aftsolve a program for multi kinetic modeling of apatite fission track data aftsolve is a computer program for deriving thermal history information from apatite fission track data it implements a new fission track annealing model that takes into account the known kinetic variability among different apatite species to fully utilize this model a fission track worker must obtain data that can be used to infer the kinetic characteristics of each apatite grain from which a measurement was taken such data can consist of etch figure lengths or chemical composition the benefit of this overall approach is that it allows useful information to be derived from previously unusable analyses extends the practical range of geological temperatures constrained by fission track analyses and increases overall confidence in model predictions aftsolve also incorporates the effects of fission track orientation relative to the apatite crystallographic c axis variation in initial track length and the biasing effect of 2 5 2 cf irradiation for enhancing confined horizontal track length detection aftsolve is written for windows operating systems and has a graphical interface that allows interactive input of thermal histories and real time generation of estimates for fission track length distributions and ages for up to six simultaneously modeled kinetic populations it also includes procedures for estimating the range of time temperature histories that are statistically consistent with a data set and constraints entered by the user

regolith and megaregolith formation of h chondrites thermal constraints on the parent body abstract spectral reflectivity data and its location near an orbital resonance suggest that asteroid 6 hebe may be the source body for h chondrites the second largest meteorite group recent spacecraft images of asteroids and theoretical modeling indicate that contrary to previous ideas asteroids can retain thick regoliths we model the thermal evolution of a hebe sized object coated with a thick insulating regolith and heated by26al and other long lived radionuclides the heat conduction equations for spherically symmetric objects were solved using finite difference approximations we assumed a three layer structure with regolith and megaregolith overlying a rocky core the three layers differed in bulk density porosity and thermal conductivity interior peak temperatures were set to match metamorphic temperatures of h6 chondrites the regolith has a major influence on thermal history and the results are very different from those for a simple rocky body published by various authors regolith insulation produces a uniform interior peak temperature of 1250 k and moves the petrographic type boundaries close to the surface of the parent body petrologic types 3 6 can be produced within 10 km of the asteroid s surface with only moderate 1 km regolith thicknesses the calculations indicate that h4 h6 formation would be consistent with the cooling rate estimates and pb pb formation ages if the material originated in the near surface regions we suggest that many if not all h chondrites could have been formed in a megaregolith and thick regolith their observed properties are consistent with this environment especially the abundance of regolith breccias and h chondrites of all petrologic types with implanted solar wind gases

rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the moon the southern highlands of the moon comprise superposed ejecta layers individually as thick as a few kilometers from the major basins smaller 1 16 km diameter impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks we observe a significant difference in the population of radar bright craters 1 16 km and larger in diameter between regions of the southeastern near side highlands north and south of lat 48 s there are about one third more radar bright craters north of this line than to the south broadly coincident with the mapped boundary between southern deposits mapped as pre nectarian age and those of nectarian imbrian age to the north the radar bright crater population is consistent with a megaregolith thickness of 1 5 km in the north and 2 5 km in the south a difference we attribute to south pole aitken basin ejecta

the lunar wide effects of basin ejecta distribution on the early megaregolith the lunar surface is marked by at least 43 large and ancient impact basins each of which ejected a large amount of material that modified the areas surrounding each basin we present an analysis of the effects of basin formation on the entire lunar surface using a previously defined basin ejecta model our modeling includes several simplifying assumptions in order to quantify two aspects of basin formation across the entire lunar surface 1 the cumulative amount of material distributed across the surface and 2 the depth to which that basin material created a well mixed megaregolith we find that the asymmetric distribution of large basins across the moon creates a considerable nearside farside dichotomy in both the cumulative amount of basin ejecta and the depth of the megaregolith basins significantly modified a large portion of the nearside while the farside experienced relatively small degrees of basin modification following the formation of the large south pole aitken basin the regions of the moon with differing degrees of modification by basins correspond to regions thought to contain geochemical signatures remnant of early lunar crustal processes indicating that the degree of basin modification of the surface directly influenced the distribution of the geochemical terranes observed today additionally the modification of the lunar surface by basins suggests that the provenance of lunar highland samples currently in research collections is not representative of the entire lunar crust identifying locations on the lunar surface with unique modification histories will aid in selecting locations for future sample collection

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

thermal evolution model for the h chondrite asteroid instantaneous formation versus protracted accretion abstract we present a model of the thermal evolution of asteroids assuming an onion shell model for the h chondrite parent body we obtain constraints for the h chondrite asteroid parameters by fitting empirical h chondrite cooling ages of estacado guarena kernouve mt browne richardton allegan nadiabondi ste marguerite and forest vale by using a genetic algorithm for parameter optimisation the model improves previous calculations on the thermal history calculated in the instantaneous accretion approximation considering sintering and porosity dependent heat conduction the model is extended to include a finite growth time of the parent body to study whether the meteoritic record constrains the duration of the growth phase of the parent body where it assembles most of its mass it is found that only short accretion times of up to 0 1 ma are compatible with the empirical data on h chondrite cooling histories best fit models yield excellent agreement with the cooling age data particularly they indicate that i 26 al was the major heat source driving metamorphism while 60 fe contributed rather marginally ii maximum temperatures remained below partial melting temperatures throughout the body indicating that no partial differentiation occurred on the h chondrite parent asteroid iii the h chondrite asteroid formed 2 ma after cais briefly after most ordinary chondrite chondrules formed if 26 al abundance defines a chronological sequence

fundamentals of fission track thermochronology fission track ft analysis has developed into one of the most useful techniques used throughout the geologic community to reconstruct the low temperature thermal history of rocks over geological time the ft method is based on the accumulation of narrow damage trails i e fission tracks in uranium rich mineral grains e g apatite zircon titanite and natural glasses which form as a result of spontaneous nuclear fission decay of 238u in nature price and walker 1963 fleischer et al 1975 the time elapsed since fission tracks began to accumulate is estimated by determining the density of accumulated tracks in a particular material in relation to the uranium content of that material chemical etching can be used to enlarge fission tracks that have formed within a mineral in order to make them readily observable under an ordinary optical microscope price and walker 1962 if a host rock is subjected to elevated temperatures fission tracks that have formed up to that point in time are shortened progressively and eventually erased by the thermal recovery i e annealing of the damage fleischer et al 1975 because thermal diffusion basically governs the annealing process the reduction in ft length is a function of heating time and temperature importantly fission tracks are partially annealed over different temperature intervals within different minerals this characteristic allows for the construction of time temperature paths of many different rock types by a plotting ft and other isotopic ages from different minerals versus their closure temperatures which is applicable in the case of a monotonous cooling history e g wagner et al 1977 zeilter et al 1982 and or by b the inverse modeling of observed ft age and confined track length data e g corrigan 1991 lutz and omar 1991 gallagher 1995 ketcham et al 2000 see also ketcham 2005

can fission track annealing in apatite be described by first order kinetics abstract it is becoming commonplace to suggest that fission track annealing in apatite can be described by first order kinetics that is the decrease in track density and or length with time during isothermal annealing can be described by a single exponential equation a review of the literature reveals that there is no convincing evidence for such an assertion but more than ample evidence to the contrary simple graphical analysis of available annealing data from the literature clearly shows that the single exponential fails to adequately describe the data more rigorous statistical model fitting is not attempted due to the generally inadequate specification of annealing data in the literature most published annealing data suffer from flaws in experimental design that render them of limited value in defining viable kinetic models for the annealing process consideration of the processes taking place during annealing suggests that there is no good reason why first order kinetics would be expected to describe fission track annealing and more complex models should be considered in order to provide reliable data capable of yielding practically useful kinetic models laboratory annealing experiments should be designed to meet a number of strict criteria in identifying viable models the need is for those which adequately describe the data rather than for physically meaningful models although models should preferably be kept as simple as possible

comment on experimental evidence for the pressure dependence of fission track annealing in apatite by a s wendt et al earth planet sci lett 201 2002 593 607 abstract the pressure experiments reported by wendt et al are cast in such a way as to question the fundamental basis underpinning apatite fission track thermochronology implying the possibility of a large systematic error in previous interpretations we find however that the study is severely compromised by fundamental errors in both experimental design and execution as well as lacking in consideration of a substantial body of previous work in the area of fission track annealing studies wendt et al have not attempted to extrapolate the results of their experiments to geologically relevant heating times and temperatures this has been the fundamental test that all previous annealing models have had to pass their study clearly fails this test moreover if significant pressure dependence does exist then it is implicit in existing deep borehole consistent annealing models whilst such models are interpolated in terms of temperature the borehole test means that they also accommodate any other factors which correlate with temperature including pressure the implications of the results reported by wendt et al have been overstated far from being intimidating we find that they have little relevance either to previous studies of apatite fission track annealing and extrapolations based on them or to the routine application of fission track analysis to elucidate thermal history information under geological conditions

