on the presence of oh defects in the zircon type phosphate mineral xenotime y ree po4 the infrared ir spectra of gem quality xenotime crystals containing considerable amounts of rare earth elements rees are characterized by sharp and strongly pleochroic absorption bands in the 3650 3350 cm 1 region xenotime presents the interesting case of a nominally anhydrous mineral where the oh stretching frequency region of weakly hydrogen bonded oh groups is overlapped by absorption bands of rees especially of dysprosium the spectra of partially metamict samples are dominated by a broad band centered at around 3450 cm 1 the water content of the unmetamict samples ranges from 5 to 10 wt ppm and for the partially metamict samples from 370 wt ppm to 1 7 wt h2o

records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

the incorporation of pb into zircon abstract the incorporation of pb into zircons grown from pb rich solutions was evaluated using three different approaches 1 high temperature growth of large crystals from pb silicate melts 2 hydrothermal overplating of thin epitaxial layers on substrates of natural zircon and 3 growth of small homogeneously nucleated crystals from aqueous fluids the melt grown zircons 50 400 m were crystallized from pbo sio2 zro2 p2o5 liquid at atmospheric pressure by cooling from 1430 to 1350 c in the p2o5 free system despite 66 wt pbo in the melt these zircons contain 3 atom pb with apparent zircon fluid partition coefficients of 4 2 and 2 6 respectively for pb4 and pb2 in contrast to the case of melt grown zircons available p is excluded from the aqueous epitaxial zircon suggesting that charge balance is accomplished by h instead small 2 5 m zircons grown by cooling aqueous solutions pbo sio2 zro2 p2o5 from 800 c or 900 c contain 0 25 0 5 atom pb 2 4 wt pbo yielding apparent dpb values of 0 2 0 3 available p5 is incorporated in a 2 1 ratio with pb2 suggesting a specific charge balance mechanism 2p5 pb2 2si4 zr4 however pb enters the zircon even when p is unavailable so h may again play a charge balancing role because of the rapid polythermal modes of zircon growth and the high pb content of the experimental systems the apparent partition coefficients should not be viewed as equilibrium values but as qualitative indicators of pb compatibility under various growth circumstances the overall results are consistent with the low but variable levels of non radiogenic common pb in natural zircons the increased compatibility of pb in fluid grown low temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet metamorphic rocks i e high concentrations of common pb

carbonates in fractures of martian meteorite allan hills 84001 petrologic evidence for impact origin carbonates in martian meteorite allan hills 84001 occur as grains on pyroxene grain boundaries in crushed zones and as disks veins and irregularly shaped grains in healed pyroxene fractures some carbonate disks have tapered mg rich edges and are accompanied by smaller thinner and relatively homogeneous magnesite microdisks except for the microdisks all types of carbonate grains show the same unique chemical zoning pattern on mgco3 feco3 caco3 plots this chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process the heterogeneous distribution of carbonates in fractures tapered shapes of some disks and the localized occurrence of mg rich microdisks appear to be incompatible with growth from an externally derived co2 rich fluid that changed in composition over time these features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual mg rich fluid that was squeezed along fractures carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate we infer that carbonates in fractures formed from grain boundary carbonates associated with plagiociase that were melted by impact and dispersed into the surrounding fractured pyroxene carbonates in fractures which include those studied by mckay et al 1996 could not have formed at low temperatures and preserved mineralogical evidence for martian organisms

seasonal changes in beach morphology along the sheltered coastline of perth western australia abstract seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology the coastline of perth western australia is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology however these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions in summer when northward sediment transport prevails due to sea breeze activity beaches located south of coastal structures headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle these beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south in contrast beaches located north of obstacles will become narrower during summer and wider during winter the usefulness of the dimensionless fall velocity h b w s t where hb is the breaker height ws is the sediment fall velocity and t is the wave period as a predictor of presence absence of bar morphology and beach type was investigated it was found that fluctuates around the threshold of bar formation 1 5 2 over a variety of time scales daily weekly and seasonally these temporal variations in in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions as a result bar occurrence and beach type can not be readily predicted using along the perth coast

distinguishing depositional environments in shallow water mixed bio siliciclastic deposits on the basis of the degree of heterolithic segregation gelasian southern italy abstract a new sedimentological technique has been tested in gelasian cross stratified bio siliciclastic deposits cropping out in southern italy lucanian apennine to disentangle paleoenvironmental and model reconstructions for shallow water mixed sediments the proposed method suggests the use of the bioclastic siliciclastic ratio bls and the segregation index s i to evaluate the percentage of the dominant elastic component in a mixed deposit and in order to estimate the degree of the heterolithic segregation between bioclastic and siliciclastic particles the principle of this method is based on the experimental evidence that bioclasts and quartz grains reveal a different physical behavior if entrained by a hydraulic flow of a given energy thus a different internal texture of bioclastic and terrigenous particles in the same sedimentary deposit is regarded as the result of the variation in the energy of a number of hydrodynamic processes that characterize shallow marine settings including waves currents and tides the studied mixed sediments subdivided into facies associations exhibit stratigraphic shoaling upward successions recording repeated transitions from offshore to shoreface environments these trends are documented through the vertical variations in the degree of heterolithic segregation between mixed clastic particles quantified by using the s i introduced in the present study the assessment of the degree of heterolithic segregation is thus proposed as a proxy to distinguish depositional environments related to different water depths in shallow water mixed systems

formation and relative ages of maskelynite and carbonate in alh84001 the morphology and stoichiometry of feldspathic glass in the martian meteorite alh84001 indicates it is maskelynite a diaplectic glass rather than a flowed glass although this glass was heterogeneously affected by a tertiary set of processes an impact event with shock pressures in excess of 31 gpa was needed to convert the original plagioclase an36ab60or4 to maskelynite carbonate is intimately associated with the maskelynite and the carbonate s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite textures also suggest carbonate was produced at the expense of maskelynite in a dissolution precipitation reaction that involved a carbonic fluid this fluid system is tentatively estimated to have been active for at least a few years at temperatures 300 c based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems which is the only analogue currently available this reaction does not need to be mitigated by microbial life neither are bacteria needed to produce the radiating textures and globular forms of carbonate which may instead reflect kinetic phenomena associated with crystal nucleation and growth because the carbonate was produced at the expense of maskelynite it is younger than maskelynite which has previously been shown to have last degassed 3 92 0 04 ga turner et al 1997 however the specific age of the carbonate and the carbonic fluid system remains unknown

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

analysis of threshold and incipient conditions for sediment movement prediction of threshold conditions and incipient motion is the essential issue for the study of sediment transport this work compares existing empirical threshold curves proposed for shields diagram a method based on the concept of probability of sediment movement and an empirical method based on movability number these methods are used to predict the incipient motion conditions for experimental runs taken from various studies most of the experimental data used in this work have not been used before in derivation of alternative formulations for shields diagram and other methods the empirical threshold curves based on the shields entrainment function was the least successful at predicting the measured incipient motion conditions while the use of the movability number gives good predictions of critical shear velocity compared with experimental data

migration and sedimentology of longshore sandwaves long point lake erie canada this paper describes the formation migration and sedimentology of sandwaves along the distal end of long point a 40 km long spit in lake erie some seven to nine sandwaves occur in a zone over the last 14 km of the spit they are characteristically 50 100 m wide at the downdrift end range in length from 350 m to 1500 m and migrate alongshore at rates that are typically 100 300 m year 1 measurements over a 7 year period show two forms of alongshore sandwave migration 1 a migratory jump and 2 downdrift accretion the migratory jump is commonly 200 500 m year 1 and results from the onshore migration and welding of an inner nearshore bar to the downdrift end of the sandwave this in turn leads to emergence of the bar over a distance of several hundred metres downdrift of the sandwave and isolation of the trough landward of the bar infilling of the trough abstracts large volumes of sediment from the local sediment transport system and may affect movement of the sandwave in the following year or movement of the next sandwave downdrift downdrift accretion commonly results in migration of 50 150 m year 1 and results from the refraction of waves around the distal end of the sandwave and episodic accretion of small swash bars this mechanism occurs less frequently and appears to reflect a local condition of lower sediment abundance often triggered by a large migratory jump in the previous year the process of bar emergence and infilling produces a distinct suite of sedimentary structures associated with the infilling of the landward trough and building of the sandwave berm the initial shoreline perturbation that generates the sandwave results from onshore migration and welding of inner nearshore bars and the development and growth of the sandwaves is promoted by refraction of highly oblique waves

review of methods to measure short time scale sediment accumulation this review paper aims at providing a list of methods available to measure sediment accumulation in underwater or intertidal environments over short term periods in the order of hours to months at the most both in situ and under laboratory conditions methods are classified based on two criteria a whether they measure sediment accumulation or sediment elevation change and b whether they allow continuous or discontinuous measurements the main characteristics of each method are outlined along with its vertical resolution its accuracy its temporal resolution its spatial coverage capacity and an indication of its relative cost typical examples of applications are also provided the purpose of this review is to be a starting point for readers who need an overview of existing techniques that measure short term sediment accumulation in order to guide and facilitate for instance the selection of an appropriate technique for a given application with set requirements

