneath volcanos and earthquake rupture zones along faults

constraints from geochemistry and sr nd isotopes for the origin of albitite deposits from central sardinia italy major and trace element contents and sr nd isotope ratios were determined in albitite albitized and unaltered late variscan granitoid samples from the world class na feldspar deposits of central sardinia italy the albite deposit of high economic grade has geological textural and chemical features typical of metasomatic alteration affecting the host granitoids albitization locally accompanied by chloritization and epidotization was characterized by strong leaching of mg fe k and geochemically similar trace elements and by a significant increase of na ca and p were moderately leached in the most metasomatized rocks other major si ti ca and trace elements u th y and zr along with light lree and middle mree rare earth elements behaved essentially immobile at the deposit scale the nd isotope ratios 0 512098 to 0 512248 do not provide information on the emplacement age of the unaltered late variscan granitoids on the other hand their sr isotope ratios fit an errorchron of 274 29 ma 1 error in fair agreement with all published ages of sardinian variscan granitoids the very low rb content of albitized rocks precludes application of the rb sr radiometric system to determine the age of albitization the sm nd system is not applicable either because the 143nd 144nd ratios of albitized rocks and unaltered granitoids overlap the overlap confirms that sm and nd were substantially immobile during albitization on the other hand the measured 87sr 86sr ratios of the albitized rocks are appreciably lower than those of the unaltered host granitoids whereas their initial sr isotope ratios are higher this seems to suggest that a albitization was induced by non magmatic fluids rich in radiogenic sr and b albitization occurred shortly after the granitoid emplacement this conclusion is supported by nd isotopes because unaltered granitoids and albitites fit the same reference isochron at 274 ma the fluids acquired radiogenic sr by circulation through the lower paleozoic metasedimentary basement specifically it is estimated that sr supplied by the non carbonatic basement represents about 22 wt of total sr in albitite

u pb isotopic dating of titanite microstructures potential implications for the chronology and identification of large impact structures identifying and dating large impact structures is challenging as many of the traditional shock indicator phases can be modified by post impact processes refractory accessory phases such as zircon while faithful recorders of shock wave passage commonly respond with partial u pb age resetting during impact events titanite is an accessory phase with lower pb closure temperature than many other robust chronometers but its potential as indicator and chronometer of impact related processes remains poorly constrained in this study we examined titanite grains from the sudbury ontario canada and vredefort south africa impact structures combining quantitative microstructural and u pb dating techniques titanite grains from both craters host planar microstructures and microtwins that show a common twin host disorientation relationship of 74 about in the vredefort impact structure the microtwins deformed internally and developed high and low angle grain boundaries that resulted in the growth of neoblastic crystallites u pb isotopic dating of magmatic titanite grains with deformation microtwins from the sudbury impact structure yielded a 207pb 206pb age of 1851 12 ma that records either the shock heating or the crater modification stage of the impact event the titanite grains from the vredefort impact structure yielded primarily pre impact ages recording the cooling of the ultra high temperature ventersdorp event but domains with microtwins or planar microstructures show evidence of u pb isotopic disturbance despite that the identified microtwins are not diagnostic of shock metamorphic processes our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record

seasonal variations of seismic velocities in the san jacinto fault area observed with ambient seismic noise s u m m a r y we observe seasonal seismic wave speed changes dv v in the san jacinto fault area and investigate several likely source mechanisms velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations we study the interrelation between dv v records frequency dependent seismic noise properties and nearby environmental data of wind speed rain ground water level barometric pressure and atmospheric temperature the results indicate peak to peak seasonal velocity variations of 0 2 per cent in the 0 5 2 hz frequency range likely associated with genuine changes of rock properties rather than changes in the noise field phase measurements between dv v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations the other considered environmental effects produce secondary variations that are superimposed on the thermal based changes more detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth