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

crater relaxation on titan aided by low thermal conductivity sand infill abstract titan s few impact craters are currently many hundreds of meters shallower than the depths expected assuming these craters initially had depths equal to that of similar size fresh craters on ganymede and callisto moons of similar size composition and target lithology then some process has shallowed them over time since nearly all of titan s recognized craters are located within the arid equatorial sand seas of organic rich dunes where rain is infrequent and atmospheric sedimentation is expected to be low it has been suggested that aeolian infill plays a major role in shallowing the craters topographic relaxation at titan s current heat flow was previously assumed to be an unimportant process on titan due to its low surface temperature 94 k however our estimate of the thermal conductivity of titan s organic rich sand is remarkably low 0 025 w m 1 k 1 and when in thick deposits will result in a thermal blanketing effect that can aid relaxation here we simulate the relaxation of titan s craters afekan soi and sinlap including thermal effects of various amounts of sand inside and around titan s craters we find that the combination of aeolian infill and subsequent relaxation can produce the current crater depths in a geologically reasonable period of time using titan s current heat flow instead of needing to fill completely the missing volume with 100 sand only 62 71 and 97 of the volume need be sand at the current basal heat flux for afekan soi and sinlap respectively we conclude that both processes are likely at work shallowing these craters and this finding contributes to why titan overall lacks impact craters in the arid equatorial regions

ureilite petrogenesis a limited role for smelting during anatexis and catastrophic disruption a popular model for ureilites assumes that during anatexis in an asteroidal mantle pressure buffered equilibrium smelting partial reduction coincident with partial melting engendered their conspicuous mafic silicate core mg diversity 75 96 mol several mass balance problems arise from this hypothesis smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of fe metal and copious proportions of co gas the most serious problem concerns the yield of co gas if equilibrium smelting produced the ureilites entire 21 mol range in olivine core mg the proportion of gas within the asteroidal mantle assuming plausibly low pressure 80 bar should have reached 85 vol based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy a runaway co leaky process that can loosely be termed smelting appears to have occurred probably triggered by a major impact the runaway scenario appears likely because by le ch telier s principle co leakage would tend to accelerate the smelting process also the copious volumes of gas produced by smelting would have led to explosive mass leaky eruptions into the vacuum surrounding the asteroid loss of mass would mean diminution of interior pressure which would induce further smelting leading to further loss of mass basalt and so on such a disruptive runaway process may have engendered the ureilites distinctive reduced olivine rims but the only smelting according to this scenario was a short lived disequilibrium process that reduced only the olivine rims not the cores and the ureilites were cooling not melting during the abortive smelting episode

critical crater diameter and asteroid impact seismology if impact stress reverberation is the primary gradational process on an asteroid at global scales then the largest undegraded crater records an asteroid s seismological response the critical crater diameter dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size it is solved for by combining relationships for crater growth and for stress attenuation the computation for dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters in comparison to their size whereas large asteroids have giant undegraded craters dcrit can even exceed the asteroid diameter in which case all craters are local and the asteroid becomes crowded with giant craters dcrit is the most recent crater to have formed on a blank slate when it is equated to the measured diameter of the largest undegraded crater on known asteroids peak particle velocities are found to attenuate with the 1 2 1 3 power of distance less attenuative than strong shocks and more characteristic of powerful seismic disturbances this is to be expected since global degradation can result from seismic cm s 1 particle velocities on small asteroids attenuation as modeled appears to be higher on asteroids known to be porous although these are also bodies for which different crater scaling rules might apply

evidence for unetchable gaps in apatite fission tracks abstract the apparent shape of etched confined fission tracks in apatite depends on their crystallographic orientation on the direction of observation and on the distribution of longitudinal etching velocities along the track overetched blade like tracks which are approximately perpendicular to the c axis sometimes exhibit an extremely discontinuous etching behaviour resulting from the existence of so called unetchable gaps the full length of such tracks might not be revealed under normal etching conditions in the context of thermal history analysis the occurrence of unetchable gaps is very important for the understanding of the relationship between measured track length distributions and areal track densities in contrast to previous statements concerning durango apatite it can be shown that the phenomenon of fragmentation is not restricted to short tracks with a length of proportionality between the reductions of the areal track density and the etchable track length as has been assumed in some models dealing with projected track length distributions does not describe the true behaviour of strongly annealed apatites

apatite fission track analysis of oligocene strata in south texas u s a testing annealing models analysis of apatite fission track data from forty seven surface and subsurface samples of fine grained sandstone to pebbly conglomerate from the oligocene catahoula gueydan frio and vicksburg formations documents the response of fission track ages and track length distributions to simple post depositional thermal histories characterized by monotonic heating data from outcrop samples document that the apatite component was derived from source areas characterized by rapid cooling through the temperature range from 120 to 50 c during the early tertiary the apatite population is characterized by a log normal chlorine distribution mean 0 3 wt 0 3 wt consisting predominantly of low cl apatites 80 is 0 5 wt mean track lengths from core samples decrease systematically with increasing depth and temperature the decrease in apatite fission track ages with increasing depth and temperature is less systematic due to the scatter in inherited ages at the time of deposition no tracks were observed in apatite from samples at temperatures above 130 c mean track lengths for the south texas data agree well with those for a similar data set from the otway basin southeastern australia for present day temperatures of 70 c at higher temperatures mean lengths are 1 to 9 m longer than those observed from the otway basin this discrepancy is interpreted to reflect up to 40 ma longer residence times at maximum burial temperatures for the otway basin samples standard deviations about mean lengths are also lower than observed in the otway basin study consistent with the wider variation of cl contents mean 0 8 wt 0 7 wt that characterize apatite from the otway basin recently published apatite fission track annealing models predict significantly different mean track length vs present day temperature profiles for thermal histories believed to be representative of those experienced by sampled south texas strata because of the large uncertainties attributed to present day temperatures 10 c the predicted mean length vs temperature trends for these models essentially fall within or on the uncertainty envelope for the observed trend residuals of predicted minus observed mean lengths suggest that the previously published carlson composite parameter model using an initial mean length of 15 8 m instead of 16 5 m or the crowley et al model for track length reduction down to 11 m provide the best estimates of mean track length reduction over geologic time scales for near end member f apatite populations

growth and evolution of asteroids asteroids are what is left of the precursors to the terrestrial planets they are stunning in their diversity ranging from charcoal black worlds the size of a hilltop spinning like a carnival ride to dog bone shaped metallic remnants of some cataclysmically disrupted planetary core to worlds as stately as ceres and vesta and fragments thereof to garden variety fractured and blocky nuggets that dominate near earth space asteroid belts are common around sun like stars when properly seen as unaccreted residues as scraps on the floor of the planetary bakery the diversity of asteroids can be fully appreciated for to paraphrase tolstoy accreted planets are all alike every unaccreted planet is unaccreted in its own way

natural long term annealing of the zircon fission track system in vienna basin deep borehole samples constraints upon the partial annealing zone and closure temperature abstract fission track ft analysis of zircon from cretaceous and tertiary sandstone samples in vienna basin boreholes has provided constraints for the thermal stability of the zircon ft system over a geological timescale confined track lengths and ages were measured for samples from depths of 1 5 7 5 km the deepest with a present environmental temperature of 200 c mean track lengths range from 10 3 to 10 8 m indistinguishable within error from the unannealed reference length of 10 7 0 1 m 1 standard error thus showing no sign of systematic downhole reduction central zircon ages are consistently older than stratigraphic ages of parent rocks with no single grain ages significantly younger than their respective stratigraphic ages such ft age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition because the present geothermal regime as well as sample burial depths have been near constant for at least the past 5 m y the lower temperature limit of the zircon ft partial annealing zone is 200 c for a heating duration of the order of 5 10 m y such evidence from long term natural annealing is compatible with a zircon ft partial annealing zone cf 200 350 c derived by the extrapolation of laboratory annealing results based on onfined track length measurements but cannot alone discriminate between different annealing models for the zircon ft closure temperature a perhaps over simplified concept these results are consistent with the oft quoted temperature of 250 c for cooling rates of 10 100 c m y

impact excavation on asteroid 4 vesta hubble space telescope results hubble space telescope images of asteroid 4 vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole color measurements within the 13 kilometer deep crater are consistent with excavation deep into a high calcium pyroxene rich crust or olivine upper mantle about 1 percent of vesta was excavated by the crater formation event a volume sufficient to account for the family of small vesta like asteroids that extends to dynamical source regions for meteorites this crater may be the site of origin for the howardite eucrite and diogenite classes of basaltic achondrite meteorites