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

temperatures on mars from 40ar 39ar thermochronology of alh84001 the thermal history of martian meteorite alh84001 has critical implications for its petrological and deformational history the age of its trapped atmospheric gases the timing of the martian dynamo and possibly the evolution of martian surface temperatures during the last 4 billion years gyr feldspathic glass in alh84001 has been dated using 40 k 40 ar and 40 ar 39 ar chronometry by several laboratories there is general agreement that these chronometers were last reset sometime between 3 9 and 4 3 billion years ago ga using the 40 ar 39 ar data from bogard and garrison meteorit planet sci 34 1999 451 473 to model several limiting thermal histories of the meteorite here we show that most of alh84001 feldspathic glass has probably not been heated to more than v350 500 c and shocked to peak pressures vs 1 gpa since the glass was last melted this indicates that most of alh84001 has been well below these temperatures since 3 9 4 3 ga since these temperatures are below the curie point of magnetite much of the magnetization recently identified in alh84001 carbonate weiss et al earth planet sci lett this issue must have been acquired by v4 ga this also provides an explanation for why alh84001 contains a sample of an apparently ancient martian atmosphere that is less evolved relative to that on present day mars our calculations also suggest that for the last 4 gyr average surface temperatures on mars may not have been much higher than the present cold conditions b 2002 elsevier science b v all rights reserved

a 4 gyr shock age for a martian meteorite and implications for the cratering history of mars analyses of meteorites that originated on mars provide important insights into the geological and atmospheric evolution of the planet such analyses have hitherto been restricted to relatively young martian rocks1 the oldest martian meteorites have an age of approximately 1 3 billion years but the recently recognized2 martian meteorite allan hills 84001 which is distinct from the other martian meteorites2 4 shows evidence for a much older age5 6 here we report an analysis of the shock alteration history of this meteorite based on argon isotope dating from which we derive a shock age of 4 0 0 1 billion years the age and geological history of this meteorite suggest that it came from the heavily cratered noachian age terrains of mars s southern hemisphere and it may thus provide an absolute chronology for this region of the planet independent of that inferred from the cratering record the shock age of the meteorite also coincides with that of the so called lunar cataclysm a relatively short period during which many of the craters on the moon are believed to have formed supporting the idea7 that intense bombardment was widespread throughout the inner solar system between 3 9 and 4 1 billion years ago

low p t caledonian resetting of u rich paleoproterozoic zircons central sweden uranium rich zircons from a paleoproterozoic high grade deformation zone in the fennoscandian shield central sweden show an almost complete resetting of the u pb system in early phanerozoic time a mylonitic gneiss in the deformation zone contains two types of highly discordant 70 u rich zircons large brown cloudy prisms and small milky white irregularly shaped grains the gneiss also contains mostly clear prismatic zircon of lower u content with mildly discordant to concordant u pb ages laser raman spectroscopy reveals that the dark cathodoluminescent areas in brown zircons have a highly metamict crystal structure whereas the structures of both the dark cathodoluminescent milky white grains and the bright cathodoluminescent clear prisms have higher degrees of crystallinity age dates obtained by u pb sims analysis of 40 zircons of the three types described above range continuously from concordant at 1871 11 ma to 98 discordant at 384 15 ma the strongly discordant zircons clearly have suffered severe disturbance at about the time of the caledonian orogeny however caledonian metamorphic temperatures and pressures in this region did not exceed 150 200 c and 1 3 kbar too low to strongly disturb the u pb systematics in non metamict zircon by thermal means alone independent evidence indicates that saline fluids were circulating in the paleoproterozoic basement rocks at this time possibly driven by hydrological gradients generated in front of the encroaching caledonian orogenic wedge these low temperature saline fluids are inferred to be responsible for causing both strong pb loss in the mostly metamict brown zircons via a diffusive process and the formation of small milky white zircon via a low temperature recrystallization or dissolution re precipitation process

ion microprobe u th pb dating of phosphates in martian meteorite alh 84001 phosphates in martian meteorites are important carriers of trace elements although they are volumetrically minor minerals po4 also has potential as a biomarker for life on mars here we report measurements of the u th pb systematics of phosphates in the martian meteorite alh 84001 using the sensitive high resolution ion microprobe shrimp installed at hiroshima university japan eleven analyses of whitlockites and 1 analysis of apatite resulted in a total pb u isochron age of 4018 81 ma in the 238u 206pb 207pb 206pb 204pb 206pb 3 d space and a 232th 208pb age of 3971 860 ma these ages are consistent within a 95 confidence limit this result is in agreement with the previously published ar ar shock age of 4 0 0 1 ga from maskelynite and other results of 3 8 4 3 ga but are significantly different from the sm nd age of 4 50 0 13 ga based on the whole rock and pyroxene taking into account recent studies on textural and chemical evidence of phosphate our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 81 ma and that since then alh 84001 has not experienced a long duration thermal metamorphism which would reset the u pb system in phosphates

petrology on mars petrologic investigations of martian rocks have been accomplished by mineralogical geochemical and textural analyses from mars rovers with geologic context provided by orbiters and by laboratory analyses of martian meteorites igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition and ultramafic cumulates alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes suggesting global magmatic evolution relatively uncommon feldspathic rocks represent the ultimate fractionation products and granitic rocks are unknown sedimentary rocks are of both clastic mudstone sandstone conglomerate all containing significant igneous detritus and chemical evaporitic sulfate and less common carbonate origin high silica sediments formed by hydrothermal activity sediments on mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments metamorphic rocks have only been inferred from orbital remote sensing measurements metabasalt and serpentinite have mineral assemblages consistent with those predicted from low pressure phase equilibria and likely formed in geothermal systems shock effects are common in martian meteorites and impact breccias are probably widespread in the planet s crustal rocks the martian rock cycle during early periods was similar in many respects to that of earth however without plate tectonics mars did not experience the thermal metamorphism and flux melting associated with subduction nor deposition in subsided basins and rapid erosion resulting from tectonic uplift the rock cycle during more recent time has been truncated by desiccation of the planet s surface and a lower geothermal gradient in its interior the petrology of mars is intriguingly different from earth but the tried and true methods of petrography and geochemistry are clearly translatable to another world

examination of reference concentration under waves and currents on the inner shelf 1 we examine reference concentration using three different data sets of near bed suspended sediment concentration observed under combined waves and currents the data include observations made at 15 and 20 m depth off dounreay scotland uk and observations obtained at 13 m depth off duck north carolina usa these data accommodate different dynamic conditions from wave dominated conditions at dounreay to wind driven current dominated conditions at duck and sediment properties median size of bed sediment ranging from 120 to 350 m near bed concentration profiles to elevations of about 80 cm were obtained using acoustic backscatter sensors with 1 cm resolution the reference concentrations cr at 1 cm were then evaluated by regressing the observed suspended sediment concentrations against a rouse type model bed shear stresses associated with each estimate of cr were estimated using the wave current interaction model of grant and madsen existing equations for reference concentration based on shear stress alone fail to accommodate all cr estimates from different environments we introduce a new empirical relationship between cr and the product of shields and inverse rouse numbers these dimensionless parameters represent the ratio of bed shear stress and submerged particle weight and the ratio of shear velocity and particle settling velocity respectively the new formula adjusts the amount of mobile sediment at the bed related to the shields number to that available for suspension at the reference height related to the inverse rouse number the new formula for reference concentration accommodates observations from different environments suggesting that it may have wide applicability on sandy inner shelves