origin of the matauri bay halloysite deposit northland new zealand at the matauri bay halloysite deposit economically valuable halloysite rich clays are hosted by a sanidine rhyolite dome ar ar dated at 10 1 0 03 ma the rhyolite dome intrudes an older basalt and is overlain by alluvial sediments and a younger basalt 4 0 0 7 ma a blanket like halloysite rich zone is restricted to depths of 10 30 m from the present day erosion surface primary sanidine and plagioclase phenocrysts in rhyolite are completely leached out in the halloysite rich zone but are only partially leached out at greater depth halloysite was formed by hydrolysis and cation leaching of sanidine and plagioclase phenocrysts and groundmass glass in the rhyolite resulting in loss of k ca na and si and enrichment in oh loi 6 10 and al2o3 20 30 relative to least altered rhyolite with 1 8 loi and 14 5 al2o3 oxygen and hydrogen isotope data indicate the halloysite is supergene rather than hydrothermal in origin which is consistent with the absence of pyrite alunite and other acid sulphate type hydrothermal minerals and with the blanket like alteration profile the dominance of halloysite over kaolinite was favoured by water saturated weathering conditions during the late miocene pliocene subtropical weathering regime in northland

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

petrology of philippine geothermal systems and the application of alteration mineralogy to their assessment abstract philippine geothermal systems occur in the vicinity of large holocene calc alkaline volcanic complexes wells drilled in these areas encountered multiple intrusions the latest dikes are the subsurface manifestations of the youngest heat source commonly at least two hydrothermal regimes are juxtaposed in a single area with the latest being in equilibrium with the present temperature and chemical regime alteration by neutral ph water is pervasive and abundant a contact metamorphic aureole also occurs near intrusives alteration due to acid sulfate fluids is generally confined to permeable structures neutral ph alteration is divided into four zones on the basis of key clay minerals and two subzones are defined by calc silicates these are the smectite ambient to 180 c transition 180 230 c illite 230 320 c and biotite 270 340 c zones subzones are defined by epidote 250 340 c and amphibole 280 340 c the four main zones of acid alteration are kaolinite ambient to 120 c dickite kaolinite 120 200 c dickite pyrophyllite 200 250 c and pyrophyllite illite 230 320 c where relict high temperature alteration reaches the surface the area being drilled is usually the outflow zone of the present system these hydrothermal mineral assemblages are used 1 as geothermometers 2 to assist in determining the depth at which the production casing will be set during drilling 3 to estimate fluid ph and other chemical parameters 4 to predict possible corrosion and scaling tendencies of the fluids 5 as a measure of permeability and possible cold water influx into wells 6 as a guide to field hydrology and 7 to estimate roughly the thickness of the eroded overburden

industrial mineral occurrences associated with cenozoic volcanic rocks of sardinia italy geological mineralogical geochemical features and genetic implications the most important industrial mineral im deposits associated with the cenozoic volcanics of sardinia include kaolin bentonite and zeolites though of less significance perlite and potassium feldspar are also important in local and national markets bentonite and to a lesser extent kaolin and perlite form economically viable deposits and zeolites appear as very important industrial mineral commodities for future economic development the market situation for potassium feldspar is still under evaluation for use in ceramics all these deposit types occur as alteration products of cenozoic volcanic rocks which were emplaced during three main volcanic cycles in the eocene ec 55 to 51 ma oligocene miocene omc 32 to 13 ma and pliocene pleistocene ppc 5 to 0 1 ma kaolin bentonite zeolites and potassium feldspar occur as alteration products of the omc volcanites perlite occurs as an alteration product of the ppc volcanites all deposits have generally derived from the alteration of the most acidic members of the cenozoic volcanic rocks rhyolites to rhyodacites but differences among the associated ore forming phenomena are recognized hydrothermal sensu stricto deuteric alteration and weathering are the most important processes involved in ore formation weathering although of minor importance can also contribute to enhance the alteration of the deposits formed by hydrothermal and deuteric processes the extent and type of occurrences are mainly controlled by the local geological and structural features and the environmental geochemical conditions kaolin occurs in three main districts at romana and tresnuraghes it is hosted within ignimbritic sequences while at serrenti furtei it is found in pyroclastic flows and ash flow tuffs kaolin formed by hydrothermal processes occurs in high sulfidation romana and serrenti furtei and low sulfidation tresnuraghes type epithermal systems the alteration is proximal to the feeder conduits and structural setting controls kaolin formation bentonite is widespread throughout sardinia deposits occur as stratiform mineral bodies in situ altered within poor welded pumice rich and ash rich layers and also as distal fall out deposits as lenticular bodies interlayered within the miocene sedimentary sequences the former originated by deuteric alteration and formation is mainly controlled by lithology whereas the second type originated by weathering which altered the volcanic ash lenses deposited in lacustrine and or fluvial deltaic environments

