paleotemperatures at the lunar surfaces from open system behavior of cosmogenic 38ar and radiogenic 40ar abstract the simultaneous diffusion of both cosmogenic 38 ar and radiogenic 40 ar from solid phases is controlled by the thermal conditions of rocks while residing near planetary surfaces combined observations of 38 ar 37 ar and 40 ar 39 ar ratios during stepwise degassing analyses of neutron irradiated apollo samples can distinguish between diffusive loss of ar due to solar heating of the rocks and that associated with elevated temperatures during or following impact events the data provide quantitative constraints on the durations and temperatures of each process from sequentially degassed 38 ar 37 ar ratios can be calculated a spectrum of apparent 38 ar exposure ages versus the cumulative release fraction of 37 ar which is particularly sensitive to conditions at the lunar surface typically over 10 6 10 8 year timescales due to variable proportions of k and ca bearing glass plagioclase and pyroxene with variability in the grain sizes of these phases each sample will have distinct sensitivity to and therefore different resolving power on past near surface thermal conditions we present the underlying assumptions and the analytical and numerical methods used to quantify the ar diffusion kinetics in multi phase whole rock analyses that provide these constraints for apollo 15 samples 15016 15556 and 15596 we find apparent 40 ar 39 ar plateau ages between 3 21 and 3 28 ga and evidence for diffusive loss of radiogenic 40 ar primarily from k bearing glass from 38 ar 37 ar spectra normalized to the apparent ca k ratios we also find evidence of diffusive loss of cosmogenic 38 ar that requires elevated temperatures either during or after surface exposure using 39 ar and 37 ar we construct multiple phase multiple diffusion domain mp mdd models to quantify the diffusion kinetics of ar from a range of macroscopic grain sizes of each phase while diffusive loss of 40 ar can be explained by brief reheating conditions after crystallization e g during an impact event we find that both the radiogenic 40 ar and cosmogenic 38 ar abundances can be explained by internally consistent thermal conditions expected for solar heating of the rocks at the lunar surface these conditions correspond to effective diffusion temperatures edt i e the temperature corresponding to the mean diffusivity over temperature variability between 65 and 81 c with an error weighted mean of 77 0 1 3 c despite differences in diffusion kinetics and large differences in surface exposure duration between the three samples 56 621 ma this edt corresponds to a maximum daytime temperature of 96 c which is in excellent agreement direct measurements of temperature at the apollo 15 landing site the open system behavior in both a radiogenic and a cosmogenic nuclide provides tests for internal consistency in best fitting solutions of time integrated thermal conditions of rocks collected from planetary surfaces this thermal information is important for the study of habitable conditions at planetary surfaces and has implications for ar based geochronology and paleomagnetic observations applied to planetary materials

role of fluid mixing and wallrock sulfidation in gold mineralization at the semna mine area central eastern desert of egypt evidence from hydrothermal alteration fluid inclusions and stable isotope data abstract the semna gold deposit is one of several vein type gold occurrences in the central eastern desert of egypt where gold bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks the semna gold deposit is related to a series of sub parallel quartz veins along steeply dipping wnw trending shear zones which cut through tectonized metagabbro and granodiorite rocks the orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre existing fault segments textural structural and mineralogical evidence including open space structures quartz stockwork and alteration assemblages constrain on vein development during an active fault system the ore mineral assemblage includes pyrite chalcopyrite subordinate arsenopyrite galena sphalerite and gold hydrothermal chlorite carbonate pyrite chalcopyrite and kaolinite are dominant in the altered metaggabro whereas quartz sericite pyrite kaolinite and alunite characterize the granodiorite rocks in the alteration zones mixtures of alunite vuggy silica and disseminated sulfides occupy the interstitial open spaces common at fracture intersections partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins petrographic and microthermometric studies aided recognition of co 2 rich h 2 o rich and mixed h 2 o co 2 fluid inclusions in the gold bearing quartz veins the h 2 o co 2 inclusions are dominant over the other two types and are characterized by variable vapor liquid ratios these inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids the generally light 34 s of pyrite and chalcopyrite may suggest a magmatic source of sulfur spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226 267 c and 350 1100 bar under conditions transitional between mesothermal and epithermal systems the semna gold deposit can be attributed to interplay of protracted volcanic activity dokhan volcanics fluid mixing wallrock sulfidation and a structural setting favoring gold deposition gold was transported as au bisulfide complexes under weak acid conditions concomitant with quartz sericite pyrite alteration and precipitated through a decrease in gold solubility due to fluid cooling mixing with meteoric waters and variations in ph and f o 2