the use of settling velocity in defining the initiation of motion of heavy mineral grains under unidirectional flow the results of critical threshold experiments on four commonly occurring heavy minerals are described the data are presented in conjunction with comparable quartz data using the non dimensional shields curve and the movability number u ws the results indicate that critical shear stress for material of high density is overestimated by use of shields curve under smooth boundary conditions grain settling velocity is found to be a good indicator for the critical shear stress for grains of a wide density range a physical explanation for the results is proposed

grains in motion a review abstract this review paper aims to present a practical integrated procedure for the prediction of sediment transport rates in both the continental and marine environment based on the settling velocity of particles such studies have important applications in diverse fields such as civil and coastal engineering sedimentology and environmental geology where they can assist in flood control and the prevention of contaminant dispersal by wind and water as well as the siltation of dams and harbours the formation of placer deposits including gold diamonds and other heavy minerals is also controlled by sediment transport processes so that these studies can form the basis for more efficient exploration programs the first part of the paper discusses some basic principles important in sediment transport followed by an overview of published methods to determine the settling velocity of differently shaped particles including natural grains the application of settling velocity to predict the entrainment threshold of sediments on plane horizontal and inclined beds by unidirectional currents and oscillatory waves is then discussed which finally leads to the determination of sediment transport rates over plane and rippled beds the validity of this approach to natural conditions is tested against published field data where possible other widely used methods are also critically discussed pointing out severe problems in current sampling technology and the calculation of bedload transport in natural environments

hydrothermal zircon and baddeleyite in val d or archean mesothermal gold deposits characteristics compositions and fluid inclusion properties with implications fortiming of primary gold mineralization zircon and baddeleyite occur within quartz tourmaline veins at four gold deposits in the val d or district of the archean abitibi southern volcanic zone host rocks have experienced intense metasomatic enrichment of zr hf y and rare earth elements the zircons contain primary inclusions of quartz tourmaline pyrite albite k mica scheelite and gold and gold occurs in primary fluid inclusions in zircons magmatic zircons in host rocks do not have this suite of inclusions consequently a wall rock inheritance model for the vein zircons is implausible compositionally the zircons feature pronounced interzone and intergrain variations of hf y yb th and u and sporadic anomalous ce contents of 1100 ppm distinct from magmatic counterparts two principal types of primary fluid inclusion occur in the vein zircons type 1 h2o co2 inclusions have low salinities variable quantities of co2 and homogenization temperatures of 260 380 c and type 2 co2 rich inclusions contain minor h2o and ch4 the vei

martian surface paleotemperatures from thermochronology of meteorites the temporal evolution of past martian surface temperatures is poorly known we used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite alh84001 we found that the nakhlites have not been heated to more than 350 c since they formed our calculations also suggest that for most of the past 4 billion years ambient near surface temperatures on mars are unlikely to have been much higher than the present cold 0 c state

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

magnetite like nanocrystals formed by laser driven shocks in siderite abstract laser driven shock experiments were carried out on mn and mg bearing natural crystalline siderite under vacuum raman spectroscopy and transmission electron microscopy were performed on samples recovered from shock pressures between 8 4 gpa and 25 9 gpa two different iron oxides were observed in the laser impact crater hematite already present in the starting sample and a spinel structured phase both associated with iron carbonates this nanometer sized mn bearing magnetite like phase results from shock induced decarbonation of siderite high pressure shocks such as meteorite impacts are a plausible mechanism for generating nanocrystals of magnetite from fe carbonate bearing terrestrial and extraterrestrial materials

tracing the fluid evolution of the kiruna iron oxide apatite deposits using zircon monazite and whole rock trace elements and isotopic studies the ore genesis of the paleoproterozoic iron oxide apatite deposits in the vicinity of kiruna in northern sweden is poorly understood despite a century long mining history and 2500 mt of iron ore with grades of 30 to 70 wt fe produced in the region to date zircon grains from the ore recently dated at ca 1874 ma show very different appearances compared to zircon from surrounding host rocks ca 1880 ma and related intrusions ca 1880 and ca 1874 ma particularly an inclusion rich rim in contrast zircon from the host rocks and a proximal granite intrusion exhibit typical igneous growth zoning electron microprobe results show near stoichiometric composition for zr si and hf in the host rock zircon grains the ore zircon crystals have low analytical totals with significant concentrations of ca fe y and p and infrared spectroscopy showed several weight percent of water these ore zircon grains further show fe rich inclusions zones and or veins in elemental x ray maps and light rare earth elements lree enrichment transmission electron microscopy tem shows that the lree are not due to micro or nano inclusions in the zircon but are likely hosted as lree oxides in amorphous regions of the grains based on these characteristics the rims on ore zircon grains are interpreted to be of hydrothermal origin uranium pb in monazite from the ore measured by sims suggests a secondary event influencing the area at ca 1624 ma a period of known geologic activity in fennoscandia electron microprobe x ray mapping of these monazite grains shows no zoning and relatively low u and th concentrations stark contrasts are visible between the ore depleted mantle influence and host rocks crustal influence in the whole rock lu hf and sm nd data the depleted mantle signature of the ore could be related to the kiruna greenstone group as a potential source region for the iron the sm nd isotopic composition of monazite from the ore shows a crustal influence and indicates that the younger event has not disturbed the whole rock sm nd signature of the ore the hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source

olivine in martian meteorite allan hills 84001 evidence for a high temperature origin and implications for signs of life olivine from martian meteorite allan hills alh 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass the inclusions are irregular in shape and range in size from 40 pm to submicrometer some of the inclusions are elongate and boudinage like the olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite the olivine exhibits a very limited range of composition from fo65 to fob6 n 25 the 6180 values of the olivine and orthopyroxene analyzed by ion microprobe range from 4 3 to 4 5 3960 and are indistinguishable from each other within analytical uncertainty the mineral chemistries 0 isotopic data and textural relationships indicate that the olivine inclusions were produced at a temperature so0 c it is unlikely that the olivines formed during the same event that gave rise to the carbonates in alh 84001 which have more elevated and variable 6180 values and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine the reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel if the olivine was formed by either reaction during a postcarbonate heating event the implications are profound with regards to the interpretations of mckay el ul 1996 due to the low diflbsion rates in carbonates this rapid high temperature event would have resulted in the preservation of the fine scale carbonate zoning while partially devolatilizing select carbonate compositions on a submicrometer scale brearley 1998a this may have resulted in the formation of the minute magnetite grains that mckay et ul 1996 attributed to biogenic activity

hydrous species in zircon a survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed the region of ir spectra of oh bearing crystalline zircon from 4000 to 2500 cm 1 exhibits anisotropic oh bands resulting from at least three oh sites one at vacant tetrahedra and two that may be at si occupied tetrahedra one site associated with si occupied tetrahedra is the most important producing ir absorption bands at 3420cm 1 e c and 3385cm 1 e c oh maybe introduced during crystal growth by a coupled substitution of the form m 3 h 4 zr 4 the sites at vacant tetrahedra account for considerably less oh producing relatively weak ir absorption bands at 3510 cm 1 e c e c oh may be introduced during crystal growth by the hydrogrossular substitution of the form 4 oh sio 4 4 an additional oh band at 3270 cm 1 appears to be associated with an occupied tetrahedron heating experiments produced two more oh environments and indicate that h can migrate among the sites at high temperatures some crystalline samples are entirely free of oh ir spectra of metamict zircon exhibit broad asymmetric isotropic oh bands similar to h 2o bands but the absence of the h 2o bending mode band indicates that oh is the dominant hydrous species present ir spectra of partially metamict zircon samples retain some anisotropy the breadth of their oh bands is a function of degree of metamictizatron the maximum oh observed in any metamict or partially metamict zircon sample was approximately 0 1wt expressed as h 2o sucho h is probably secondary the maximum primary oh observed is 0 05 wt partially metamict zircon from a hydrothermal vug crystalline crustal zircon contains more than an order of magnitude less oh the maximum oh content observed in crystalline zircon 0 01 wt i s in a mantle zircon sample from a kimberlite suggesting high activity of h 2o in the mantle additional oh is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite fluid inclusions containing h 2o occur in some samples this may be a partial explanation of the high h 2o contents reported for some radiation damaged zircon based on loss on heating measurements