subsidence breccias in kimberlite pipes an application of fractal analysis abstract a particular variety of volcanogenic country rock breccia is described a contact breccia that has been identified at venetia river ranch and wimbledon kimberlite pipes the contact breccia is clast supported with no juvenile kimberlite component has tightly packed angular fragments with occasional rounding of smaller particles and has a shear fabric dipping towards the center of each kimberlite pipe or volcanic event clasts have preferred orientations parallel to the fabric photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis the fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism clast size distribution fractal dimensions in the coarser particles greater than circa 3 cm range from greater than 3 for nonsheared breccia down to circa 2 3 for the sheared breccia breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high energy explosions followed by collapse and abrasion by subsequently gravity induced rockmass subsidence all studied contact breccias produced a distinctive fractal signature in the finer particles

multiple parent bodies of ordinary chondrites abstract thermal histories of chondrite parent bodies are calculated from an initial state with material in a powder like form taking into account the effect of consolidation state on thermal conductivity the very low thermal conductivity of the starting materials makes it possible for a small body with a radius of less than 100 km to be heated by several hundred degrees even if long lived radioactive elements in chondritic abundances are the only source of heat the maximum temperature is determined primarily by the temperature at which sintering of the constituent materials occurs the thermal state of the interior of a chondrite parent body after sintering has begun is nearly isothermal near the surface however where the material is unconsolidated and the thermal conductivity is much lower the thermal gradient is quite large this result contradicts the conventional onion shell model of chondrite parent bodies but because the internal temperature is almost constant through the whole body it supports a multiple parent bodies model according to which each petrologic type of chondrite comes from a different parent body

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

oxidation during metamorphism of the ordinary chondrites subtle but progressive changes in the compositions and relative abundances of olivine pyroxene and metal with increasing metamorphic grade in equilibrated types 4 6 ordinary chondrites indicate that these meteorites experienced oxidation of metallic fe during heating oxygen fugacities calculated from these mineral assemblages increase with petrologic type the hypothesis of oxidation during chondrite metamorphism differs from a commonly accepted idea that chondritic fe was reduced by graphite the mineralogies of unequilibrated type 3 chondrites do not conform to the progressions in equilibrated chondrites allowing the possibility that fe may first have been reduced through reaction with graphite at the onset of metamorphism although there are conflicting observations supporting both oxidation and reduction in these meteorites as temperatures increased to levels appropriate for types 4 6 chondrites the activity of graphite may have been lowered by its dissolution in taenite and in any case oxygen fugacities appear to have been well within the stability field of graphite precluding its reaction at these temperatures oxidation state was largely controlled by equilibrium between ferromagnesian silicates and metal oxygen fugacities calculated from chondrite mineral equilibria are 2 3 log units below previously reported intrinsic o2 measurements we propose that progressive oxidation during metamorphism was promoted by interaction with small amounts of an oxidizing vapor derived by heating ices originally accreted into the parent asteroids assuming that this vapor was pure h2o the watenrock weight ratio required to account for the observed oxidation of fe in h and l chondrites is very modest less than several per mil progressive oxidation in the metamorphic sequence resulted from different degrees of reaction with such a fluid possibly caused by temperature variations or evolution of the composition of the fluid as it permeated through the body the presence of even minor amounts of a vapor phase during metamorphism affects interpretations of the volatile trace element and oxygen isotopic fractionations observed in ordinary chondrites and inferred to have formed by nebular or parent body processes

diffusion creep of dry melt free olivine 1 deformation experiments were conducted on fine grained 3 6 m fully synthetic fo90 olivine aggregates in a gas medium apparatus at 300 mpa confining pressure and temperatures of 1150 1360 c the strain rates of the solution gelation derived and therefore genuinely melt free dry samples are about two orders of magnitude lower than the strain rates for nominally melt free aggregates at the same pressure and temperature conditions and grain size benchmark deformation tests with anita bay dunite and mild steel reproduce published data the creep strength of melt added sol gel olivine is similar to the published creep strength of dry melt bearing olivine derived from natural rocks nonlinear least squares fits to the melt free deformation data give an activation energy of 484 kj mol a stress exponent of 1 4 and a grain size exponent of 3 over a range of stresses from 15 to 210 mpa these results suggest that small amounts of melt may be similarly effective in reducing the creep strength of upper mantle rocks as small amounts of water however a possible contribution of grain boundary composition to the observed differences in rheology in the absence of melt cannot be conclusively ruled out by the current experiments

an inverse method of modeling thermal histories from apatite fission track data abstract apatite fission track ft ages and track length distributions are important sources of information about the thermal histories of rocks recent advances in the understanding of track annealing in apatite provide a solution to the forward problem of predicting ft ages and track length distributions that result from a given thermal history in this paper a method is presented to address the inverse problem estimation of the thermal history from ft data the inverse method uses an iterative approach the downhill simplex to systematically modify a starting thermal history to achieve satisfactory statistical agreement between the predicted and observed ft age and track length distribution however because of analytical uncertainties a unique thermal history does not exist monte carlo simulations are used to take into account the uncertainties in the data and yield a spectrum of possible thermal histories the results are based on the isothermal annealing model of laslett et al 1 chem geol isot geosci sect vol 65 but because the method uses forward predictions and not an analytical formula specific to a single annealing model alternative annealing models could be used within the same framework this method is an improvement in interpreting fission track data because the range of thermal histories permitted by the data can be evaluated to demonstrate the method it is applied to several sets of test data created by forward modeling of idealized thermal histories the results show that the age at which the sample cooled through its closure temperature and the most general features of the thermal history are revealed however data from partially annealed samples have wide spectra that reflect large uncertainties in the form of the thermal history for such samples ft data alone are not likely to provide strong tests of geologic hypotheses conversely geologic information may play a crucial role in constraining the thermal history spectrum in application to uplift related cooling this inverse method can provide much more realistic assessments of the cooling rate and its uncertainty than are otherwise possible

the southern uplands controversy report of a discussion meeting held at burlington house on 12 february 1986 the idea for a discussion meeting followed from remarks made by professor c h holland 1986 in his presidential address to this society in 1985 when he expressed concern about the uncritical application of the accretionary prism model to the southern uplands of scotland it has since become apparent that divergent views are held both among those against the model and those in favour of it this meeting served to air some of these different opinions even if it has not immediately resolved many of the arguments most of the contributors to the meeting see the society newsletter for january 1985 have produced written papers for this volume and so far only one or two have modified their views between writing their abstract and submitting their paper the concept of an accretionary prism model developed gradually a comparison of scotland with burma suggested that the southern uplands were like western burma where trench and oceanic sediments are being underthrust by subduction processes mitchell mckerrow 1975 with the development of modern analogues from the pacific seeley et al 1974 karig sharman 1975 the model was applied to the southern uplands with more emphasis mckerrow et al 1977 leggett et al 1979 a b 1982 moseley 1977 1978 was one of the first to express doubts about this model he believed that the iapetus ocean had closed by the end of the ordovician and thus like many others

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

numerical simulations of the differentiation of accreting planetesimals with 26al and 60fe as the heat sources numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26al and 60fe as the heat sources planetesimal accretion was started at chosen times up to 3 ma after ca al rich inclusions cais were formed and was continued for periods of 0 001 1 ma the planetesimals were initially porous unconsolidated bodies at 250 k but became sintered at around 700 k ending up as compact bodies whose final radii were 20 50 100 or 270 km with further heating the planetesimals underwent melting and igneous differentiation two approaches to core segregation were tried in the first labelled a the core grew gradually before silicate began to melt and in the second labelled b the core segregated once the silicate had become 40 molten in a when the silicate had become 20 molten the basaltic melt fraction began migrating upward to the surface carrying 26al with it the 60fe partitioned between core and mantle the results show that the rate and timing of core and crust formation depend mainly on the time after cais when planetesimal accretion started they imply significant melting where accretion was complete before 2 ma and a little melting in the deep interiors of planetesimals that accreted as late as 3 ma the latest melting would have occurred at 10 ma the effect on core and crust formation of the planetesimals final size the duration of accretion and the choice of 60fe 56fe initial were also found to be important particularly where accretion was late the results are consistent with the isotopic ages of differentiated meteorites and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 ma after cais

post impact thermal evolution of porous planetesimals abstract impacts between planetesimals have largely been ruled out as a heat source in the early solar system by calculations that show them to be an inefficient heat source and unlikely to cause global heating however the long term localized thermal effects of impacts on planetesimals have never been fully quantified here we simulate a range of impact scenarios between planetesimals to determine the post impact thermal histories of the parent bodies and hence the importance of impact heating in the thermal evolution of planetesimals we find on a local scale that heating material to petrologic type 6 is achievable for a range of impact velocities and initial porosities and impact melting is possible in porous material at a velocity of 4 km s burial of heated impactor material beneath the impact crater is common insulating that material and allowing the parent body to retain the heat for extended periods millions of years cooling rates at 773 k are typically 1 1000 k ma matching a wide range of measurements of metallographic cooling rates from chondritic materials while the heating presented here is localized to the impact site multiple impacts over the lifetime of a parent body are likely to have occurred moreover as most meteorite samples are on the centimeter to meter scale the localized effects of impact heating cannot be ignored