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

argon diffusion in pyroxenes implications for thermochronometry and mantle degassing abstract the rate at which argon diffuses in pyroxenes is relevant to several problems in the earth and planetary sciences pyroxenes are among the most abundant minerals in the earth s upper mantle and are important hosts for potassium and radiogenic 40 ar 40 ar additionally pyroxenes commonly contain 25 of the total 40 ar within meteorites and therefore hold great potential for quantifying the timing duration and thermal conditions of events experienced by meteorites and their parent bodies we conducted detailed feedback controlled laser heating diffusion experiments using synthetically produced 37 ar in gem quality clinopyroxene cpx and orthopyroxene opx crystals stepwise heating between 850 and 1350 c yielded linear arrhenius arrays from which activation energies e a of 379 2 4 1 and 371 0 6 0 kj mol and pre exponential factors d o of 1 36 10 4 6 15 10 5 1 12 10 4 and 5 73 10 2 2 25 10 2 3 72 10 2 were determined for cpx and opx respectively these e a s are an order of magnitude higher than previously reported values obtained with a different experimental approach watson et al 2007 and indicate that ar diffusion within pyroxenes is strongly temperature dependent at high temperatures ar diffusion in pyroxenes is rapid which indicates that diffusive 40 ar distributions observed within meteoritic pyroxenes may provide information about brief high temperature shock heating events in contrast subsequent low t conditions should minimally influence these concentration gradients the experimental ar diffusion kinetics in opx and cpx correspond to closure temperatures of 600 700 and 700 800 c respectively for 10 c ma cooling these results suggest that diffusive equilibration through 0 1 0 5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures 1300 c and is therefore unlikely to inhibit mantle degassing

argon 40 argon 39 age spectra of apollo 17 highlands breccia samples by laser step heating and the age of the serenitatis basin we have obtained high resolution 21 63 steps 40ar 39ar age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from apollo 17 samples collected from the pre imbrian highlands in the easternmost part of the serenitatis basin the samples include poikilitic melt rocks inferred to have been formed in the serenitatis basin forming impact aphanitic melt rock whose compositions vary and whose provenance is uncertain and granulite gabbro and melt clasts three of the poikilitic melts have similar age spectrum plateau ages 72395 96 3893 16 ma 2 72535 7 3887 16 ma 76315 150 3900 16 ma with a weighted mean age of 3893 9 ma which we interpret as the best age for the serenitatis basin forming impact published 40ar 39ar age spectrum ages of apollo 17 poikilitic melts are consistent with our new age but are much less precise two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of 3869 ma 72558 7 and 3743 ma 77135 178 plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 16 ma to 3937 16 ma and have probably been affected by the ubiquitous older clasts and by post formation heating impact events plateau ages from two of the aphanitic melt blobs and two granulites in sample 72255 fall in the narrow range of 3850 16 ma to 3869 16 ma with a weighted mean of 3862 8 ma two of the aphanitic melt blobs from 72255 have ages of 3883 16 ma and 3894 ma whereas a poikilitic melt clast of different composition from the serenitatis melts has an age of 3835 16 ma which is the upper limit for the accretion of 72255 these data suggest that either the aphanitic melts vary in age as is also suggested by their varying chemical compositions or they formed in the 72255 accretionary event about 3 84 3 85 ga and older relict material is responsible for the dispersion of ages in any case the aphanitic melts do not appear to be serenitatis products our age for the serenitatis impact shows on the basis of the isotopic age evidence alone that serenitatis is 20 25 ma and probably 55 60 ma older than imbrium 3870 ma and probably 3836 ma dalrymple and ryder 1993 noritic granulite sample 78527 has a plateau age of 4146 17 ma representing a minimum age for cooling of this sample in the early lunar crust so far there is no convincing evidence in the lunar melt rock record for basin forming impacts significantly older than 3 9 ga