argon 39 argon 40 ages and trapped argon in martian shergottites chassigny and allan hills 84001 abstract argon isotopic abundances were measured in neutron irradiated samples of martian meteorites chassigny allan hills alh 84001 alh 77005 elephant moraine eet 79001 yamato y 793605 shergotty zagami and queen alexandra range que 94201 and in unirradiated samples of alh 77005 chassigny gives a 39ar 40ar age of 1 32 0 07 ga which is similar to radiometric ages of the nakhlites argon 39 argon 40 data for alh 84001 indicate ages between 3 9 and 4 3 ga a more precise definition of this age requires detailed characterization of the multiple trapped ar components in alh 84001 and of 39ar recoil distribution all six shergottite samples show apparent 39ar 40ar ages substantially older than the 165 200 ma range in ages given by other isotope dating techniques shergottites appear to contain ubiquitous ar components acquired from the martian atmosphere the martian mantle and commonly terrestrial atmospheric contamination zagami feldspar also suggests inherited radiogenic 40ar these data analyses indicate that the recent martian atmospheric component trapped in shergottites has a 40ar 36ar ratio possibly as low as 1750 and no greater than 1900 these ratios are less than the value of 3000 500 reported by viking the 40ar 36ar ratio for the martian mantle component is probably 500 but is poorly constrained the correlation between trapped 40ar 36ar and 129xe 132xe ratios in shergottite impact glasses and unirradiated samples of alh 77005 shows considerable scatter and suggests that the 36ar 132xe ratio in the martian components may vary resolution of martian atmospheric 40ar 36ar ratio at different time periods i e at 4 0 and 0 2 ga is also difficult without an understanding of the composition of various trapped components

the sources of water in martian meteorites clues from hydrogen isotopes abstract h isotope measurements of carbonate phosphate feldspathic and mafic glasses and post stishovite silica phase in the shergottites zagami shergotty sau 005 dag 476 alha 77005 and eeta 79001 as well as in chassigny and alh 84001 show that all these phases contain deuterium enriched water of extraterrestrial origin the minerals and glasses analyzed may contain an initial primary hydrogen component but their isotopic composition was modified to varying degrees by three different processes interaction with a fractionated exchangeable water reservoir on mars hydrogen devolatilization by impact melting and terrestrial contamination positive correlations between d and water abundance in feldspathic glass and post stishovite silica in zagami shergotty and sau 005 is indicative of mixing of a high d component 3000 4000 and a less abundant low d component 0 the high d component is primarily derived from the martian exchangable reservoir but may also have been influenced by isotopic fractionation associated with shock induced hydrogen loss the low d component is either a terrestrial contaminant or a primary magmatic component the negative correlation between d and water abundances in mafic and feldspathic glasses in alh 84001 alha 77005 and eeta 79001 is consistent with the addition of a low d terrestrial contaminant to a less abundant high deuterium martian component the low d of magmatic glass in melt inclusions suggests that the d of martian parent magma was low and that the initial h isotope signature of mars may be similar to that of earth

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds experiments and modelling experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady oscillatory flows derived critical shear stresses are compared with the mixed bed entrainment model of wiberg smith 1987 as predicted by the model coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains entrainment occurs generally at lower shear stresses than predicted by the model especially under unidirectional flows this may be the result of grains resting in unusually unstable positions during the experiments because the beds are unworked at the start of the experiments the model of wiberg and smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined the pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data the measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture the results are poorly described by the pivoting angle model presented by wiberg smith 1987 and are better represented by a model of the form d di d50 after li komar 1986 where and are empirical constants the threshold model is found to be more effective using the improved pivoting relationship the entrainment of grains is found to be easier beneath unidirectional flows than combined flows in accordance with previous authors findings a suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow the results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near threshold flows

microscale carbon isotope variability in alh84001 carbonates and a discussion of possible formation environments abstract the carbonates in martian meteorite alh84001 preserve a record of aqueous processes on mars at 3 9 ga and have been suggested to contain signatures of ancient martian life the conditions of the carbonate formation environment are critical for understanding possible evidence for life on mars the history of water on mars and the evolution of the martian atmosphere despite numerous studies of petrographic relationships microscale oxygen isotope compositions microscale chemical compositions and other minerals associated with the carbonates formation models remain relatively unconstrained microscale carbon isotope analyses of alh84001 carbonates reveal variable 13c values ranging from 27 to 64 the isotopic compositions are correlated with chemical composition and extent of crystallization such that the mg poor early formed carbonates are relatively 13c depleted and the mg rich later forming carbonates are 13c enriched these data are inconsistent with many of the previously proposed environments for carbonate formation and a new set of hypotheses are proposed specifically two new models that account for the data involve low temperature

entrainment threshold of natural grains in liquids determined empirically from dimensionless settling velocities and other measures of grain size abstract grain size plays a crucial role in sediment transport but different methods of size analysis do not yield comparable results particle diameters obtained by settling tube for example cannot be employed in equations based on sieve sizes although the majority of experiments on the initiation of sediment motion has been based on sieve data settling tube analysis is probably superior as both the settling rate and the entrainment threshold of grains are determined by the same factors of size shape and density in this paper empirical equations are provided which allow the determination of a non dimensional critical shear stress directly from the equivalent sedimentation diameter the equations apply to the entrainment of well sorted grains with a high sphericity on flat sediment beds and are valid for particles and liquids of different densities

settling velocities and entrainment thresholds of biogenic sands shell fragments under unidirectional flow settling velocities and entrainment thresholds of biogenic sedimentary particles under unidirectional flow conditions are derived on the basis of settling tower and laboratory flume experiments material consisting predominantly of equant blocks shell fragments of cerastoderma edule density s 2800 kg m 3 or of mica like flakes and elongate rods mytilus edulis fragments s 2720 kg m 3 are used in separate series of experiments differences in the measured settling and threshold properties are related primarily to particle shape the selection of a characteristic length scale for non spherical grains is investigated by comparing two approaches used to define the grain size d of the sediment samples grain settling and sieve analysis that are used to derive data for the threshold criteria in terms of the shields and movability diagrams the empirical curves effectively predict the threshold conditions for any grain shape provided that grain size is defined in terms of grain settling velocity however a functional distinction is made between the characteristic hydraulic grain size defined by grain settling for grain transport applications and the actual physical grain size defined by sieve analysis

types of mixing and heterogeneities in siliciclastic carbonate sediments abstract mixed siliciclastic carbonate deposits consist of a suite of different types of mixing between the two components from bed core plug to stratigraphic seismic scales producing a high vertical and lateral lithological variability mixed deposits results from the interaction of siliciclastic input and coeval carbonate production controlled by temporal and or spatial factors although mixed deposits are very diffuse in the geological record studies about these deposits are scrappy and not well encoded accordingly mixed deposits represent a labyrinth for researchers who want to investigate them for the first time in this paper different types of mixing compositional versus strata controlled by different allocyclic e g sea level climate and or autocyclic e g depositional processes factors that operate at different scale are documented mixing is recognized and described at three main scales of observation bed core plug scale lithofacies well log scale and stratigraphic seismic scale i compositional mixing reflects the contemporaneous accumulation of the two heterolithic fraction in space and time this type of mixing is observable at lamina to bed scale locally producing depositional structures diagnostic for particular depositional environments ii strata mixing results from the alternation of the two heterolithic fraction in time this type of mixing is observable at lithofacies to stratigraphic scale and can be related to depositional processes climatic variations and or relative sea level changes a correct identification of these different types of mixing and the scale of their occurrence is crucial in revealing i physical processes that control the sedimentation ii environmental factors that influence the carbonate factory related to the siliciclastic dispersal mechanisms and iii internal heterogeneity of the resulting sedimentary deposit furthermore the petroleum industry is interested to unravel new insights about internal properties of mixed siliciclastic carbonate systems e g porosity permeability and to reconstruct predictive 3d models for the related reservoirs the correct prediction of internal heterogeneity and the recognition of lateral and vertical compartmentalization have an important impact on hydrocarbon exploration and exploitation