conditions for pore water convection within carbonaceous chondrite parent bodies implications for planetesimal size and heat production abstract the critical rayleigh number for convective flow of pore water through a permeable spherical body is derived the result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production if the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water then the present calculations suggest that the parent bodies were less than 80 km in diameter the maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water rock ratios in altered carbonaceous chondrites pore water would have flowed through parent bodies larger than 120 km in diameter with typical rock like permeabilities convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other

a 4 2 billion year old impact basin on the moon u pb dating of zirconolite and apatite in lunar melt rock 67955 abstract a sharp rise in the flux of asteroid size bodies traversing the inner solar system at 3 9 ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments the prevalence of 3 9 ga crystallization ages for lunar impact melt breccias and u pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short lived cataclysmic episode of late heavy bombardment at that time here we report u pb isotopic compositions of zirconolite and apatite in coarse grained lunar melt rock 67955 measured by ion microprobe that date a basin scale impact melting event on the moon at 4 22 0 01 ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later significant impacts prior to 3 9 ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish other isotopic evidence such as 40 ar 39 ar ages of granulitic lunar breccias regolith fragments and clasts extracted from fragmental breccias and re os isotopic compositions of lunar metal is also suggestive of impact related thermal events in the lunar crust during the period 4 1 4 3 ga we conclude that numerous large impactors hit the moon prior to the canonical 3 9 ga cataclysm that some of those pre cataclysm impacts were similar in size to the younger lunar basins and that the oldest preserved lunar basins are likely to be significantly older than 3 9 ga this provides sample based support for dynamical models capable of producing older basins on the moon and discrete populations of impactors an extended period of basin formation implies a less intense cataclysm at 3 9 ga and therefore a better opportunity for preservation of early habitable niches and hadean crust on the earth a diminished cataclysm at 3 9 ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance

experimental evidence for the pressure dependence of fission track annealing in apatite abstract fission track analysis has seen a major expansion in application to general geological problems reflecting its advances in understanding the temperature dependence of track annealing and track length distributions however considerable uncertainties still persist in particular concerning the stability of fission tracks subjected to the interaction of environmental physical parameters e g pressure temperature stress and in extrapolation of laboratory data to geological time scales in this work we studied the fading behavior of spontaneous fission tracks in basic apatite hexagonal ca5 po4 3 oh f cl when exposed simultaneously to laboratory pressures temperatures and stress over varying time spans the experiments showed that track fading is a complex recovery mechanism which is extremely sensitive to the coupling of these three parameters in particular a strong decrease in the fission track fading rate was observed as a function of increasing pressure and a nearly temperature independent dramatic increase in fission track recovery was observed as a function of stress consequently 1 the stability field of fission tracks in apatite increases towards temperatures higher than 110 c depending on the absolute pressure 2 closure ages in apatite are underestimated 100 for an ideal geothermobarometric gradient 3 related exhumation and erosion rates are overestimated above the closure temperature and underestimated below the closure temperature and 4 since the widely applied statistical description of thermally induced fading kinetics does not account for the influences of either pressure or stress and is based on fission track annealing data produced at ambient pressure the accuracy in extrapolating fission track data to geological time scales and in their application to dynamical systems must be cast into doubt

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

towards zircon fission track thermochronology reference framework for confined track length measurements abstract horizontal confined track hct lengths and angles to the crystallographic c axis were measured for both unannealed spontaneous and induced fission tracks ft in zircons to establish a reference framework of hct length measurement induced track lengths were analysed on eleven zircon separates from various rock types including four age standards and spontaneous lengths on three zircon standards were analysed in general the observed hct lengths in each sample are slightly anisotropic with shorter tracks having greater angles to the c axis in addition all samples show large anisotropy in the number of observed hct s particularly for induced tracks as an observed mean hct length is controlled by the combined influence of these two anisotropies selective measurement of tracks that have specified azimuth angles to the c axis e g 60 90 is recommended in annealing studies or thermochronological applications mean spontaneous and induced hct lengths of 60 90 are approximately concordant for individual age standards although slightly controlled by the difference in overetching effects due to the different etching criteria adopted this indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon in contrast to the case in apatite mean lengths of induced hct s do not differ significantly between the zircons analysed therefore the overall mean and standard deviation values of hct lengths for 60 90 azimuth angles i e 10 50 0 04 1 standard error m and 0 76 0 03 m for spontaneous tracks and 10 73 0 04 and 0 63 0 03 m for induced tracks respectively offer reliable references for hct length measurement in zircon ft thermochronology

dating pseudotachylyte of the nojima fault using the zircon fission track method 1 the zircon fission track analysis of pseudotachylyte answered how old the nojima fault is which was activated during the 1995 kobe earthquake in japan the time of seismic fault motions at the crustal interior can be constrained by dating pseudotachylyte formation we succeeded in separating zircons from the pseudotachylyte layer of the fault as well as in dating them by the fission track method here we show that the layer has a significantly younger age 56 4 ma than the initial cooling age of the host rock suggesting that the fault was already initiated at 56 ma much older than the previous estimate based on the spatial distribution of sediments around the fault

quantified mineralogical evidence for a common origin of 1929 kollaa with 4 vesta and the hed meteorites abstract in this paper we present the first correlation of derived mineral abundances of v class asteroid 1929 kollaa 4 vesta and the hed meteorites we demonstrate that 1929 kollaa has a basaltic composition consistent with an origin within the crustal layer of 4 vesta and show a plausible genetic connection between kollaa and the cumulate eucrite meteorites these data support the proposed delivery mechanism of hed meteorites to the earth from vesta and provide the first mineralogical constraint derived from the observation of a small v class vesta family asteroid on the crustal thickness of 4 vesta

geological partial annealing zone of zircon fission track system additional constrains from the deep drilling miti nishikubiki and miti mishima abstract fission track ft analysis of zircon from rhyolite and sandstone samples of ministry of international trade and industry miti nishikubiki and miti mishima boreholes has given additional constraints on the temperature range of the zircon partial annealing zone zpaz over a geological time scale the advantages of these samples are the following 1 they have been exposed to a stable temperature for 1 million years and 2 it is possible to obtain data at successive temperatures ages and confined track lengths were measured in samples from depths of 3 5 5 6 km with a current environmental temperature of 223 c at the deepest track lengths in samples below 200 c are still long and ft ages are compatible with their sedimentary ages showing no evidence of annealing for a heating duration of 1 million years samples at 205 c seem to yield shortened tracks as a result of annealing because of a low number of zircon crystals only four track lengths were measured for the deepest sample whose mean length was shortest among analysed samples these data are consistent with previous results from ultra deep boreholes

tectonic development of the southern belt of the southern uplands accretionary complex detailed structural studies in the wenlock age southern belt of the southern uplands have identified major controls on tectonic development original thickness and composition of sediment control the style of faulting thin poorly consolidated interbedded sandstone mudstone sequences form broad sheared zones while thicker well consolidated sandstone dominated sequences fail along thin bedding parallel brittle fractures with vein infill the structural evolution of the sheared zones provides an insight into melange development depth of burial within the accretionary pile governs the development of cleavage and intensity of post fl deformational phases the intensity of imbrication within accreted material probably relates to subduction rate with early wenlock sequences being highly imbricated and mid wenlock sediments preserved in internally coherent fault bounded packets sinistral strike slip movement controls the development of steeply plunging folds sub horizontal stretching directions and sideways facing bedding cleavage relationships and strongly influences fabric development in sheared zones determination of the relative importance of these factors permits a fuller understanding of the deformational history

