chemical modification of projectile residues and target material in a memin cratering experiment in the context of the memin project a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite campo del cielo as projectile accelerated to 4 56 km s 1 and a block of seeberger sandstone as target material the ejecta collected in a newly designed catcher are represented by 1 weakly deformed 2 highly deformed and 3 highly shocked material the latter shows shock metamorphic features such as planar deformation features pdf in quartz formation of diaplectic quartz glass partial melting of the sandstone and partially molten projectile mixed mechanically and chemically with target melt during mixing of projectile and target melts the fe of the projectile is preferentially partitioned into target melt to a greater degree than ni and co yielding a fe ni that is generally higher than fe ni in the projectile this fractionation results from the differing siderophile properties specifically from differences in reactivity of fe ni and co with oxygen during projectile target interaction projectile matter was also detected in shocked quartz grains the average fe ni of quartz with pdf about 20 and of silica glasses about 24 are in contrast to the average sandstone ratio about 422 but resembles the fe ni ratio of the projectile about 14 we briefly discuss possible reasons of projectile melting and vaporization in the experiment in which the calculated maximum shock pressure does not exceed 55 gpa

three dimensional crustal structure of eastern north america extracted from ambient noise 1 group velocity dispersion curves of surface waves extracted from ambient seismic noise are inverted to find 3 d shear wave structure of the crust beneath eastern north america the 3 d model consists of one sediment layer and another six layers with fixed depths at 5 7 5 10 15 25 and 43 km velocities of the seven layers together with the thickness of the sediment layer are determined by the simulated annealing method we found that almost all failed ancient rifting events e g the reelfoot rift ouachita triple junction and the midcontinent rift and rifting related events e g the ozark uplift and the nashville dome are associated with high velocity bodies in the middle and lower crust our results also suggest the existence of a triple junction like high velocity body centered around the new madrid and the wabash valley seismic zones with the reelfoot rift the ozark uplift and the nashville dome being on its southwestern northwestern and southeastern arms respectively we also found that the western limb of the midcontinent rift mcr extends southwestward to western oklahoma and texas and the eastern limb of the mcr extends southeastward into western ohio the appalachian mountains are characterized by high velocity upper crust underlain with relatively low velocity middle and lower crust all major seismic zones are associated with either divergent or convergent events the new madrid seismic zone and wabash valley seismic zone are clearly associated with the failed reelfoot rift both the eastern tennessee seismic zone and ouachita orogen seismic zone are located along convergent boundaries

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

the role of regolith redistribution in influencing the evolution of the shapes of asteroids major surface fissures and relatively large scale angular surface irregularities are expected to have been present on many asteroids at early stages in their histories as a byproduct of at least two processes impact disruption and reassembly into rubble piles for all classes of asteroid and for carbonaceous chondrite parent bodies aqueous alteration which led to the low bulk densities currently being observed for asteroids however in all cases where high enough resolution images exist such abrupt deep irregularities are not observed we model the spatial redistribution of impact generated regolith on an asteroid with an idealized irregular shape to show how the complex gravitational field of such a body will lead to the systematic infilling of deep valleys in the surface our analysis emphasizes the high efficiency with which regolith redistribution can act to disguise the internal structures of asteroids with sizes in the 20 100 km range

the lunar regolith chemistry mineralogy and petrology the lunar regolith a several meter thick layer of unconsolidated debris forms the interface between the moon and its space environment the regolith forms from lithic sources by the destructional processes of comminution and the constructional processes of agglutinate formation in this manner a steady state soil can develop and will remain in a type of dynamic equilibrium until the system is disturbed by burial or mixing with new soil mixing takes place mainly on a local scale vertically through meters and horizontally over several kilometers lateral transport is a relatively inefficient process and therefore most of the lunar soil components were derived locally there is no evidence for preferential transport of the finer soil fractions relative to the coarser fractions the truly exotic derived at distances of tens of kilometers component is generally in low abundance 1 therefore remotely sensed data should provide a reasonably accurate picture of bedrock geology the chemical composition of the regolith varies with grain size the 10 m fraction is chemically fractionated from the bulk soil in most cases the 10 m fraction is enriched in al2o3 cao na2o k2o light rare earth elements and th and depleted in mgo feo mno and sc these systematics are largely a result of simple comminution with feldspar and friable mesostasis from local source rocks concentrating in the finest fraction there is evidence that agglutinate glass forms preferentially by fusion of the finest soil fractions therefore agglutinate glass like the 10 m soil fraction appears to be fractionated from bulk soil compositions highland soils are distinct from mare soils both chemically and petrologically soil samples collected near mare highland contacts are mixtures of the two lithologies mineral chemical systematics of monomineralic fragments in the lunar soil provide important clues to soil provenance they can in some cases sort out the highland mare contributions and in special cases can even further pinpoint a lithic source in theory lunar regolith cores can serve as solar probes recording the sun s activity over billions of years in practice this has generally not been the case the formation and evolution of most cores is still poorly understood for most we have not established how they formed for example how many depositional and erosional events were involved what the time span involved in the accretion of the sampled soil column was and how long that soil column has remained in situ without this information it is difficult if not impossible to read the sun s record more interdisciplinary studies on cores must be carried out before they can be used as solar probes