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

carbonate formation events in alh 84001 trace the evolution of the martian atmosphere carbonate minerals provide critical information for defining atmosphere hydrosphere interactions carbonate minerals in the martian meteorite alh 84001 have been dated to 3 9 ga and both c and o triple isotopes can be used to decipher the planet s climate history here we report 17o 18o and 13c data of alh 84001 of at least two varieties of carbonates using a stepped acid dissolution technique paired with ion microprobe analyses to specifically target carbonates from distinct formation events and constrain the martian atmosphere hydrosphere geosphere interactions and surficial aqueous alterations these results indicate the presence of a ca rich carbonate phase enriched in 18o that formed sometime after the primary aqueous event at 3 9 ga the phases showed excess 17o 0 7 that captured the atmosphere regolith chemical reservoir transfer as well as co2 o3 and h2o isotopic interactions at the time of formation of each specific carbonate the carbon isotopes preserved in the ca rich carbonate phase indicate that the noachian atmosphere of mars was substantially depleted in 13c compared with the modern atmosphere

evidence for magma carbonate interaction beneath syrtis major mars 1 we used three methods spectral index mapping linear spectral mixture analysis and factor analysis and target transformation to determine if carbonate decomposition products including lime cao periclase mgo portlandite ca oh 2 and brucite mg oh 2 are present on the surface of mars using an expanded spectral library that includes decomposition products of calcite and magnesite we deconvolved a binned emissivity data set from the mars global surveyor thermal emission spectrometer mgs tes the deconvolution model indicates that anhydrous carbonates are not present at kilometer scales above the tes detection limit however carbonate decomposition products may be present at or above the tes detection limit in northeast syrtis major and several other small regions on mars the model results are supported by spectral index mapping and factor analysis and target transformation of tes data in this region the volcanic setting of the detections may indicate the interaction of syrtis major lavas with subsurface carbonates or alternatively the eruption of carbonate bearing lavas that were subsequently devolatilized by impacts or later lava flows

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

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

reverse age zonation of zircon formed by metamictisation and hydrothermal fluid leaching abstract understanding the stability of radiation damaged zircon is crucial to the application of zircon to u pb geochronology especially for zircons rich in radioactive nuclides electron microprobe and laser raman spectral analyses show that zircons from the ejinao sodalite syenite in southeast china are composed of cores without zonation and oscillatory rims the cores are rich in uranium and have experienced increased metamictisation an internal sieved structure and radial cracks were formed by radiation damage and radiation induced volume expansion infrared spectral analyses show that zircon from the ejinao sodalite syenite contains large amounts of oh and h 2 o this finding suggests that the zircon crystals experienced hydrothermal alteration the pb loss of zircons is ascribed to their interaction with a hydrothermal fluid derived from the ejinao syenite radiogenic pb 4 situated in recoil damaged sites has been significantly reduced to pb 2 by the hydrothermal fluid becoming incompatible in the crystal lattice and highly mobile in solution consequently it has been leached out from the zircon this pb loss leads to diverse 206 pb 238 u ages from 134 ma to 147 ma and severe pb loss from the inner core even produces reverse age zonations with cores approximately 5 ma younger than the rims the real crystallisation age for zircons from the ejinao sodalite syenite should be approximately 145 ma considering the entire range of analytical data obtained by both the shrimp and la icpms methods

interaction of weathering solutions with oxygen and u pb isotopic systems of radiation damaged zircon from an archean granite darling range batholith western australia zircons from archean granites from the darling range batholith in the yilgarn craton of western australia have been shown to have complexly discordant u pb systems with a strong component of zero age disturbance the only geological event that has affected the granites in recent times is the pervasive regional weathering our aim in this study was to investigate the effects of weathering on the u pb and oxygen systems of the zircons and to this end we report secondary ion mass spectrometry sims results of oh oxygen and u pb isotope systems of six typical zircons from a sample of the granite these results confirmed the presence of oh in highly radiation damaged parts of the zircons demonstrating fluid interaction within grains the presence of oh was accompanied by significant changes in the 18o 16o ratios the data suggest trends where 18o values in individual grains both increase and decrease with increasing oh sims measurements showed the u pb systems are variably and unsystematically discordant in radiation damaged parts of the zircons particularly those with elevated oh contents the complex u pb systems are interpreted in terms of multiple disturbance events between 450 and 0 ma involving low temperature fluid induced movement of radiogenic pb decoupled from parent u and th within the radiation damaged zircons together with some pb loss

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

field investigation of suspended sediment clouds under plunging breakers abstract breaking waves nearshore force the suspension of beach material and so play an important role in surf zone sedimentological regimes the analysis of cinemato graphical records of wave breaking and bottom sediment response for eight plunging and eight plunging transition breakers has enabled the study of sand cloud initiation evolution and decay the characteristics of the suspended sediment clouds can be explained in terms of breaker energy dissipation some of the sand cloud features are common to both plunging and plunging transition breakers although the total spatial and temporal history of the cloud is controlled by the water surface profile evolution during breaking observations of breaker kinematics in the aerated region after breaking highlight the importance of breaker induced vortices in controlling the initiation concentration elevation and longevity of suspended material in the surf zone

sand grain threshold in relation to bed stress history an experimental study besides particle size density and shape the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow such flow effectively rearranges the grains at and below the bed surface causing them to become more resistant to subsequent erosion the effects of the stress history leading up to the critical condition for sediment movement are investigated for unidirectional flows generated in a recirculating laboratory flume the sediment beds investigated consisted of cohesionless quartz sand grains with mean grain diameters of 0 194 mm fine sand 0 387 mm medium sand and 0 774 mm coarse sand with narrow particle size distributions the critical threshold shear velocity target value for the three beds was established within 2 5 min of increasing the flow from zero velocity the subsequent experiments were performed under prethreshold velocities at 70 for 5 10 20 40 and 80 min exposure duration 80 for 5 10 20 40 and 80 min exposure duration 90 and 95 for 5 10 20 40 80 and 120 min exposure duration of the target value following exposure to these different prethreshold conditions the flow was increased then to reach actual critical conditions within a period of 2 5 min the critical condition for the initiation of sediment movement was established using visual observation supplemented by video recordings according to the yalin criterion the results show that if the exposure duration to prethreshold velocities remains constant then the critical shear velocity increases with increasing prethreshold velocity likewise if the prethreshold velocity remains constant then the critical shear velocity increases with increasing exposure duration in some circumstances the critical shear velocity was found to increase by as much as 27 an empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand sized sediment beds this is prior to their inclusion into bedload transport formulae for an improved prediction of the magnitude and nature of transport