pyroxene selective impact smelting in ureilites abstract a characteristic feature of ureilite meteorites is reduction of feo but the reduction is usually confined to the rims of olivine in the lar 04315 lap 03587 and almahata sitta ureilites pyroxene was extensively reduced by impact smelting in lar 04315 the impact caused nearly all of the original pigeonite to melt or otherwise become sufficiently structurally compromised to allow smelting and yet a minor proportion of the pyroxene escaped smelting and survived with its original composition en 74 1 wo 10 2 olivine mosaicism confirms that lar 04315 experienced a major shock event the smelted pyroxenes also show a distinctive patchiness in their interference colors although each grain s basic optical continuity often including twinning is still discernible they also have reduced compositions are ubiquitously porous 15 and contain sprinklings of fe metal and felsic glass for the most part the olivine underwent only very slight reduction much of the small pyroxene component of lap 03587 shows the same oddly porous texture lar 04315 also contains large traces of silica and felsic glass with a typical composition of in wt 61 sio 2 23 al 2 o 3 11 cao 3 7 na 2 o glass these two phases together form selvages that line the walls of many of the largest voids in the rock silica is a by product of pyroxene smelting the felsic glass probably derives largely from interstitial basaltic melt that predated the impact however the comparatively stiff surrounding included silica may have promoted unusually high melt retention within lar 04315 through the smelting episode one aspect of which was a major stream out through the same large voids of co x gas the impact smelted pyroxene of lap 03587 is enigmatic because this ureilite also features little shocked euhedral graphite laths and no olivine mosaicism the fine grained ureilitic component of almahata sitta appears to have likewise formed by impact smelting but with more extensive melting of pyroxene especially a ca rich pyroxene component more pulverization and melting of olivine and more displacement of both however in places the original coarse equant ureilite texture is still discernible in relict form ordinarily an impact shock melts olivine before or at least no later than pyroxene but in the case of lar 04315 and lap 03587 the great shock event evidently occurred when the material was already anatectic or very nearly so and thus the difference in melting temperature between pyroxene and olivine 300 degrees lower for pyroxene was decisive if literature inferences of extremely fast cooling rates implying shallow burial depths are accurate the proportion of co x gas generated by ureilite smelting exceeded by a very large factor of order 10 3 but possibly much greater the volume represented as porosity in the final ureilites the outflow of so much gas may have by near surface explosive expansion and jetting enhanced the thoroughness of the impact triggered catastrophic impact disruption of the parent asteroid

thermobarometric data from a fossil zircon partial annealing zone in high pressure low temperature rocks of eastern and central crete greece abstract a fossil partial annealing zone of fission tracks in zircon is described from high pressure low temperature hp lt rocks of the phyllite quartzite unit pq on the island of crete greece correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data and with new experimental annealing results allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples the zircon fission track ft ages of samples from the pq across crete range from original detrital signature through reduced to completely reset the annealing is the result of a single heating period related to the hp lt metamorphism with near peak temperatures lasting for only a few million years some time between 24 1 and 20 1 ma in eastern crete where rocks have experienced temperatures of 300 50 c and pressures of 0 8 0 3 gpa zircon ft ages range from 414 24 to 145 10 ma ages above 300 ma occur mostly near the east coast of the island in rocks which have not been heated to above ca 280 c and probably represent a pre variscan source track lengths are already indicative of a substantial annealing at this temperature most of the zircon ft ages from eastern crete scatter within error around the stratigraphic age samples with apparent zircon ft ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca 320 c towards the west a sudden decrease to very young ages ranging from 17 2 to 18 1 ma reflects a complete resetting at ca 350 c short tracks however are still observed throughout the central and western part of the island ages are consistently below 22 ma thermobarometric data for this area indicate maximum temperatures of 400 50 c and pressures of 1 0 3 gpa only samples from western crete which have been exposed to 400 50 c show exclusively long tracks consequently the high temperature limit of the zircon partial annealing zone zpaz appears to be between 350 and 400 c a significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0 5 gpa and at different temperatures which fit the curves previously obtained by other authors at ambient pressure

on estimating contributions of basin ejecta to regolith deposits at lunar sites we have developed a quantitative model for predicting characteristics of ejecta deposits that result from basin sized cratering events this model is based on impact crater scaling equations housen schmitt and holsapple 1983 holsapple 1993 and the concept of ballistic sedimentation oberbeck 1975 and takes into account the size distribution of the individual fragments ejected from the primary crater using the model we can estimate for an area centered at the chosen location of interest the average distribution of thicknesses of basin ejecta deposits within the area and the fraction of primary ejecta contained within the deposits model estimates of ejecta deposit thicknesses are calibrated using those of the orientale basin moore hodges and scott 1974 and of the ries basin horz ostertag and rainey 1983 observed densities of secondary craters surrounding the imbrium and orientale basins are much lower than the modeled densities similarly crater counts for part of the northern half of the copernicus secondary cratering field are much lower than the model predicts and variation in crater densities with distance from copernicus is less than expected these results suggest that mutual obliteration erases essentially all secondary craters associated with the debris surge that arises from the impacting primary fragments during ballistic sedimentation if so a process other than ballistic sedimentation is needed to produce observable secondary craters regardless our ejecta deposit model can be useful for suggesting provenances of sampled lunar materials providing information complementary to photogeological and remote sensing interpretations and as a tool for planning rover traverses e g haskin et al 1995 2002

natural long term annealing of the zircon fission track system around a granitic pluton we measured 24 ages and 14 length distributions of fission tracks in zircons separated from basement sandstones around a granitic pluton to investigate natural long term annealing behaviors of the zircon fission track zft system samples were collected from three cretaceous coherent units of the shimanto accretionary complex shikoku japan along an eastward traverse at distances of between 1 6 and 10 6 km from the intrusion contact of 15 ma takatsukiyama granite exposing about 5 km in diameter sampling was carried out around the expected position of the transition zone of track retention i e the fossil partial annealing zone fpaz based on previous regional zft age distributions approaching the contact original zft ages of 100 ma show a continuous reduction to 15 ma at 3 km distance their single grain age distributions accordingly change from widespread multimodal patterns reflecting provenance ages to unimodal ones characterizing the secondary heating by the granite intrusion mean and distributions of zft lengths also show notable variations within the fpaz as a result of mixing both preexistent tracks that were shortened by the miocene heating and unannealed ones having original long lengths that have been formed and accumulated after the cooling of the pluton these facts along with observed track segmentation and annealing anisotropy suggest that the shortening patterns of tracks in nature a heating duration of 105 106 years estimated from thermal modeling are indistinguishable from those previously demonstrated at laboratory timescales this basically validates the conventional approach of extrapolating laboratory based kinetic parameters to geologic timescales the observed fpaz and thermal modeling results in conjunction with laboratory heating data previously reported and recent constraints from an ultradeep borehole favor a relatively narrow temperature range for the partial annealing zone i e 230 to 330 c for a heating duration of 106 years

positioning the titanite fission track partial annealing zone abstract as part of the ktb kontinentale tiefbohrung programme in germany titanite sphene fission track data were acquired for samples to a depth of 9000 m where the host rock temperature is 265 c at this depth age and confined track length information suggest that the partial annealing zone for the fission tracks in titanite was not reached by reconstructing the thermal history and backstacking of an uplifted partial annealing zone it is suggested that the in situ partial annealing zone is likely to be found in the region 265 310 c this value is more precise than previous estimates of the titanite partial annealing zone and it is also higher than previous estimates microprobe analysis indicates that the chemical composition of the titanites is rather homogeneous throughout the whole drilled section

ejecta blocks on 243 ida and on other asteroids abstract seventeen positive relief features 45 150 m across are identified as probable blocks in galileo high resolution images of ida their presence provides direct evidence for regolith retention on asteroids the spatial distribution maximum size and integrated volume of the blocks are consistent with those of blocks associated with craters on the earth the moon phobos and deimos the concordance suggests that the features are impact ejecta fragments and that cratering mechanics on ida an object of average diameter 32 km are similar to those applying on previously studied rocky bodies the blocks that lie within or near the rims of craters lascaux and mammoth were likely mobilized in the low velocity tail portion of the excavation flow that formed those craters a few blocks located near smaller craters may have migrated some distance away from their source possibly by impact induced spallation hopping rolling and or sliding some blocks on ida could be surviving fragments from the koronis parent body accreted after its breakup the lifetime of 102 m sized boulders against collisional disruption is estimated to be in the 108 109year range consistent with ages considered for the largest and oldest craters on ida extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks 15 and 70 m across could be present on dactyl and gaspra respectively in both cases too small to be identified in available galileo images blocks 100 m in size could be present on 433 eros and km sized megablocks on 4 vesta

crater ejecta scaling laws fundamental forms based on dimensional analysis a model of crater ejecta is constructed using dimensional analysis and a recently developed theory of energy and momentum coupling in cratering events general relations are derived that provide a rationale for scaling laboratory measurements of ejecta to larger events specific expressions are presented for ejection velocities and ejecta blanket profiles in two limiting regimes of crater formation the so called gravity and strength regimes in the gravity regime ejecta velocities at geometrically similar launch points within craters vary as the square root of the product of crater radius and gravity this relation implies geometric similarity of ejecta blankets that is the thickness of an ejecta blanket as a function of distance from the crater center is the same for all sizes of craters if the thickness and range are expressed in terms of crater radii in the strength regime ejecta velocities are independent of crater size consequently ejecta blankets are not geometrically similar in this regime for points away from the crater rim the expressions for ejecta velocities and thickness take the form of power laws the exponents in these power laws are functions of an exponent that appears in crater radius scaling relations thus experimental studies of the dependence of crater radius on impact conditions determine scaling relations for ejecta predicted ejection velocities and ejecta blanket profiles based on measured values of are compared to existing measurements of velocities and debris profiles

constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling abstract paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system the existence timing intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies thus paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system however models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited here we perform asteroid thermal evolution models across suites of radii accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields we find that dynamo activity in both types of asteroid is delayed until 4 5 5 5 myr after calcium aluminium rich inclusion formation due to the partitioning of 26al into the silicate portion of the body during differentiation and large early surface heat fluxes followed by a brief period

snowdon basalts and the cessation of caledonian subduction by the longvillian basalts of the bedded pyroclastic formation snowdon volcanic group north wales of longvillian caradoc age are heterogeneous in trace element composition forming a mixing trend between ocean island basalt oib type and magmatic arc basalt the two end member compositions were partial melts of two distinct upper mantle sources the oib end member is similar to icelandic oib and is the only known example of basalt of this type in the ordovician silurian welsh basin since ocean island basalt magma type is thought never to erupt above well established subduction zones the eruption of the longvillian oib provides a minimum age for the end of subduction of iapetus oceanic lithosphere below the welsh basin a model for the evolution of the mantle below the welsh basin is proposed which accounts for the geochemistry of magmas erupted from the tremadoc to the early silurian

ejecta megaregolith accumulation on planetesimals and large asteroids megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10 i have modeled global average ejecta accumulation as a function of the largest impact size with no explicit modeling of time in conjunction with an assumed cratering size distribution exponent b the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith the largest impactor mass ratio is a major fraction of the catastrophic disruption mass ratio and in general the largest crater s diameter is close to the target s diameter total accumulation is roughly 1 5 of and proportional to the target s radius global accumulations estimated by this approach are higher than in the classic housen et al 1979 study by a factor of roughly 10 this revision is caused mainly by higher typical largest crater size for b 2 the single largest crater typically contributes close to 50 of the total of new nonrecycled ejecta megaregolith can be destroyed by sintering a process whose pressure sensitivity makes it effective at lower temperature on larger bodies planetesimals 100 km in diameter may be surprisingly well suited about as well suited as bodies two to three times larger in diameter for attaining temperatures conducive to widespread melting a water rich composition may be a significant disadvantage in terms of planetesimal heating as the shallow interior may be densified by aqueous metamorphism and will have a low sintering temperature

regoliths on small bodies in the solar system a regolith is defined as a layer or mantle of loose incoherent rocky material of whatever origin that nearly everywhere forms the surface of the land and rests on coherent bedrock the regoliths on many planetary bodies are the result of continual impacts which transform coherent surfaces into fragmental debris the present investigation is concerned with the special case of regolith formation and evolution on small objects such as asteroids and meteorite parent bodies first order models of regolith evolution on asteroidal surfaces are constructed on the basis of data provided by studies of lunar samples and meteorites it appears that regolith formation proceeds by deposition of discrete layers of the widely spread ejecta primarily from the larger impacts moderate size 100 300 km diameter asteroids are covered by modest regoliths of the order of one km in depth small rocky asteroids develop negligible regoliths

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

thermochronological constraints on the timing and magnitude of miocene and pliocene extension in the central wassuk range western nevada apatite fission track and u th he thermochronological data provide new constraints on the timing of faulting and exhumation of the wassuk range western nevada where east dipping normal faults have accommodated large magnitude ene wsw oriented extension extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the grey hills and central wassuk range these fault blocks display westward tilts of 60 and expose preextensional paleodepths of up to 8 5 km based on the structural reconstruction of tilted preextensional tertiary andesite flows that unconformably overlie mesozoic basement rocks apatite fission track and u th he thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at 15 ma fission track modeling results indicate rapid fault block exhumation occurred between 15 and 12 ma which is in agreement with miocene volcanic rocks that bracket the tilting history in addition fission track and u th he data suggest reduced rates of cooling following major extension as well as renewed cooling related to active high angle faulting along the present day range front starting at 4 ma thermochronological data from structurally restored fault blocks indicate a preextensional miocene geothermal gradient of 27 5 c km the thermochronological constraints on the timing of extensional faulting and the eruptive history in the wassuk range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension and where synextensional magmatism is suppressed during the highest rates of extension

the global effects of impact induced seismic activity on fractured asteroid surface morphology abstract impact induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids in this study we use a series of linked seismic and geomorphic models to investigate the process in detail we begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact these synthetic seismograms are then applied to a model of regolith resting on a slope and the resulting downslope motion is computed for a full range of impactor sizes next this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure showing how topographic erosion accumulates as a function of time and the number of impacts finally these results are applied in a stochastic cratering model for the surface of an eros like body same volume and surface area as the asteroid with craters formed by impacts and then erased by the effects of superposing craters ejecta coverage and seismic shakedown this simulation shows good agreement with the observed 433 eros cratering record at a main belt exposure age of 400 200 myr including the observed paucity of small craters the lowered equilibrium numbers loss rate production rate for craters less than 100 m in diameter is a direct result of seismic erasure which requires less than a meter of mobilized regolith to reproduce the near observations this study also points to an upper limit on asteroid size for experiencing global surface modifying seismic effects from individual impacts of about 70 100 km depending upon asteroid seismic properties larger asteroids will experience only localized regional seismic effects from individual impacts

terrane boundaries in ireland inferred from the irish magnetotelluric profile and other geophysical data this paper presents explanations for some important geophysical features associated with terrane boundaries in the iapetus suture zone in ireland the acquisition of data along the irish magnetotelluric profile imp provided the basis for a re assessment of magnetic gravity and seismic refraction results and further resolved sub surface structures associated with terrane development during the caledonian orogeny the magnetotelluric data located two upper crustal conductors associated with the fair head clew bay and virginia magnetic lineaments one of the conductors probably extends along strike to scotland and its top surface may be associated with the t reflector seen on the north east coast nec deep seismic reflection profile the other conductor has a southeastern edge directly beneath a major throw in seismic basement and defines the northern boundary of the midland valley terrane in ireland the conductors and magnetic lineaments are probably due to black shales in sedimentary basins which have been thrust under the avalonian and laurentian continents during the closure of the iapetus ocean the magnetotelluric data also suggest that the southern uplands magnetic lineament represents a crustal break but the navan tipperary magnetic lineament is probably the consequence of a shallow volcanic block sedimentary trough structure

impact origin of the vesta family the compelling petrographic link consolmagno and drake 1977 gaffey 1983 between basaltic achondrite meteorites and the 530 km diameter asteroid 4 vesta has been tempered by a perceived difficulty in launching rocks from this asteroid s surface at speeds sufficient to bring them to earth wasson and wetherill 1979 without obliterating vesta s signature crust i address this impasse in response to recent imaging zellner et al 1996 dumas and hainaut 1996 of a 450 km impact basin across vesta s southern hemisphere thomas et al 1997 and model the basin forming collision using a detailed two dimensional hydrocode with brittle fracture including self gravitational compression cf asphaug and melosh 1993 a 42 km diameter asteroid striking vesta s basaltic crust atop a denser mantle and iron core at 5 4 km s launches multikilometer fragments up to 600 m s without inverting distal stratigraphy according to the code this modeling together with collisional dynamical rheological and exposure age timescales marzari et al 1996 welten et al 1996 and observations of v type asteroids binzel and xu 1993 suggests a recent 1 ga impact origin for the vesta family and a possible vesta origin for earth approaching v type asteroids cruik shank et al 1991

the geology of 433 eros the global high resolution imaging of asteroid 433 eros by the near earth asteroid rendezvous near shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small s type asteroid eros displays a variety of surface features and evidence of a substantial regolith large scale facets grooves and ridges indicate the presence of at least one global planar structure directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object as with other small objects impact craters dominate the overall shape as well as the small scale topography of eros depth diameter ratios of craters on eros average 0 13 but the freshest craters approach lunar values of 0 2 ejecta block production from craters is highly variable the majority of large blocks appear to have originated from one 7 6 km crater shoemaker the interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of phobos this lack of mechanical boundaries and the abundant evidence of regolith in nearly every high resolution image suggests a gradation in the porosity and fracturing with depth the density of small craters is deficient at sizes below 200 m relative to predicted slopes of empirical saturation this characteristic which is also found on parts of phobos and lunar highland areas probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith eros displays a variety of regolith features such as debris aprons fine grained ponded deposits talus cones and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith these processes serve to mix materials in the upper loose fragmental portion of the asteroid regolith in the instance of ponded materials and crater wall deposits there is evidence of processes that segregate finer materials into discrete deposits the near observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the near shoemaker spacecraft