penecontemporaneous metamorphism fragmentation and reassembly of ordinary chondrite parent bodies the occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three h l ll parent asteroids of 100 km radius e g miyamoto et al 1982 this conventional onion shell model predicts an inverse relation between petrologic type and metallographic cooling rate but none has been observed scott and rajan 1981 devised a metamorphosed planetesimal model to explain this discrepancy whereby metamorphism occurs in planetesimals a few kilometers in radius which then accrete to form r 100 km parent bodies metallographic cooling rates are then controlled by burial depth thermal and collisional constraints on the metamorphosed planetesimal model are examined here and the model is found to be applicable only to highly insulating 26al rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface an alternative model is presented here in which onion shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled if reassembly times are short days then metallographic cooling rates would be determined by burial depth in the reaccreted parent body this model unlike previous ones can explain both coherent and incoherent cooling rates of breccia clasts by collisions during or after metamorphism respectively

the formation and alteration of the renazzo like carbonaceous chondrites i implications of bulk oxygen isotopic composition abstract to better understand the role of aqueous alteration on the cr chondrite parent asteroid a whole rock oxygen isotopic study of 20 meteorites classified as renazzo like carbonaceous chondrites cr was conducted the cr chondrites analyzed for their oxygen isotopes were dhofar 1432 elephant moraine eet 87770 eet 92042 eet 96259 gao guenie b graves nunataks gra 95229 gra 06100 grosvenor mountains gro 95577 gro 03116 lapaz ice field lap 02342 lap 04720 meteorite hills met 00426 north west africa nwa 801 pecora escarpment pca 91082 queen alexandra range que 94603 que 99177 and yamato 793495 y 793495 three of the meteorites asuka 881595 a 881595 gra 98025 and met 01017 were found not to be cr chondrites the remaining samples concur petrographically and with the well established oxygen isotope mixing line for the cr chondrites their position along this mixing line is controlled both by the primary oxygen isotopic composition of their individual components and their relative degree of aqueous alteration combined with literature data and that of this study we recommend the slope for the cr mixing line to be 0 70 0 04 2 with a 17 o intercept of 2 23 0 14 2 thin sections of al rais shisr 033 renazzo and all but 3 samples analyzed for oxygen isotopes were studied petrographically the abundance of individual components is heterogeneous among the cr chondrites but feo poor chondrules and matrix are the most abundant constituents and therefore dominate the whole rock isotopic composition the potential accreted ice abundance physico chemical conditions of aqueous alteration e g temperature and composition of the fluid and its duration control the degree of alteration of individual cr chondrites combined with literature data we suggest that lap 02342 was exposed to lower temperature fluid during alteration than gra 95229 with only two falls terrestrial alteration of the cr chondrites complicates the interpretation of their whole rock isotopic composition particularly in the most aqueously altered samples and those with relatively higher matrix abundances we report that que 99177 is the isotopically lightest whole rock cr chondrite known 18 o 2 29 17 o 4 08 possibly due to isotopically light unaltered matrix which shows that the anhydrous component of the cr chondrites is isotopically lighter than previously thought although it experienced aqueous alteration que 99177 provides the best approximation of the pristine cr chondrite parent body s oxygen isotopic composition before aqueous alteration took place using this value as a new upper limit on the anhydrous component of the cr chondrites water rock ratios were recalculated and found to be higher than previously thought ratios now range from 0 281 to 1 157 we also find that according to their oxygen isotopes a large number of cr chondrites appear to be minimally aqueously altered although sample heterogeneity complicates this interpretation