origin of the duvertepe kaolin alunite deposits in simav graben turkey timing and styles of hydrothermal mineralization abstract the duvertepe district located on the western end of the simav graben is the largest known fossil hydrothermal kaolin deposit in turkey active hot springs occur to the north of simav city eynal citgol nasa and geothermal fields occur to the west of the graben hisaralan kaolin in the duvertepe district formed at the expense of miocene rhyolites rhyodacites and tuffs emplaced in a tectonically active system undergoing n s extension upward fan shaped deposits of silicification and kaolin are found along fault zones and define areas of enhanced kaolinization silica sinters located above the kaolin zones suggest that hydrothermal fluids followed the fracture systems and mark outflow of geothermal waters fractured volcanic rocks rich in glass shards bear sufficient porosity and permeability for hydrothermal circulation two mineral facies kaolinite and alunite dominate the deposits which respectively include 1 kaolinite alunite quartz and 2 alunite opal ct quartz halloysite kaolinite crystals are well formed hexagonal vermiform habits and alunite crystals are idiomorphic rhombohedral forms needle and tubular shaped halloysites are common in the alunite facies the 34 s values for alunite range from 1 55 to 6 18 kaolinites have d ranging from 49 0 to 94 3 and the 18 o values range from 5 8 to 14 8 calculated formation temperatures of kaolin mineralization using 18 o values suggest that hydrothermal alteration occurred in the range of 38 to 129 c independent k ar and 40 ar 39 ar ages of alunite are concordant and indicate that alteration occurred from 20 1 to 20 6 ma in the southern part of the graben and 17 3 to 19 2 ma in the northern part these results suggest that metasomatism by steam heated hydrothermal fluids sourced by rhyolitic magma is responsible for the duvertepe kaolin and alunite deposit origins

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

thermal modeling of shock melts in martian meteorites implications for preserving martian atmospheric signatures and crystallization of high pressure minerals from shock melts the distribution of shock melts in four shergottites having both vein and pocket geometry has been defined and the conductive cooling time over the range 2500 c to 900 c calculated isolated 1 mm2 pockets cool in 1 17 s and cooling times increase with pocket area an isolated vein 1 7 mm in northwest africa nwa 4797 cools to 900 c in 4 5 s interference between thermal haloes of closely spaced shock melts decreases the thermal gradient extending cooling times by a factor of 1 4 to 100 this is long enough to allow differential diffusion of ar and xe from the melt small pockets 1 mm2 lose 2 2 ar and 5 2 xe during cooling resulting in a small change in the ar xe ratio of the dissolved gas over that originally trapped with longer cooling times there is significant fractionation of xe from ar and the ar xe ratio increases rapidly the largest pockets show less variation of ar xe and likely preserve the original trapped gas composition considering all of the model calculations even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse i e 0 01 s the crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite

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

quartz alunite alteration in the western part of the virginia range washoe and storey counties nevada large areas of miocene volcanic rocks in the western part of the virginia range washoe and storey counties nevada were altered to quartz alunite and clay dominated mineral assemblages and all significant precious metal and mercury deposits throughout the range including the comstock lode ramsey and talapoosa districts occur in or adjacent to extensively altered andesites and dacites of the miocene alta and kate peak formations radioisotopic analyses of alunite and precious metal vein minerals bracket the ages of quartz alunite alteration at approximately 16 to 9 ma and vein mineralization at approximately 14 to 10 ma closely corresponding to kate peak formation eruptive rock ages that range from approximately 15 to 10 ma oxygen deuterium and sulfur isotope analyses of alunite quartz and water extracted from fluid inclusions in quartz alunite altered rocks in the western part of the virginia range and fluid inclusion microthermometry show that the altering fluids were composed of magmatic water and mixtures of meteoric and magmatic water and that the alteration took place in the temperature range approximately 205 degrees to 265 degrees c fluid inclusion water isotopic analyses indicate that comstock lode district veins were also deposited by a mixed magmatic meteoric water fluid and that subsurface altered intrusions such as the molybdenite bearing diorite in the washington hill alteration zone can be identified by isotopic analyses of water in surface samples sources and thermal characteristics of fluids involved in quartz alunite alteration and gold mineralization in the ramsey district similar to that at goldfield nevada and for other deposits in the virginia range have not been determined but mixingof magmatic and meteoric water at moderate hydrothermal temperatures may have been important in localizing ore