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

vibrational spectroscopic study of minerals in the martian meteorite alh 84001 micro raman spectra of carbonates silica and amorphous plagioclase and both micro raman and ir reflectance spectra of phosphates in alh84001 are reported data from these vibrational techniques combined with electron microprobe analyses show that 1 the carbonates exhibit complex compositional heterogeneity on a sub micrometer scale 2 the phosphates chlorapatite and merrillite are largely anhydrous 3 amorphous silica and plagioclase experienced peak shock pressures 32 gpa and 50 gpa respectively and 4 vitreous plagioclase was quenched from a shockinduced melt after relaxation of the peak shock pressure the observed general raman band broadening of lattice and internal modes of carbonates in alh84001 indicates complex sub microscopic compositional heterogeneity and possibly structural disorder any search for biogenic markers in alh84001 must recognize the complex shock and thermal history of these minerals

petrogenesis of allan hills 84001 constraints from impact melted feldspathic and silica glasses abstract compositional and textural relationships of shock melted glasses in the allan hills alh 84001 meteorite have been examined by optical microscopy electron microprobe analysis and compositional mapping the feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in alh 84001 the feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase an27 39ab58 68or3 7 or sanidine or51ab46an3 or from a mixture of these phases mixed feldspar glasses these observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite hydrothermal precipitates or alteration products or shock melts that have undergone alkali volatilization carbonate was deposited around previously formed mixed feldspar glass clasts suggesting that carbonate deposition occurred after the shock event that formed the granular bands crushed zones in this meteorite sio2 rich glasses appear to be silica remobilized during shock with little addition of other material a petrogenetic history of alh 84001 consistent with the observations of feldspathic and silica glasses is 1 igneous crystallization and cumulate formation 2 a pre carbonate shock event that formed the granular bands crushed zones and sheared chromites and melted igneous plagioclase and sanidine to form mixed feldspar glasses 3 carbonate and silica deposition in the granular bands veining of plagioclase glasses by sio2 and deposition of carbonate around mixed feldspar and plagioclase glass clasts 4 a post carbonate shock event that resulted in invasion of carbonate by feldspathic melts shock faulting and decarbonation of carbonate high temperature mobilization of silica melts and minor dissolution of orthopyroxene by silica melts

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

the hydrous component of sillimanite polarized infrared spectra of a suite of sillimanite samples from high grade regionally metamorphosed rocks and associated quartz veins and pegmatites upper amphibolite to pyroxene granulite facies from xenoliths in basaltic rocks and from alluvial deposits indicate that hydroxyl is the dominant hydrous species bound in sillimanite absorption bands at 3556 3329 3300 and 3248 cm 1 are characteristic of many of the samples heating experiments indicate that only above 700 c is weight loss primarily due to loss of structurally bound oh complete dehydration required heating to 1400 c the maximum content of bound oh in the sillimanites was 0 02 wt h 2o equivalent the intensities of the oh features generally decrease with increasing temperatures estimated for metamorphism consistent with the expectation that water activities decrease with increasing temperature however notable exceptions to this trend suggest that secondary hydration at the unit cell scale is also a viable explanation for oh incorporation in sillimanite

relationship between aerodynamic entrainment threshold and hydrodynamic settling velocity of particles abstract data on the critical shear stress c at incipient grain motion under air flow have been carefully selected from the literature it is shown that the non dimensional shields entrainment threshold c in air is an exponential function of the dimensionless settling velocity wds in water of a sphere with a diameter equal to the sieve size as wd5 can be related to the measured dimensionless settling velocity wdm of natural grains wdm can be used directly to calculate c for quartz grains in air the equations are valid for sizes between 0 01 and at least 0 166 cm i e silt to very coarse sand the method is applicable to well sorted grains with a relatively high sphericity entrained on flat surfaces

shape entropy and settling velocity of natural grains abstract the shape of sedimentary particles has been quantified by many different form indices which have been utilized mainly in determining grain hydrodynamic behavior although the orthogonal axis dimensions required for calculating shape indices are easily measured in the case of grains larger than 2 mm this is impractical for sand and silt different data sets of the settling velocity of natural quartz grains varying in size between 0 0001 and 1 cm are used in this paper to determine their mean hofmann shape factor which defines a polynomial curve in the grain size range from clay to medium size pebbles sand exhibits the highest sphericity culminating with coarse to very coarse sand the two polynomial equations describing the curve are employed to determine the settling velocity of natural grains with a mean accuracy of 95 1 4 5 the latter is similar to that obtained for both smooth ellipsoids and angular grains which suggests that grain roundness does not play a major role in the settling velocity of particles up to a reynolds number of at least 4 200

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

ion microprobe measurements of 18o 16o ratios of phosphate minerals in the martian meteorites alh84001 and los angeles abstract oxygen isotope ratios of merrillite and chlorapatite in the martian meteorites alh84001 and los angeles have been measured by ion microprobe in multicollector mode 18o values of phosphate minerals measured in situ range from 3 to 6 and are similar to martian meteorite whole rock values as well as the 18o of igneous phosphate on earth these results suggest that the primary abiotic igneous phosphate reservoir on mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on earth this is an important first step in the characterization of martian phosphate reservoirs for the use of 18o of phosphate minerals as a biomarker for life on mars cumulative textural major element and isotopic evidence presented here suggest a primary igneous origin for the phosphates in los angeles and alh84001 textural and chemical evidence suggests that phosphates in alh84001 were subsequently shock melted in a later event

transmission electron microscopy of minerals in the martian meteorite allan hills 84001 we have studied carbonate and associated oxides and glasses in a demountable section of allan hills 84001 alh 84001 using optical scanning and transmission electron microscopy tem to elucidate their origins and the shock history of the rock massive fracture zone and fracture filling carbonates in typical locations were characterized by tem x ray microanalysis and electron diffraction in a comprehensive study that preserved textural and spatial relationships orthopyroxene is highly deformed fractured partially comminuted and essentially unrecovered lamellae of diaplectic glass and other features indicate shock pressures 30 gpa bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene carbonate crystals are 5 m in size untwinned and very largely exhibit the r3c calcite structure evidence of plastic deformation is generally found mildly only in fracture zone and fracture filling carbonates even adjacent to highly deformed orthopyroxene and appears to have been caused by low stress effects including differential shrinkage high dislocation densities like those observed in moderately shocked calcite are absent carbonate contains impact derived glasses of plagioclase silica and orthopyroxene composition indicating brief localized impact heating stringers and lenses of orthopyroxene glass in fracture filling carbonate imply flow of carbonates and crystallization during an impact periclase mgo occurs in magnesite as 30 50 nm crystals adjacent to voids and negative crystals and as 1 m patches of 3 nm crystals showing weak preferred orientation consistent with 111 mgo 0001 carb as observed in the thermal decomposition of caco3 to cao magnetite crystals that are epitaxially oriented at voids negative crystals and microfractures clearly formed in situ fully embedded faceted magnetites are topotactically oriented in general with 111 mag 0001 carb so that their oxygen layers are aligned in optically opaque rims magnetites are more irregularly shaped and except for the smallest crystals poorly aligned all magnetite and periclase crystals probably formed by exsolution from slightly non stoichiometric co2 poor carbonate following impact induced thermal decomposition any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity

the evolution of buctouche spit new brunswick canada buctouche spit is an 11 km long sandy spit on the northeast shore of new brunswick canada it is a typical flying spit having a narrow proximal section which is characterized by a single foredune that is prone to overwash and transgression and a much wider prograding distal section which is characterized by a well developed foredune backed by a series of relict foredunes an optical luminescence dating method in which infrared stimulation is used on potassium feldspars was developed for this study it is a very promising method for dating holocene deposits yielding good resolution and having simplicity of measurement the luminescence ages obtained indicate that buctouche spit extended at a rate of 4 m yr 1 between 715 45 years ago and 240 25 years ago this extension rate equates to a distal end accumulation rate of 56 000 m3 yr 1 for this period at this rate of accretion the present buctouche spit would have formed during the past 2000 years the core data indicate that the spit is 2200 years old which is consistent with the preceding estimate since 240 25 years ago the rate of sediment accumulation at buctouche spit has apparently been falling the accumulation rate between 1839 and 1945 was 23 000 m3 yr 1 less than half of the historic rate of 56 000 m3 yr 1 since 1945 the distal end of buctouche spit has not prograded extended measurably this is because buctouche spit s present vertical growth rate is at most equal to the rate of relative sea level rise at the spit and may be as little as half of the rate of relative sea level rise present sediment supply to buctouche spit is probably 16 000 m3 yr 1 it is likely that the much larger sediment supply required to sustain an historic accretion rate of 56 000 m3 yr 1 was derived from the reworking of a former sandy barrier located seaward and possibly updrift of the present spit buctouche spit is now in a limited sediment supply situation and it has become a constrained spit sediment is being eroded from the centre section of the spit transported to the distal end and then carried away by ebb tidal currents the centre section of the spit will likely breach within the next hundred years if a breach or breaches becomes permanently established in the centre section of the spit the distal end will suffer significant erosion and may be destroyed after this the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise it is concluded that the key to understanding buctouche spit s long term evolution is recognizing that it is migrating landward both by continuous processes like overwash and by the more discrete and catastrophic process of overstepping