seismic reflection constraints on the structure of the crust beneath the san bernardino mountains transverse ranges southern california a 30 km long n s seismic reflection line was shot by california consortium for crust studies calcrust across the southern mojave desert and onto the northern flank of the san bernardino mountains in southern california on the northern end of the seismic section the reflectivity increases markedly in the midcrust at a depth corresponding to a two way travel time of 4 to 5 s 12 15 km suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust the high reflectivity disappears at about 8 s 24 km and may be correlated with a change in seismic velocity in the lower crust from 6 3 km s to 6 8 km s a band of reflectivity between 9 5 and 10 s 27 30 km is believed to represent the moho the midcrustal relectivity transition and moho both deflect downward toward the san bernardino mountains uplift over the entire length of the profile the deflection of the midcrustal transition 12 appears greater than that of the moho 6 resulting in a thinning of the lower crust to the south beneath the uplift in addition the midcrustal transition coincides with the base of the seismogenic zone brittle ductile transition which is also dipping southward beneath the san bernardino mountains while the moho deflection is consistent with elastic flexure resulting from edge loading by the san bernardino mountains which have been thrust over the mojave block it is suggested that the thinning of the lower crust beneath the san bernardino mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust since a portion of the load is transmitted through the lower crust to the moho the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift 2 m y

radar constraints on asteroid regolith properties using 433 eros as ground truth abstract radar data enable us to estimate an asteroid s near surface bulk density thus providing a joint constraint on near surface porosity and solid density we investigate two different approaches to simplifying this joint constraint estimating solid densities by assuming uniform porosities for all asteroids and estimating porosities by assuming uniform mineralogy within each taxonomic class methods used to estimate asteroids near surface solid densities from radar data have not previously been calibrated via independent estimates recent spacecraft results on the chondritic nature of 433 eros now permit such a check and also support porosity estimation for s class objects we use radar albedos and polarization ratios estimated for 36 main belt asteroids and nine near earth asteroids to estimate near surface solid densities using two methods one of which is similar to the uncalibrated algorithms used in previous studies the other of which treats eros as a calibrator we also derive porosities for the same sample by assigning solid densities for each taxonomic class in advance density estimation results obtained for eros itself are consistent with the uncalibrated method being valid in the mean those derived for the full sample imply that uncalibrated solid densities are at most a few tens of percent too large on average however some derived densities are extremely low whereas most porosity estimates are physically plausible we discuss the relative merits of these two approaches

experimental impacts into chondritic targets part i disruption of an l6 chondrite by multiple impacts mr after each shot was impacted by a 3 18 mm ceramic sphere at a nominal speed of 2 km s 1 this continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot ms two chunks of bushveldt gabbro with similar initial masses were also impacted under the same conditions until mr was less than half m0 the two gabbro targets required a total of 1 51 10 7 and 1 75 10 7 erg g 1 to attain 0 27 and 0 33 mr m0 respectively the chondrite however was considerably tougher reaching 0 40 and 0 21 mr m0 only after receiving 2 37 10 7 and 3 10 10 7 erg g 1 respectively the combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series which was the number of impacts required to disrupt each gabbro target i e mr m0 0 5 unlike the behavior shown in previous regolith evolution series neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock about 3 mm an effect possibly related to the width of the shock pulse the original chondrite was so fine grained and fractured and the variance in its grain size distribution so large that effects related to grain size were relegated to the 63 m fraction impacts into alh 85017 produced abundant fine grained debris but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets the characteristic slopes of the chondrite s size distributions however were notably more constant over the entire nine impact series than those from any of the terrestrial targets a testament to the control over comminution apparently exerted by pre existing fractures and other microscopic damage in the meteorite the enhancement in the finer fraction of debris from alh 85017 indicates that ordinary chondrites in solar orbit would be very efficient contributors to the cosmic dust complex at the same time the greater resistance to disruption displayed by ordinary chondrites relative to that exhibited by igneous rocks indicates that a selection effect could be operative between the annealed ordinary chondritic breccias and relatively weaker differentiated meteorites preferential survival from their time in the regoliths of their parent bodies through their transit to earth and passage through the atmosphere suggests that meteorite collections could be biased in favor of the ordinary chondrites

shock compaction heating and collisional processes in the production of type 3 ordinary chondrites lessons from the nearly unique l3 chondrite melt breccia northwest africa 8709