origin and history of ureilitic material in the solar system the view from asteroid 2008 tc3 and the almahata sitta meteorite asteroid 2008 tc3 approximately 4 m diameter was tracked and studied in space for approximately 19 h before it impacted earth s atmosphere shattering at 44 36 km altitude the recovered samples 680 individual rocks comprise the meteorite almahata sitta ahs approximately 50 70 of these are ureilites ultramafic achondrites the rest are chondrites mainly enstatite ordinary and rumuruti types the goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid almahata sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts however we calculate that the mass of fallen material was 0 1 of the pre atmospheric mass of the asteroid based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples we infer that the lost material was mostly ureilitic therefore 2008 tc3 probably contained only a few percent nonureilitic materials similar to other polymict ureilites except less well consolidated from available data for the ahs meteorite fragments we conclude that 2008 tc3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites we therefore suggest that the immediate parent of 2008 tc3 was the immediate parent of all ureilitic material sampled on earth we trace critical stages in the evolution of that material through solar system history based on various types of new modeling and re evaluation of published data we propose the following scenario 1 the ureilite parent body upb accreted 0 5 0 6 ma after formation of calcium aluminum rich inclusions cai beyond the ice line outer asteroid belt differentiation began approximately 1 ma after cai 2 the upb was catastrophically disrupted by a major impact approximately 5 ma after cai with selective subsets of the fragments reassembling into daughter bodies 3 either the upb before breakup or one of its daughters after breakup migrated to the inner belt due to scattering by massive embryos 4 one daughter after forming in or migrating to the inner belt became the parent of 2008 tc3 it developed a regolith mostly 3 8 ga ago clasts of enstatite ordinary and rumuruti type chondrites were implanted by low velocity collisions 5 recently the daughter was disrupted fragments were injected or drifted into earth crossing orbits 2008 tc3 comes from outer layers of regolith other polymict ureilites from deeper regolith and main group ureilites from the interior of this body in contrast to other models that have been proposed this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 tc3 and other polymict ureilites

the distribution of short lived radioisotopes in the early solar system and the chronology of asteroid accretion differentiation and secondary mineralization abstract we evaluate initial 26 al 27 al i 53 mn 55 mn i and 182 hf 180 hf i ratios together with 207 pb 206 pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common closed isotopic system i e the early solar nebula the relative initial isotopic abundances of 26 al 53 mn and 182 hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values here denoted by i al ss i mn ss and i hf ss respectively i mn ss and i hf ss 9 1 1 7 10 6 and 1 07 0 08 10 4 respectively correspond to canonical i al ss 5 1 10 5 i hf ss so determined is consistent with i hf ss 9 72 0 44 10 5 directly determined from an internal hf w isochron for cai minerals i mn ss is within error of the lowest value directly measured for cais we suggest that erratically higher values measured for cais in carbonaceous chondrites may reflect proton irradiation of unaccreted cais by the early sun after other asteroids destined for melting by 26 al decay had already accreted the 53 mn incorporated within such asteroids would have been shielded from further local spallogenic contributions from within the solar system the relative initial isotopic abundances of the short lived nuclides are less consistent with the 207 pb 206 pb ages of the corresponding materials than with one another the best consistency of short and long lived chronometers is obtained for 182 hf 180 hf i and the 207 pb 206 pb ages of angrites 182 hf 180 hf i decreases with decreasing 207 pb 206 pb ages at the rate expected from the 8 90 0 09 ma half life of 182 hf the model solar system age thus determined is t ss hf w 4568 3 0 7 ma 26 al 27 al i and 53 mn 55 mn i are less consistent with 207 pb 206 pb ages of the corresponding meteorites but yield t ss mn cr 4568 2 0 5 ma relative to i al ss 5 1 10 5 and a 207 pb 206 pb age of 4558 55 0 15 ma for the lew86010 angrite the mn cr method with i mn ss 9 1 1 7 10 6 is useful for dating accretion if identified with chondrule formation primary igneous events and secondary mineralization on asteroid parent bodies all of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies for i mn ss 9 1 1 7 10 6 parent body differentiation is found to extend at least to 5 ma post t ss i e until differentiation of the angrite parent body 4563 5 ma ago or 4564 5 ma ago using the directly measured 207 pb 206 pb ages of the d orbigny clan angrites the 1 ma difference is characteristic of a remaining inconsistency for the d orbigny clan between the al mg and mn cr chronometers on one hand and the 207 pb 206 pb chronometer on the other differentiation of the iiiab iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body and at nearly the same time as one another as shown by the mn cr ages of iiiab irons and ureilites respectively the latest recorded episodes of secondary mineralization are for carbonates on the ci carbonaceous chondrite parent body and fayalites on the cv carbonaceous chondrite parent body both extending to 10 ma post t ss

resolution of the age structure of the detrital zircon populations of two lower cretaceous sandstones from the weald of england by fission track dating modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography a computer program has been written and used to identify modes in the zircon fission track age distribution within two lower cretaceous sandstone samples from the weald of southern england pronounced modes appear in one rock around 119 ma 160 ma 243 ma and 309 ma and in the other around 141 ma 175 ma 257 to 277 ma and 394 to 453 ma the geological implications of these quite dissimilar zircon age spectra are discussed it is concluded that they support the palaeogeographical models of allen 1981 and indicate that the provenance of the first sample from the top ashdown sandstone member at dallington in east sussex was almost entirely southerly while that of the second from the netherside sand member at northchapel in west sussex was more varied but predominantly westerly and northerly

comment on parent body depth pressure temperature relationships and the style of the ureilite anatexis by p h warren maps 47 209 227 ureilites are carbon rich ultramafic olivine dominantly low ca pyroxene achondrites with poorly understood petrogenesis one major problem concerns the origin of extensive variation in feo content olivine core fo values ranging from approximately 75 to 95 among the individual ureilites the two main competing hypotheses to explain this variation are 1 equilibrium smelting in which ureilite fo values were established by pressure dependent depth linked carbon redox reactions on the ureilite parent body during partial melting or 2 nebular inheritance in which the variation in feo contents was derived from ureilite precursors and was preserved during partial melting the paper parent body depth pressure temperature relationships and the style of the ureilite anatexis by warren 2012 discusses a series of topics related to ureilite petrogenesis in each case an argument is presented within the context of smelting versus nonsmelting models collectively these arguments create the impression that there are many valid arguments against smelting the purpose of this comment is to point out flaws in some of these arguments and or to show that the issues they address are independent of smelting versus nonsmelting models both equilibrium smelting and nebular inheritance simple anatexis models face challenges in explaining all the properties of ureilites but both remain viable

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

vesta and extensively melted asteroids why hed meteorites are probably not from vesta abstract most researchers hold that the hed clan of differentiated meteorites originated on vesta largely based on the assumption that nearly all v type asteroids with basaltic reflection spectra are fragments spalled off vesta although it is a reasonable working hypothesis that most of the v type asteroids in the vesta family originated on vesta the spectra are not unique enough to confirm this a sizable fraction may have been produced during the destruction of a differentiated asteroid in the same large region of dynamic space observations of asteroids in the inner asteroid belt show that more than half of the v type asteroids do not belong to the vesta dynamic family iron meteorite evidence shows that at least 26 asteroids experienced extensive melting and would have generated basalts and other differentiated stony meteorites most iron meteorites show high degrees of elemental fractionations that lead to the conclusion that they experienced fractional crystallization it is probable that all these bodies generated basalts there are 9 of these magmatic iron meteorite groups and test criteria mainly based on extreme fractionations indicate that an additional 17 disrupted asteroids hosted fractionally crystallized cores and thus that 26 asteroids experienced extensive melting this estimate is much lower than previous estimates that included nonmagmatic irons within expected planetary heterogeneities the o isotopic composition of heds is the same as that in oxides from iiiab irons the largest magmatic group of iron meteorites e cr 54 values are also very similar in iiiabs and heds the o and cr isotopic ties are much stronger than the spectral tie thus the working hypothesis should be that heds are from the iiiab parent asteroid remote elemental analysis could confirm that heds are not from vesta if future remote analysis measures k contents 0 6 mg g this will indicate that heds did not originate on vesta