petrogenesis of the northwest africa 4734 basaltic lunar meteorite we report the petrography mineralogy trace element abundance geochemistry and pb pb geochronology of the lunar meteorite northwest africa nwa 4734 and make a comparison with the lapaz icefield lap 02205 02224 low ti lunar basaltic meteorites nwa 4734 is an unbrecciated low ti mare basalt composed mainly of subophitic textured pyroxene 60 vol and plagioclase 30 pyroxene plagioclase and olivine exhibit large compositional variations and intra grain chemical zoning pyroxene and plagioclase in nwa 4734 have rare earth element ree concentrations and patterns similar to those of the laps the crystallization age of nwa 4734 determined in situ in baddeleyite is 3073 15 ma 2 sigma nearly identical to that of the laps 3039 12 ma nwa 4734 and the laps have similar textures modal abundances mineral chemistry and crystallization ages and are most likely source crater paired on the moon

symplectites derived from metastable phases in martian basaltic meteorites the martian basalts los angeles que 94201 shergotty and zagami contain several late stage mineral types including oxides sulfides phosphates and associated silicate assemblages symplectites consisting of two and three phase assemblages are present in los angeles que 94201 and shergotty the three phase symplectite is composed of hedenbergitic pyroxene fayalitic olivine and an sio 2 polymorph and the two phase symplectite consists of fayalitic olivine and an sio 2 polymorph these symplectites are commonly found in close association with the ca phosphate merrillite general formula ca 9 mg fe 2 po 4 7 scanning electron microscopy was used to examine the distribution and occurrence of the symplectites and point count and electron microprobe data were used to recast symplectites in terms of pyroxene stoichiometry the reconstructed three phase symplectite compositions plot close to lunar metastable pyroxferroite on the pyroxene quadrilateral diagram and indicate that pyroxene compositions ranged into the forbidden region and eventually crystallized pyroxferroite which subsequently broke down to the three phase symplectite upon cooling two phase symplectites which occur directly adjacent to merrillite may be the result of breakdown of metastable ferrosilite the crystal chemistry of merrillite and its close association with the symplectites indicates that during crystallization of pyroxene co crystallizing merrillite contributes to depletion of ca and mg and the formation of metastable pyroxferroite and ferrosilite

dating phosphates of the strongly shocked suizhou chondrite impacts on undifferentiated asteroidal bodies provide invaluable information for understanding the solar system evolution nevertheless dating early and small scale impact events is technically challenging uranium lead u pb systematics of u bearing phases within shock induced melt veins smvs of chondrites may be significantly disturbed by localized heating thus providing opportunities to date these impact events as one of the major u hosts in chondrites apatite in the suizhou l6 chondrite has been compositionally and structurally modified to varying degrees by shock metamorphism apatite grains in the host remained largely intact during the impact and have recorded the initial thermal cooling time similar to 4550 ma on their parent body apatite grains in regions less than 100 mu m bordering the smvs or in relatively cold regions within the smvs were partly decomposed to tuite driven by the localized transient heating within the smvs their u pb systematics were disrupted to varying extents apatite in regions close to the center of the smvs has been completely transformed to tuite which yields an age of 4481 30 ma 2 sigma providing an upper limit to the impact event this study clearly demonstrates that by integrating in situ u pb isotope analysis with detailed microstructural and compositional analysis of phosphates it is possible to deduce the timing of early and small scale celestial impact events and hence create a more comprehensive understanding of the impact history of the solar system