a record of impacts preserved in the lunar regolith abstract the absolute chronology of meteoroid impacts on the moon is largely quantified by only a few 40 ar 39 ar plateau ages of rocks thought to be associated with specific impact events stoffler et al 2006 we demonstrate a more broadly applicable approach by using high resolution 40 ar 39 ar thermochronometry to investigate the physical conditions responsible for partially reset k ar systems in lunar rocks seven rocks from apollo 16 regolith sample 63503 have plateau ages of either 3 9 billion yr ga or 4 2 ga and all experienced varying degrees of partial resetting concordance between diffusion kinetics and the degree of resetting among all samples shows that these observations are best explained by a heating event 3 3 ga ago that lasted between 10 3 s at 600 c and 20 yr at 300 c we conclude that partial resetting of the k ar systems in these samples record an impact event 3 3 ga ago that mixed several preexisting ejecta units in the cayley plains if partially reset 40 ar 39 ar ages of other lunar highland samples also constrain the timing of late stage reheating associated with impact events they constitute an additional record of impacts preserved in the lunar regolith a review of existing datasets from this perspective reveals that episodic pulses in the impactor flux in the inner system are common and most likely related to dynamical events in the asteroid belt or outer solar system

solar and cosmogenic argon in dated lunar impact spherules abstract we have studied lunar impact spherules from the apollo 12 and apollo 14 landing sites examining the isotopic composition of argon released by stepwise heating elsewhere we reported the formation ages of these spherules determined by the 40ar 39ar isochron method here we discuss solar and cosmogenic argon from the same spherules separating these two components by correlating their partial releases with the releases of calcium derived 37ar on a cosmochron diagram we use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial the solar argon is seen to be separated into isotopically lighter and heavier fractions which presumably were implanted to different depths in the spherules the abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux such as the suprathermal tail of the solar wind the fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time over the history of the solar system however the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age we believe that most of the spherules are saturated with solar argon having reached a condition in which implantation by the solar wind is offset by losses from solar wind sputtering and diffusion

quantifying the diffusion kinetics and spatial distributions of radiogenic 4he in minerals containing proton induced 3he apatite titanite and olivine samples were bombarded with a 150 mev proton beam to produce 10 8 atoms mg of spallation 3he high precision stepped heating experiments were then performed in which the artificial 3he and for apatite and titanite the natural radiogenic 4he were measured to characterize the diffusive behavior of each isotope helium 3 diffusion coefficients are in excellent agreement with concurrently and or previously determined he diffusion coefficients for each mineral our results indicate that proton induced 3he is uniformly distributed and that radiation damage associated with a proton fluence of 5 x 10 14 protons cm 2 does not cause noticeable changes in 4he diffusion behavior in at least apatite and titanite proton induced 3he can therefore be used to establish he diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural 4he distribution is inhomogeneous in addition step heating 4he 3he analysis of a mineral with a uniform synthetic 3he concentration provides a means by which a natural 4he distribution can be determined