grain size evolution and fractionation trends in an experimental regolith the comminution of blocky planetary surfaces into fine grained regoliths was simulated by impacting a fragmental gabbro target 200 times with stainless steel projectiles diameter 6 35 mm mass 1 2g vi 1 35 km s initially the target fragments were 2 32 mm in size with a mean diameter of 15 2 mm which was reduced to 0 4 mm after 200 shots for the first 60 shots comminution was linear with cumulative kinetic energy and thus essentially linear with time the comminution efficiency however decreased at later stages this could be due to a change in fracture mechanism from predominantly intergranular to predominantly intragranular and or due to a change in partitioning of the projectile s kinetic energy from collisional fragmentation or large fragments at early times to cratering in a progressively finer grained medium at later stages mineral specific preferential comminution is abundantly observed in the modal make up of various grain size fractions plagioclase comminutes more readily than pyroxenes plagioclase initially present at 54 vol comprises typically 65 70 of the 125 250 m fraction pyroxenes initially at 33 are depleted to 20 25 this fractionation occurs in the earliest samples analyzed and is not a time dependent phenomenon bulk chemical analyses reveal a corresponding increase in al2o3 cao and na2o and a decrease in feo and mgo the finest grain sizes are generally the most severely affected by differential comminution similar if not exactly identical trends are abundantly documented for lunar soil samples these experimental findings illustrate that differential comminution is capable of producing fractionation in the finest grain sizes they diminish the need for prominent lateral and vertical transport of components otherwise foreign to a lunar soil sampling site as postulated by some workers on the basis of multivariate chemical mixing models

the active southwest margin of the colorado plateau uplift of mantle origin during cenozoic time the colorado plateau was raised about 2 km above sea level the most recent and best documented uplift of the plateau 1 km has been concentratedatitssouthwestmarginbetween6 and 1 ma whereas the eastern colorado plateau may have been at high elevations sinceeocenetime tobetterunderstandthe recent tectonic activity at the southwest marginofthecoloradoplateau wecompile detailed crustal thickness and density informationfromseismicandgravitydatafor a region that includes northwest arizona and the southern tip of nevada this information is used to isolate the mantle contribution to uplift wefind that there is relatively low density mantle underlying the southernmarginoftheplateauinnorthwest arizona which could result from about 60 80 km of thinning of the dense mantle lithosphere combined with about 100 cof heating through a 100 km thick mantle layer the available estimates from earthquake source seismology in or near the studyareaarecompatiblewiththisestimate of lithospheric thinning we speculate that uplift may result from subduction related thinning of the continental lithosphere

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

clay mineraloid weathering products in antarctic meteorites abstract clay mineraloids are associated with gypsum k fe sulfates possibly jarosite cryptocrystalline rust and possibly with zeolites and or secondary alkali feldspars as terrestrial weathering products in stony meteorites from the allan hills alh and elephant moraine eet collection areas in victoria land antarctica weathering formed fracture fillings in fusion crusts of eucrites eeta79004 and eeta79005 diogenites alha77256 and eeta79002 shergottite eeta79001 and h5 chondrite alh82102 and decomposition products of glass and plagioclase in interior samples of achondrites clay mineraloids displayed massive to incipient vermicular forms with elemental compositions that suggest smectite and mica like phases deposits were too small to yield pure material for phase identification although x ray diffractometry and differential scanning calorimetry of bulk samples suggested that the clay mineraloids were not well crystallized a typical structural formula calculated on a smectite basis follows k 0 26 ca 0 04 al 0 97 ti 0 02 fe 3 0 77 mg 0 03 si 4 00 o 10 oh 2 n 2 o net charge 0 28 excessions 0 58 si 0 02 cl 0 09 so 4 0 02 po 4 hydrocryogenic diagenesis can initiate clay mineral formation on timescales of 10 4 10 6 y even with only limited supplies of liquid water cosmochemists must take care not to confuse possible weathering induced elemental and isotopic fractionations in antarctic meteorite samples with signatures of pre terrestrial cosmic processes given proper sample selection and testing though occurrence of clay mineraloid and other weathering products should probably not impede progress in studies of antarctic meteorites