laboratory investigations into the threshold of movement of natural sand sized sediments under unidirectional oscillatory and combined flows the threshold of movement of sediment obtained from sandbanks within the bristol channel uk is investigated under unidirectional oscillatory and combined flows the experiments were undertaken in a recirculating unidirectional laboratory flume containing an oscillating plate to simulate wave action with movement along the same axis as the unidirectional flows the sand samples consisted of cohesionless quartz grains with median grain sizes between 0 315 and 0 513 mm the experiments were performed under flow velocities measured at 2 cm above the bed ranging between 0 and 24 cm s 1 and oscillatory currents wave periods of 5 12 and 15 s ranging from 0 to 28 cm s 1 the critical conditions for the initiation of sediment movement were assessed by visual observation using the yalin criterion the results show that under unidirectional flow there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions in the case of oscillatory flows the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves under combined flows wave period is shown to control threshold conditions with the unidirectional and oscillatory flow components combining in a linear fashion for long period 12 s and 15 s waves in contrast in the presence of short period 5 s waves the unidirectional and oscillatory components of the flow appear to decouple for high orbital velocities in both cases the effect of the wave period on threshold diminishes

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

the use of grain size trend analysis in macrotidal areas with breakwaters implications of settling velocity and spatial sampling density abstract in a macrotidal environment with offshore breakwaters elmer west sussex a new approach for the identification of the sediment transport pathways with grain size trend analysis gsta was undertaken using statistical parameters mean sorting and skewness directly derived from settling velocities distributions the same samples were analysed with sieving quarter and half phi resolution and gsta was performed again in order to directly compare the resultant sediment transport directions derived with the two analytical techniques furthermore both regular and irregular sampling distributions were used to recalculate gsta hydrodynamic data were collected in different locations around the breakwaters and net sediment transport directions were calculated in order to assess the accuracy of the sediment transport pathway directions derived with the different analytical techniques the accuracy of settling velocity in determining the statistical parameters of the grain size distribution is identified especially for the fine medium sand sediments settling velocities produced better results than the sieving the quarter phi resolution producing the poorer results in comparison with the coarser half phi resolution the results for the different spatial sampling strategies are found to depend upon the number of samples utilised that shows that the accuracy of the gsta is based upon the ability of representing adequately the spatial distribution of the sediment parameters

grain scale variations in trace element composition of fluid altered zircon acasta gneiss complex northwestern canada sem back scattered electron images of zircon separates from a 2 9 ga granitic dyke from the acasta gneiss complex slave province reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains electron microprobe analyses show that altered zircon domains are depleted in zr and si relative to unaltered zircon while retaining a constant zr si ratio sims trace element analyses indicate that lree are preferentially enriched in the altered domains altered zircon contains elevated concentrations of ba up to 580 ppm ca up to 2 weight of the element and common pb 50 ppb the presence of lree ca ba and common pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid and not by primary incorporation during crystallization we propose that the alteration related elements are located in amorphous zircon domains

electron microprobe dating as a tool for determining the closure of th u pb systems in migmatitic monazites high spatial resolution dating of monazite by the electron probe microanalyzer epma enables systematic and detailed studies of small minerals like zircon monazite records the complex history undergone by the host rocks recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages our work shows that a significant number of individual spot analyses is required to reach such precise information i e more than 30 40 data using the examples of monazites from three migmatites and one granite we show how to select the most efficient method of age calculation according to the u and th geochemistry of the grains or grain domains that we are trying to date three situations may be met 1 monazites exhibiting significant th u ratio variation 2 monazites exhibiting a fairly constant th u ratio but significant u th heterogeneity and 3 monazites of constant u and th concentrations for the first case a precise mean age can be calculated using a method of data reduction in the th pb f u pb diagram whereby a precision of 5 10 ma 2 is commonly achieved for the second case an isochron age can be calculated according to the pb f th method with a common precision of around 20 ma 2 whereas for the third case a simple weighted average age can be calculated using these approaches coupled with a back scattered electron image study we demonstrate that inheritance is probably as common for monazite as for zircon in addition the combination of high spatial resolution and precise age determination show the limited extent of pb diffusion in monazite finally an example from a migmatite from southern french guiana demonstrates the especially robust behavior of the th u pb system in monazite this system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons the monazite yielded exactly the same age as the protolith zircons

variability of diffusion of argon in albite pyroxene and olivine in shocked and unshocked samples abstract we analyzed the argon diffusion parameters of albite and pyroxene as a function of shock as well as an olivine of unknown shock pressure in albite shock pressures of 30 60 gpa all have an equal effect on the diffusion parameters lowering the activation energy e from 45 to 25 kcal mol and the log d0 a2 from 4 to 1 s 1 in a mixture of low ca and high ca pyroxene similar shock pressures also have an equal effect lowering the e from 130 to 80 kcal mol and the log d0 a2 from 14 to 7 s 1 however given that some shocked and unshocked pyroxene splits have similar release patterns making a clear distinction between the two is difficult additionally splits of pure unshocked high ca pyroxene have an e of 100 kcal mol and a log d0 a2 of 10 s 1 intermediate to the values of mixed pyroxene when shocked and unshocked the olivine fa18 has an e of 45 kcal mol and a log d0 a2 of 1 s 1 somewhat similar to that of feldspar these results will be useful in interpreting complex argon release patterns from shocked multi mineral samples too fine grained for easy mineral separation such as chondrites

pivoting analyses of the selective entrainment of sediments by shape and size with application to gravel threshold measured variations of pivoting angles with grain size shape reliability and angularity and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting with a bed of uniform grain sizes as employed experimentally to establish the standard threshold curves such as that of shields the threshold condition depends on grain shape and fabric the analysis demonstrates quantitatively that there should be a series of nearly parallel threshold curves depending on grain pivoting angles for a given grain size the order of increasing flow strength required for entrainment is spheres smooth ellipsoids depending on their reliability angular grains and imbricated ellipsoids depending on their imbrication angles the relative threshold values for these different grain shapes and fabric are predicted according to their respective pivoting angles but remain to be directly tested by actual threshold measurements the pivoting angle of a grain also depends on the ratio of its size to those it rests upon this dependence permits an evaluation of selective entrainment by size of grains from a bed of mixed sizes the condition generally found in natural sediments the pivoting model predicts systematic departures from the standard threshold curves for uniform grain sizes such departures have been found in recent studies of gravel threshold in rivers and offshore tidal currents the pivoting model is compared with those threshold data with reasonable agreement however more controlled measurements are required for a satisfactory test of the model it is concluded that variations in pivoting angles for grain entrainment are significant to the processes of selective sorting by grain size and shape

high temperature mars to earth transfer of meteorite alh84001 martian meteorites provide crucial insights into martian evolution and interplanetary mass transfer including the potential for exogenesis alh84001 is the oldest martian meteorite discovered so far and has been used to derive important conclusions about martian surface temperatures and very low temperature mars to earth transfer to better constrain the thermal evolution and shock metamorphic history of alh84001 we applied u th he thermochronometry to single grains of phosphate merrillite from alh84001 the u th he ages of individual phosphate grains in alh84001 range from 60 ma to 1 8 ga with a weighted mean of 830 ma this broad age distribution reflects multiple diffusion domains and requires a relatively high temperature resetting event younger than 60 ma these new data are combined with the published whole rock maskelynite as a main ar reservoir 40 ar 39 ar age spectra which show 5 8 fractional loss of radiogenic 40 ar since 4 0 ga he diffusion in both terrestrial and extraterrestrial apatite has a significantly higher activation energy 138 184 kj mol than ar diffusion in maskelynite 75 kj mol leading to an important kinetic crossover in fractional loss contours for these systems taken together the phosphate u th he and whole rock 40 ar 39 ar ages require both very low surface temperatures on mars and one or more short lived high temperature shock events after 4 0 ga we suggest that the last shock event occurred with ejection of alh84001 from mars and reached a peak temperature of approximately 400 c these results undermine the proposed low temperature ejection hypothesis for alh84001 but support long lived extremely cold martian surface temperatures 2007 elsevier b v all rights reserved