argon diffusion in plagioclase and implications for thermochronometry a case study from the bushveld complex south africa plagioclase is not only the most abundant mineral in the earth s crust but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material applying the k ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings nonetheless plagioclase has rarely been used for thermochronometry largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low k concentration here we describe an analytical technique that generates high precision 40 ar 39 ar data and quantifies ar diffusion kinetics of low k minerals we present results of five diffusion experiments conducted on single crystals of plagioclase from the bushveld complex south africa the observed diffusion kinetics yield internally consistent thermochronological constraints indicating that plagioclase is a reliable thermochronometer individual grains have activation energies of 155 178 kj mol and ln d0 a 2 varies between 3 5 and 6 5 these diffusion parameters correspond to closure temperatures of 225 300 c for a 10 c ma cooling rate age spectra generally conform to single domain diffusive loss profiles suggesting that grain scale diffusion dominates argon transport in this fairly simple plagioclase conjointly examining several single grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the bushveld complex cooled rapidly and monotonically from magmatic temperature to 300 c over 3 ma followed by protracted cooling to ambient crustal temperatures of 150 200 c over 600 ma 2009 elsevier ltd all rights reserved

systematic variations of argon diffusion in feldspars and implications for thermochronometry abstract coupled information about the time dependent production and temperature dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of earth and planetary processes to better assess the accuracy of thermal models an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable here we present step heating ar diffusion experiments conducted on feldspars with diverse compositions structural states and microstructural characteristics the experiments reveal systematic variations in diffusive behavior that appear closely related to these variables with apparent closure temperatures for 0 1 1 mm grains of 200 400 c assuming a 10 c ma cooling rate given such variability there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling sample specific data are required diffusion experiments conducted on oriented cleavage flakes do not reveal directionally dependent diffusive anisotropy to within the resolution limits of our approach approximately a factor of 2 additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain scale whereas feldspars containing hydrothermal alteration and or incoherent sub grain intergrowths do not arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments these structural transitions along with sub grain domain size variations cause deviations from linearity i e upward and downward curvature on arrhenius plots an atomistic model for arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression the resulting implications for accurately extrapolating laboratory derived diffusion parameters to natural settings and over geologic time are discussed we find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain mdd models fit to arrhenius plots for exsolved alkali feldspar where the inferred ar partial retention zones may be spuriously hot

magnetic fidelity of lunar samples and implications for an ancient core dynamo abstract some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo however a longstanding problem has been interpreting the unstable alternating field af demagnetization behavior observed in the remaining majority of the apollo lunar sample suite similar unstable behavior has also been observed for many meteorites it is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties it is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample s parent body to address this issue we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors mare basalts 15556 15016 12017 10020 and troctolite 76535 we demonstrate that the effects of spurious anhysteretic remanent magnetization arm during af demagnetization in combination with multidomain magnetic carriers and magnetic remanence anisotropy are likely responsible for the apparently poor magnetic behavior of many lunar samples therefore the unstable af demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo nevertheless spurious arm may explain the observation of surprisingly high 1 t apollo era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

early lunar magnetism it is uncertain whether the moon ever formed a metallic core or generated a core dynamo the lunar crust and returned samples are magnetized but the source of this magnetization could be meteoroid impacts rather than a dynamo here we report magnetic measurements and 40ar 39ar thermochronological calculations for the oldest known unshocked lunar rock troctolite 76535 these data imply that there was a long lived field on the moon of at least 1 microtesla approximately 4 2 billion years ago the early age substantial intensity and long lifetime of this field support the hypothesis of an ancient lunar core dynamo

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

chemical composition and origin of apollo 15 impact melts to better understand the apennine front we have analyzed 14 apollo 15 impact melt rocks including hand samples rake samples and coarse fines for major fused bead microprobe and trace element inaa abundances the melt rocks show a remarkable diversity of chemical composition although all but one have typical kreep incompatible element patterns they appear to form five chemical groups all but the most kreep rich of which may represent single impacts three groups are of low k fra mauro composition none correspond to very high alumina vha compositions the compositions are unlike nearly all other impact melts and regoliths from highland sites indicating a distinct provenance for the apollo 15 samples serenitatis basin impact melt compositions apollo 17 poikilitic boulders are not duplicated although one apollo 17 aphanitic melt sample 73215 is similar to one of the apollo 15 groups the apollo 15 group consisting only of 15445 and 15455 remains the best candidate for imbrium basin impact melt while the most kreep rich group probably consists of melts produced in varied post imbrium impacts into the apennine bench formation apollo 15 kreep basalt flows the most populated group was previously unrecognized and could be either pre or post imbrium basin it is possible that it not 15445 15455 represents imbrium melt one unique sample has a non kreep incompatible element pattern with low abundances and a positive eu anomaly its target was rather primitive and would not appear to be of regional extent or present in the area in post imbrium times it is either a pre imbrian melt or exotic the apennine front does not consist of a monolithic lkfm composition but shows a much wider diversity of compositions than was previously recognized