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

geology geochemistry and origin of high sulfidation cu au mineralization in the nansatsu district japan the nansatsu district of southern kyushu has been the site of calc alkaline volcanism for the last 10 m y shifting eastward with time associated hydrothermal activity followed deposition of the volcanic host rocks by about 0 5 m y and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions resulting in the formation of high sulfidation cu au deposits at kasuga iwato and akeshi the orebodies consist of 95 wt percent sio 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride sulfate waters these are inferred to have formed when magmatic vapors containing hcl and so 2 condensed into meteoric water the residual silica now quartz orebodies are best developed where the host was initially permeable the margins of the quartz bodies are abrupt with narrow 1 2 m halos representing the reaction front of acid waters isochemically dissolving the host rock the halo comprises alunite strongly zoned in na and k with p rich cores dickite and or kaolinite or pyrophyllite grading out into illite and interlayered illite smectite clays and finally propylitic alteration this pattern is characteristic of deposits of this type throughout the world for example at summitville colorado and lepanto philippines mineralization occurred after initial leaching by the vapor condensates with metals transported by a dense magmatic fluid mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies ore grades are restricted to the quartz bodies gold is most closely associated with enargite and pyrite later minerals include covellite and then sulfur the last stage of activity was steam heated with descending waters oxidizing sulfides to goethite and locally remobilizing au into fractures this varies in degree between deposits erosion exposed the orebodies to supergene weathering continuing the sulfide oxidation and au remobilization stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid delta 18 o 7 or 2 per thousand delta d 25 or 5 per thousand similar to nearby active volcanic discharges with local meteoric water in contrast the clays in the marginal halo have isotopic compositions indicating a delta 18 o shift of 6 to 8 per mil from local meteoric water values probably due to water rock interaction and the delta 18 o values of residual silica quartz may also be due to meteoric water domination fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent nacl equiv during late quartz growth though there is evidence in one sample for higher salinity fluid having been present up to 30 wt percent nacl equiv some inclusions contain daughter minerals of halite and sulfur the t h values of over 1 000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees c at akeshi to about 200 degrees c at iwato and 230 degrees c at kasuga the presence of vapor rich inclusions some with t hv similar to t hl indicate the presence at times of a two phase fluid in the center of the ore zones with depths of about 150 to 300 m below the paleowater table the mineralizing fluid was relatively oxidized sulfide sulfate ratio about 3 1 close to pyrite alunite coexistence under these redox conditions a ph of 3 and over a temperature range of 200 degrees to 300 degrees c aucl 2 super complexing may dominate over hau sh 2 at salinities above about 2 wt percent nacl several conditions are conducive for high sulfidation mineralization to occur 1 a crystallizing magma exsolves a fluid with lower pressure conditions favoring metal fractionation from melt to fluid 2 the exsolved fluid separates into vapor and saline liquid phases due to immiscibility with the latter being metal rich 3 the gas rich hcl and so 2 h 2 s vapor ascends to the surface with at least a portion condensed into meteoric water forming an acid fluid which leaches the host rock to create permeable zones for later mixing and 4 the dense metal bearing fluid also ascends into this leached zoned and precipitates cu sulfosalts sulfides and au upon mixing with meteoric water if the saline liquid is not released from its source adjacent to the magma due to lack of fracturing or if there is a strong hydraulic gradient caused by high relief only the vapor related stage may occur this will leave leached barren rock which is characteristic of many eroded volcanic terranes

investigating the shock histories of lunar meteorites miller range 090034 090070 and 090075 using petrography geochemistry and micro ftir spectroscopy fourier transform infrared ftir spectroscopy and cathodoluminescence cl imaging techniques combined with electron microprobe analyses have been used to determine the physical state of feldspathic phases that have been subject to varying levels of shock in the grouped lunar meteorites miller range 090034 090070 and 090075 six feldspathic phases have been identified based on spectral textural and chemical properties a specific infrared wavelength band ratio 1064 932 cm 1 equivalent to 9 40 10 73 m chosen because it can distinguish between some of the feldspathic phases can be used to estimate the pressure regimes experienced by these phases in addition ftir spatial mapping capabilities allow for visual comparison of variably shocked phases within the samples by comparing spectral and compositional data the origin and shock history of this lunar meteorite group has been determined with each of the shocked feldspathic phases being related to events in its geological evolution as such we highlight that ftir spectroscopy can be easily employed to identify shocked feldspathic phases in lunar samples estimate peak shock pressures and when compared with chemical data can be used to investigate their shock histories