shock and thermal history of martian meteorite allan hills 84001 from transmission electron microscopy microstructures in the allan hills 84001 meteorite were studied using optical and electron microscopy putting emphasis on shock effects which are widespread some orthopyroxene exhibits only 100 slip but more typical grains suffered extensive slip microfracturing and frequently contain 100 clino inversion lamellae in fracture zones shock deformation of orthopyroxene has produced all three effects in profusion together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae which were apparently created by shearing on low index planes usually 100 or 110 both types of plane are loci that pseudo planar fractures tend to follow thus the glass lamellae which have not been observed in other meteorites probably formed by frictional heating during the sliding of microscale corrugated surfaces one over another leading to local melting we infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation nanocrystals of eskolaite cr2o3 coating silica glass grains are probably also a result of shock induced thermal decomposition of chromite iron sulfides pyrite and pyrrhotite were identified tended to be associated with plagioclase glass a carbonate disk showing no evidence for shock deformation had a substructure of elongated slightly misoriented subcells in the exterior interior regions had more eqiaxed subcells both microstructures probably formed during growth but the conditions are undetermined chemical composition varied on a micron scale but the rim of the disk was more ferroan oxide precipitates and voids were widely distributed as in fracture filling carbonates if the fracture zones and opx glass are the result of strong shock as we deduce it is very unlikely that pores could have filled by carbonate long after the fracture zones formed we infer that the carbonate like the phosphate olivine pyrrhotite eskolaite and many euhedral submicrometer chromites crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase silica and orthopyroxene

constraints from zircon hf o li isotopic compositions on the genesis of slightly low δ18o alkaline granites in the taohuadao area zhejiang province se china abstract the origins of the mesozoic low 18 o a type granites in eastern china are quite controversial a new low 18 o granite the taohuadao alkaline a 1 type granite from the zhoushan archipelago offshore from zhejiang is reported in this study integrated in situ analyses of hf o li isotopes in zircon are used to provide insights on the petrogenesis and evolution of this granite and low 18 o granites in general three types of zircon have been recognized in taohuadao samples zircon with sector oscillatary zoning type 1 disturbed zoning type 2 and extremely high u type 3 grains data are carefully screened for radiation damage and post magmatic alteration and only type 1 zircon represents a magmatic origin in type 1 zircons type 1a with normal mantle 18 o zrn value of 5 23 has a positive 7 li value 2 0 3 3 while type 1b has a 18 o value of 4 81 and highly variable negative 7 li values 31 9 to 0 3 in contrast to the o li isotopes zircon is quite homogeneous in terms of hf isotopes ehf t 3 5 1 7 suggesting that the taohuadao a 1 type granites were derived from a simple source our results show that the magmatic zircon 18 o zrn values are negatively correlated with hf content but positively correlated with zr hf ratio in zircon which indicates that fractional crystallization plays an important role in producing the slightly low 18 o signature the anhydrous nature of the a 1 type granites suppressed the crystallization of biotite and hornblende while low oxygen fugacity is responsible for insufficient precipitation of magnetite both of which drive the later differentiates towards slight depletion in 18 o therefore we suggest a ferrogabbro type fractional crystallization may have played a major role in producing the chemical and oxygen isotopic variations of the taohuadao a 1 type granite and accounts for 0 5 18 o depletion the extremely depleted 7 li in type 1b zircons is most likely a result of diffusion driven fractionation mantle like o li isotopes and very low li abundance in zircon suggest a predominantly mantle origin with little if any involvement of continental crustal materials in the granite thus the taohuadao a 1 type granite was most likely produced by intensive fractionation of reduced anhydrous basaltic melts rather than by mixing of mantle and crustally derived melts as previously thought

magnetite in alh 84001 an origin by shock induced thermal decomposition of iron carbonate in martian orthopyroxenite alh 84001 pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact s transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad nanometer sized magnetite particles with cuboid irregular and teardrop morphologies frequently associated with voids the fragments of carbonate must have been incorporated into the melt at temperatures of 900 c well above the upper thermal stability of siderite feco3 which decomposes to produce magnetite and co2 below 450 c these observations suggest that most if not all of the fine grained magnetite associated with fe bearing carbonate in alh 84001 could have been formed as result of the thermal decomposition of the siderite feco3 component of the carbonate and is not due to biological activity

hydrogen incorporation and charge balance in natural zircon the water and trace element contents of natural igneous zircons were determined to constrain the mechanism of hydrogen incorporation the low radiation damage zircons were derived from fe ti oxide gabbros from the vema fracture zone 11 n mid atlantic ridge they contain up to 1212 ppmw h2o 1 9 wt y2o3 and 0 6 wt p2o5 and are generally strongly zoned ree y are partially charge balanced by p y ree3 p5 zr4 si4 but a large ree excess is present on an atomic basis this excess is closely approximated by the amount of h present in the zircons we therefore conclude that h is incorporated by a charge balance mechanism h ree3 zr4 this interpretation is consistent with ftir data of the vema zircons which shows a strongly polarised main absorption band at ca 3100 cm 1 similar to experimentally grown lu doped hydrous zircon the size of this 3100 cm 1 band scales with h and ree contents apart from a small overlapping band at 3200 cm 1 no other absorption bands are visible indicating that a hydrogrossular type exchange mechanism does not appear to be operating in these zircons because of charge balanced uptake of h p and ree in zircon the partitioning of these elements into zircon is dependent on each of their concentrations for instance dreezrc melt increases with increasing h and p contents of the melt whereas dhzrc melt increases with increasing ree content but decreases with increasing p content in addition h p ree systematics of sector zoning indicate kinetic effects may play an important role hence using h in zircon to determine the water content of melts is problematic and ree partitioning studies need to take into account p and h2o contents of the melt

zircon an immobile index in soils interpretation of any element s mobility in soils needs rigorous means of calculating losses or gains in relation to parent rock the first mass balance calculations were based on isoelement changes major elements during conversion of fresh rocks to weathered materials harrisson 1933 leneuf 1959 isovolumetric mass balance calculations were applied for the first t ime in 1955 to determine mass transfers in saprolite millot and bonifas 1955 since that time numerous works have been carried out on mass balance calculations in determining transfers during weathering brimhall and dietrich 1986 recently equations used to calculate volumetric changes and net mass gain or loss of mobile elements have been based on zirconium mostly from the mineral zircon the aim of the paper is to question the axiomatic immobility of zircon in soils and then to assess the limit of mass balance calculations in surficial weathering systems

a comparison of velocity profiles in unidirectional currents and the wave boundary layer implications for sediment entrainment abstract although current velocity profiles are commonly used to predict sediment entrainment under unidirectional currents it has not been possible to use this method for waves because the nature of velocity profiles within the wave boundary layer was unknown based upon theoretical wave modelling that allows the thickness of the boundary layer and the boundary velocity to be determined in any water depth combined with an empirical method to determine the critical boundary velocity the different forms of the law of the wall for smooth transitional and rough boundary conditions are evaluated for the wave boundary layer the results show that velocity profiles in the wave boundary layer are of the same form as in unidirectional currents but that the critical shear velocity differs from that of the latter for smooth boundaries the shear velocity necessary to entrain grains of a specific size and density is more than that required under unidirectional currents but the inverse is true for fully rough conditions this is attributed to additional forces under waves related to the balance between the static pressure and static pressure gradient