decline of the lunar core dynamo abstract recent analyses of apollo samples have demonstrated that a core dynamo existed on the moon between at least 4 25 and 3 56 billion years ago ga with surface field intensities reaching 70 t however it is unknown when the moon s magnetic field declined determining the temporal evolution of the dynamo is important because it constrains secular changes in power at the lunar core mantle boundary and by implication the moon s thermal and orbital evolution and the field generation mechanism here we present paleomagnetic data from several younger mare basalts which demonstrate that the surface magnetic field had declined precipitously to 4 t by 3 19 ga it is currently unclear whether such a rapid decrease in field strength reflects either the cessation of the dynamo during this period or its persistence beyond 3 19 ga in a drastically weakened state

cosmogenic noble gas paleothermometry we present a theoretical basis for reconstructing paleotemperatures from the open system behavior of cosmogenic noble gases produced in minerals at earth s surface experimentally determined diffusion kinetics predicts diffusive loss of cosmogenic 3he and 21ne from common minerals like quartz and feldspars at ambient temperatures incomplete retention has also been observed empirically in field studies we show that the theory of simultaneous production and diffusion that applies to radiogenic noble gases in minerals the basis of thermochronology can also be applied to cosmogenic noble gases to reconstruct past surface temperatures on earth we use published diffusion kinetics and production rates for 3he in quartz and 21ne in orthoclase to demonstrate the resolving power of cosmogenic noble gas paleothermometry with respect to exposure duration temperature and diffusion domain size calculations indicate that when paired with a quantitatively retained cosmogenic nuclide such as 21ne or 10be observations of cosmogenic 3he in quartz can constrain temperatures during surface exposure in polar and high altitude environments likewise 21ne retention in feldspars is sensitive to temperatures at lower latitudes and elevations expanding the potential geographic applicability of this technique to most latitudes as an example we present paired measurements of 3he and 10be in quartz from a suite of antarctic sandstone erratics to test whether the abundances of cosmogenic 3he agree with what is predicted from first principles and laboratory determined diffusion kinetics we find that the amounts of cosmogenic 3he present in these samples are consistent with the known mean annual temperature mat for this region of antarctica between 25 and 30 c these results demonstrate the method s ability to record paleotemperatures through geologic time

lunar equatorial surface temperatures and regolith properties from the diviner lunar radiometer experiment 1 the diviner lunar radiometer experiment onboard the lunar reconnaissance orbiter has measured solar reflectance and mid infrared radiance globally over four diurnal cycles at unprecedented spatial and temporal resolution these data are used to infer the radiative and bulk thermophysical properties of the near surface regolith layer at all longitudes around the equator normal albedos are estimated from solar reflectance measurements normal spectral emissivities relative to the 8 m christiansen feature are computed from brightness temperatures and used along with albedos as inputs to a numerical thermal model model fits to daytime temperatures require that the albedo increase with solar incidence angle measured nighttime cooling is remarkably similar across longitude and major geologic units consistent with the scarcity of rock exposures and with the widespread presence of a near surface layer whose physical structure and thermal response are determined by pulverization through micrometeoroid impacts nighttime temperatures are best fit using a graded regolith model with a 40 increase in bulk density and an eightfold increase in thermal conductivity adjusted for temperature occurring within several centimeters of the surface