experimental study on hydrothermal annealing of fission tracks in zircon abstract increasing temperature causes fading of fission tracks fts in minerals which has been quantified by many laboratory heating experiments in atmosphere in order to elucidate whether or not ft annealing at hydrothermally pressured conditions is the same as that at atmospheric condition we performed zircon heating experiments using a hydrothermal synthetic apparatus for reliable comparison we annealed the same zircon samples using the same temperature monitor and analyzed them using the same experimental procedure as the previous experiment at atmosphere yamada et al chem geol isot geosci sect 122 1995 249 the observed ft annealing characteristics are indistinguishable between atmospheric and hydrothermally heating it is therefore implied that the annealing kinetics of ft in zircon previously established can be applied to thermal history analysis of rocks that were subjected to hydrothermal conditions in nature such as those in fault and subduction zones

apatite fission track chronometry using laser ablation icp ms abstract comparison is made of apatite fission track ft ages measured by using conventional thermal neutron irradiation and laser ablation inductively coupled plasma mass spectrometry la icp ms to determine sample uranium content spontaneous track densities were determined on specified apatite grains from the durango fish canyon tuff and mount dromedary complex age standards neutron induced track densities and icp ms uranium values were determined for the same specified grains ages determined using the two approaches show reasonable consistency indicating the possibility of routine fission track analysis using la icp ms uncertainties and assumptions in the la icp ms approach include the laser ablation of chemically etched mineral surfaces the suitability of reference materials for calibration and the derivation of ft ages using the alternatives of selection of a f value or a revised zeta comparative approach the use of la icp ms in ft chronometry would dramatically increase the speed of analysis and hence sample throughput as well as avoiding the need for neutron irradiation and the production transport and handling of radioactive samples with the acknowledged dependency of ft annealing in apatite on composition such grain by grain multi element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal

impact history of eros craters and boulders abstract preliminary measurements of craters and boulders have been made in various locations on eros from images acquired during the first nine months of near shoemaker s orbital mission including the october 2000 low altitude flyover we offer some very preliminary qualitative analysis of later laf images and very high resolution images obtained during near s landing on 12 february 2001 craters on eros 100 m diameter closely resemble the saturated crater population of ida eros is more heavily cratered than gaspra but lacks the saturated giant craters of mathilde these craters and the other large scale geological features were formed over a duration of very roughly 2 gyr while eros was in the main asteroid belt between the time when its parent body was disrupted and eros was injected into an earth approaching orbit probably tens of myr ago saturation equilibrium had been expected to shape eros crater population down to very small sizes as on the lunar maria however craters

a thermal model for the differentiation of asteroid 4 vesta based on radiogenic heating a finite element code has been developed to model the thermal history of asteroid 4 vesta this is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained the results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from vesta the work solves two major problems with the hypothesis of heating by decay of26al an extinct radionuclide postulated to be a plausible heat source in the early solar system first the model demonstrates that it is possible to keep the mantle of vesta hot for 100 ma thereby explaining the observed difference in ages between cumulate and noncumulate eucrites second the simulation offers a possible explanation of why detectable excesses of26mg the decay product of26al are not observed in noncumulate eucrites the simulation draws a model chronology of vesta and predicts times relative to cai formation for accretion at 2 85 myr core formation at 4 58 myr crust formation at 6 58 myr and geochemical closure at 100 myr for a h chondrite asteroidal bulk composition decay of60fe is found to cause no perceptible difference in the thermal history of vesta even when sequestered into a central core although chondritic xenoliths have not been described in hed igneous lithologies the thermal model suggests the possibility that a veneer of unmelted near surface material should remain

evidence for a dynamo in the main group pallasite parent body understanding the origin of pallasites stony iron meteorites made mainly of olivine crystals and feni metal has been a vexing problem since their discovery here we show that pallasite olivines host minute magnetic inclusions that have favorable magnetic recording properties our paleointensity measurements indicate strong paleomagnetic fields suggesting dynamo action in the pallasite parent body we use these data and thermal modeling to suggest that some pallasites formed when liquid feni from the core of an impactor was injected as dikes into the shallow mantle of a 200 kilometer radius protoplanet the protoplanet remained intact for at least several tens of millions of years after the olivine metal mixing event

efficient early global relaxation of asteroid vesta abstract the asteroid vesta is a differentiated planetesimal from the accretion phase of solar system formation although its present day shape is dominated by a non hydrostatic fossil equatorial bulge and two large mostly unrelaxed impact basins vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating we use a finite element viscoplastic flow model coupled to a 1d conductive cooling model to calculate the expected rate of relaxation throughout vesta s early history we find that given sufficient non hydrostaticity the early elastic lithosphere of vesta experienced extensive brittle failure due to self gravity thereby allowing relaxation to a more hydrostatic figure soon after its accretion vesta reached a closely hydrostatic figure with 40 200 my after formation depending on the assumed depth of megaregolith the veneneia and rheasilvia giant impacts which generated most non hydrostatic topography must have therefore occurred 40 200 my after formation based on crater retention ages topography and relation to known impact generated features we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early vesta the long wavelength figure of this terrain suggests that before the two late giant impacts vesta had a rotation period of 5 02 h 6 3 faster than present while its spin axis was offset by 3 0 from that of the present the evolution of vesta s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non hydrostatic terrains and epochs

tectonostratigraphy of the glen app area southern uplands scotland anatomy of an ordovician accretionary complex the stratigraphy and structure of the glen app coastal area northwestern southern uplands are characterized by structural repetition of both chaotic and coherent units of the middle ordovician caradocian tappins group the chaotic unit the currarie formation comprises blocks including basalt chert and terrigenous sedimentary rocks set in folded and in places sheared red to grey siliceous mudstone the coherent unit is turbiditic and is divided into the dalreoch formation in the north and the corsewall formation in the south the former is characterized by inclined isoclinal folds and thrusts whereas the latter shows a homoclinal dip although both are largely overturned northwards small scale duplexes are observed within the coherent turbidites of the dalreoch formation and chaotic units larger duplexes identified on the basis of mapping are oriented at an oblique angle to the general trend of regional structures the chemistry of basalt blocks within the currarie formation shows a within plate type suggesting an origin as seamounts the tectono stratigraphy of the area is interpreted as the result of accretion of a substantial seamount at a subduction zone during mid ordovician time thrust interleaving of chaotic and coherent units is inferred to have taken place in and above the decollement zone while thick turbidite beds without strong folding were probably preserved below the decollement and emplaced below a large scale nappe interpretation of the glen app area as part of an accretionary complex confirms that an ocean basin remained open in part of the iapetus system at least until caradocian times

impacts on asteroids fragmentation regolith transport and disruption abstract we use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of asteroids 951 gaspra and 243 ida which were imaged by the galileo spacecraft a shock wave fractures the asteroid in advance of crater excavation flow thus for impactors larger than 100 m impacting at 5 3 km s 1 tensile strength is unimportant in these bodies whether they are initially intact or are rubble piles because of the shock induced fracture impact results are controlled by gravity therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates which had assumed that strength was controlling these processes for rock targets the rubble and regolith produced by this fracture can be jolted by the impact redistributing surface material and globally erasing craters the crater ejecta can produce tens of meters of regolith per large event likely consisting of 100 m size boulders mixed with smaller particles the response of kilometer size asteroids to impacts is qualitatively different from that of few centimeter targets in terrestrial experiments making prediction based on such experiments difficult the compositional distribution of delivered meteorites depends on the outcomes of such asteroid impacts

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

zircon fission track thermochronology and applications to fault studies zircon is one of the most useful minerals used to unravel the earth s history recorded in rocks after the success of etching fission tracks in zircon fleischer et al 1964 naeser 1969 krishnaswami et al 1974 early studies focused primarily on dating young volcanic horizons for stratigraphic purposes e g naeser et al 1973 hurford et al 1976 seward 1979 gleadow 1980 kowallis et al 1986 kohn et al 1992 subsequently fission track analysis of zircon has been extensively employed along with other radiometric dating methods such as u pb and u th he techniques e g davis et al 2003 reiners 2005 to understand the thermochronology of rocks in a variety of geological settings i e thermal history analysis of basement rocks for orogenic studies e g zeilter et al 1982 hurford 1986 kamp et al 1989 sorkhabi 1993 seward and mancktelow 1994 hasebe and tagami 2001 spikings et al 2001 thermochronology using detrital zircon grains in sedimentary rocks for provenance analysis and thermal history analysis in or near faults which will be highlighted below in this present chapter the thermal sensitivity of the zircon fission track thermochronometry is reviewed in the context of laboratory and field studies then laboratory procedures of the zircon fission track analysis using the external detector method are outlined as practiced in the fission track laboratory of kyoto university for basic fundamentals of these fission track analyses including data analysis and graphical displays see the chapter by tagami and o sullivan 2005 finally as an example of recent geological applications of the technique the results of zircon fission track analysis of the nojima fault zone japan are presented these rocks were the target of systematic drilling of an active fault system using five boreholes and a trench applications to other geological settings and the