in situ u pb dating of micro baddeleyite by secondary ion mass spectrometry abstract we introduce a technique for u pb dating of baddeleyite using secondary ion mass spectrometry sims in situ analysis of ng mass crystals that cannot be efficiently extracted by conventional mineral separation techniques average 207 pb 206 pb ages for precambrian baddeleyite crystals are within 206 pb 238 u ratios are corrected for instrumental fractionation calibrated through linear regression in a pb u relative sensitivity vs uo 2 u calibration plot calibration is performed on separated baddeleyite crystals 100 200 m in maximum dimension mounted in random crystallographic orientation 206 pb 238 u ages for baddeleyite from duluth gabbro fc4b and kovdor are accurate within 1 2 when averaging 15 30 individual spot analyses and relative sensitivities calibrated on phalaborwa baddeleyite the relative difference of 206 pb 238 u between large crystals and micro baddeleyite from fc4b is within 1 comparison between silicate glass and baddeleyite as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in pb u sensitivity in baddeleyite due to variable crystal orientations we successfully utilized oxygen flooding and a uo 2 u based calibration to significantly reduce orientation dependent bias

epithermal sinters of paleozoic age in north queensland australia silica sinters deposited from hot springs have been found associated with volcanic rocks of late devonian to late carboniferous age in north queensland australia these deposits have many features in common with modern hot spring sinters such as those of the taupo volcanic zone in new zealand they occur associated with subaerial volcanic rocks and are locally found with air fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common the associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures the silica veins contain low salinity fluid inclusions trapped at epithermal temperatures and there is evidence of boiling textures preserved in the sinters are identical to those found in modern sinters they include columnar structures similar to bacterial stromatolites described from yellowstone national park as well as striated surfaces apparently resulting from silica deposition on filamentous algae plant fossils including oxroadia gracilis are abundant criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system which may be an important guide in exploration for epithermal precious metal deposits

resolution of impact related microstructures in lunar zircon a shock deformation mechanism map abstract the microstructures of lunar zircon grains from breccia samples 72215 73215 73235 and 76295 collected during the apollo 17 mission have been characterized via optical microscopy cathodoluminescence imaging and electron backscatter diffraction mapping these zircon grains preserve deformation microstructures that show a wide range in style and complexity planar deformation features pdfs are documented in lunar zircon for the first time and occur along 001 110 and 112 typically with 0 1 25 m spacing the widest pdfs associated with 112 contain microtwin lamellae with 65 misorientation relationships deformation bands parallel to 100 planes and irregular low angle misorientation axes this geometry is consistent with a dislocation glide system with 010 during dislocation creep nonplanar fractures recrystallized domains with sharp irregular interfaces and localized annealing textures along fractures are also observed no occurrences of reidite were detected shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon pdfs form along a limited number of specific hkl planes that are perpendicular to directions of high young s modulus suggesting that pdfs are likely to be planes of longitudinal lattice damage twinned 112 pdfs also contain directions of high shear modulus a conceptual model is proposed for the development of different deformation microstructures during an impact event this shock deformation mechanism map is used to explain the relative timing conditions and complexity relationships between impact related deformation microstructures in zircon

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

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

new shock microstructures in titanite catisio5 from the peak ring of the chicxulub impact structure mexico accessory mineral geochronometers such as apatite baddeleyite monazite xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity impact processes to date little is known about the response of titanite to shock metamorphism even though it is a widespread accessory phase and a u pb geochronometer here we report two new mechanical twin modes in titanite within shocked granitoid from the chicxulub impact structure mexico titanite grains in the newly acquired core from the international ocean discovery program hole m0077a preserve multiple sets of polysynthetic twins most commonly with composition planes k1 1 11 and shear direction 1 and less commonly with the mode k1 130 1 in some grains 130 deformation bands have formed concurrently with the deformation twins indicating dislocation slip with burgers vector b can be active during impact metamorphism titanite twins in the modes described here have not been reported from endogenically deformed rocks we therefore propose this newly identified twin form as a result of shock deformation formation conditions of the twins have not been experimentally calibrated and are here empirically constrained by the presence of planar deformation features in quartz 12 5 and 17 5 gpa and the absence of shock twins in zircon 20 gpa while the lower threshold of titanite twin formation remains poorly constrained identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 gpa given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on earth microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased altered or did not manifest due to generally low 20 gpa shock pressure