a long lived lunar core dynamo paleomagnetic measurements indicate that a core dynamo probably existed on the moon 4 2 billion years ago however the subsequent history of the lunar core dynamo is unknown here we report paleomagnetic petrologic and 40ar 39ar thermochronometry measurements on the 3 7 billion year old mare basalt sample 10020 this sample contains a high coercivity magnetization acquired in a stable field of at least 12 microteslas these data extend the known lifetime of the lunar dynamo by 500 million years such a long lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior the inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory

argon diffusion in apollo 16 impact glass spherules implications for 40ar 39ar dating of lunar impact events abstract the 40 ar 39 ar technique applied to impact glass has been used to date both terrestrial and lunar impact events the ability to utilize the 40 ar 39 ar technique rests on the assumption that impact glasses are closed to the loss of daughter product 40 ar after formation diffusion experiments were performed on three apollo 16 lunar impact glasses and yielded activation energies for 39 ar of 17 to 20 kcal mol 1 and log 10 d 0 a 2 values of 5 2 to 6 0 s 1 the resulting diffusion coefficients are interpreted as minimum values and the apollo 16 glass is probably some of the least retentive of lunar glasses as the degree of non bridging oxygen is at one end of the range in lunar glasses at temperatures below the glass transition temperature i e 660 c the data can be explained by volume diffusion from a single diffusion domain modeling shows that apollo 16 composition glass could lose significant quantities of radiogenic argon 40 ar 90 100 over 20 40 myr assuming a diffusion domain size a of 75 m due to diurnal temperature variations on the lunar surface although 40 ar loss is highly sensitive to exposure duration and effective diffusion domain size modeling shows that loss from transient thermal events e g heating to 200 c for 10 2 yr duration can also cause partial resetting of apparent 40 ar 39 ar ages in small a 75 m glasses a maximum of 50 60 of 40 ar is lost over 4 ga when buried to depths corresponding to temperatures of 15 c results indicate that caution should be exercised in interpreting lunar impact glass 40 ar 39 ar ages as the assumption of closed system behavior may have been violated particularly in glasses with low fractions of non bridging oxygen

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

persistence and origin of the lunar core dynamo the lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation here we report analyses of two 3 56 gy old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least 13 t these data extend the known lifetime of the lunar dynamo by 160 my and indicate that the field was likely continuously active until well after the final large basin forming impact this likely excludes impact driven changes in rotation rate as the source of the dynamo at this time in lunar history rather our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo

facies and cycle architecture of a pleistocene marine terrace crotone southern italy a sedimentary response to late quaternary high frequency glacio eustatic changes abstract cyclic late pleistocene shallow marine deposits such as those forming the sedimentary cover of some marine terraces allow a detailed study of the architecture of nearshore to offshore cycles deposited under the influence of high amplitude glacio eustatic changes the capo colonna marine terrace located near crotone southern italy is composed of two cycles recording a low frequency retrogradational trend which is thought to be the result of a generalized base level rise during a late pleistocene interglacial stage superimposed on regional uplift cycles of the capo colonna marine terrace are composed of coarse grained lags siliciclastic shoreface deposits and mostly carbonate shelf deposits and their general stratigraphic architecture evidences relatively thin transgressive and thick regressive intervals the latter can be top truncated due to wave ravinement during a subsequent erosional transgression the architectural style of the capo colonna marine terrace cycles is thought to be linked to very rapid base level rises related to the late pleistocene high amplitude glacio eustatic changes climatic variations controlling the sediment supply rate are considered less effective in the present context present results allow a comparison with other cyclic successions developed under icehouse climatic regimes showing a discrete variability in their stratal architecture

argon retention properties of silicate glasses and implications for 40 ar 39 ar age and noble gas diffusion studies sized aggregates of glasses 47 84 wt sio2 were fused from igneous derived cohesive fault rock and igneous rock and step heated from 400 to 1 200 c to obtain their 39ar diffusion properties average e 33 400 cal mol 1 d o 4 63 10 3 cm2 s 1 at t 1 mm are analyzed glasses with compositions similar to ours may preserve their formation ages