the flux of meteorites to the earth and weathering in hot desert ordinary chondrite finds abstract the abundance of ferric iron produced by weathering in ordinary chondrites collected from hot desert regions is related to their terrestrial age enabling a variety of processes involved in meteorite weathering to be deduced fe0 fe2 fe3 content was measured quantitatively by 57fe mossbauer spectroscopy it is observed that meteorites from different regions are oxidised at different rates we also propose the tentative hypothesis that one source of scatter in total oxidation versus terrestrial age for a region is climatic change a mechanism to explain the variable response to climatic conditions involves the high initial porosity of samples being reduced as ferric oxides and other alteration products are produced reliable recovery data from a meteorite accumulation site roosevelt county new mexico usa and a quantitative measure of weathering and terrestrial age permit an estimate of the flux of meteorites to the earth s surface having established a decay constant for meteorites and accounting for the area of searches and pairing we estimate an accumulation rate of 159 falls of mass greater than 20 g per 106 km2 per yr this calculation is distinct from but in broad agreement with previous estimates from camera observations suggesting that the accumulation rate of meteorite falls to the earth s surface has remained essentially unchanged over the last 50 000 years at between 59 and 159 falls of mass exceeding 20 g per 106 km2 per yr the error for both methods is approximately a factor of 2

modeling the structure of high porosity asteroids abstract a number of asteroids have been observed to have extremely low bulk densities in the 1 3 g cm 3 range given reasonable compositions based on meteorite analogues this bulk density implies bulk porosities of 40 60 suggesting that many asteroids are rubble piles held together by self gravity and containing significant empty space perhaps more empty space than solid material simple energy balance calculations and accretional models of disrupted asteroids suggest that materials in porous asteroids may be sorted by particle size the large irregular pieces and larger voids fractures are probably located deeper inside the asteroid and the fine particle size fractions that are observed on their surfaces are restricted to the surface regolith zone the large interior voids fractures are preserved from infilling by the effects of friction on the smaller size fractions friction tends to dominate the downward pull of gravity and prevents the fine fractions from filtering into the interior of the asteroid and infilling the large fractures and voids friction may also play a role in allowing shattered asteroids to maintain their relief features and shape by resisting the movements of pieces within the object in effect providing strength to non coherent objects

dolomitization anhydrite cementation and porosity evolution in a reflux system insights from reactive transport models significant volumes of world hydrocarbon reserves occur in dolostones and the majority of these reservoirs are interpreted to be of reflux origin we used a two dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization dolomite cementation anhydrite cementation and porosity evolution in a reflux system we tested the sensitivity of reflux dolomitization to brine concentration mesohaline to near halite saturated brines near surface temperature flow rate porosity permeability feedback relationships reactive surface area and the effect of initial porosity permeability heterogeneity simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization and that mesohaline reflux dolomitization is viable reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers replacement dolomitization increased porosity up to 8 but postreplacement reflux resulted in the precipitation of minor dolomite cements overdolomitization anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25 simulated rates of replacement dolomitization by reflux are fast up to three orders of magnitude faster than seawater dolomitization the rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry temperature and reactive surface area were important controls on the rate of dolomitization whereas the feedback relationship between porosity and permeability was a relatively moderate control our reactive transport models predict the general spatial and temporal trends in dolomite porosity and anhydrite observed in some major dolomite reservoirs

the porosity and permeability of chondritic meteorites and interplanetary dust particles abstract we have investigated the porosity of a large number of chondritic interplanetary dust particles idps and meteorites by three techniques standard liquid gas flow techniques a new noninvasive ultrasonic technique and image processing of backscattered images the latter technique is obviously best suited to sub kilogram sized samples we have also measured the gas and liquid permeabilities of some chondrites by two techniques standard liquid gas flow techniques and a new nondestructive pressure release technique we find that chondritic idps have a somewhat bimodal porosity distribution peaks are present at 0 and 4 porosity a tail then extends to 53 type 1 3 chondrite matrix porosities range up to 30 with a peak at 2 the bulk porosities for type 1 3 chondrites have the same approximate range as exhibited by the matrix which indicates that other components of the bulk meteorites including chondrules and aggregates have the same average porosity as the matrix these results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar which implies that similar accretion dynamics operated through 12 orders of size magnitude permeabilities of the investigated chondrites vary by several orders of magnitude and there appears to be no simple dependence of permeability with degree of aqueous alteration chondrite type or porosity

porosity and density of ordinary chondrites clues to the formation of friable and porous ordinary chondrites densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors we report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study based on our measurements we find that no significant correlation exists between porosity and petrologic grade chemical group sample mass bulk and grain density or shock level to investigate variations in porosity and density between pieces of a meteorite we examined stones from two showers holbrook and pultusk examination of nine samples of holbrook suggests relative homogeneity in porosity and density between pieces of this shower measurements of three samples of pultusk show homogeneity in bulk density in contrast to wilkison and robinson 2000 a study that reported significant variations in bulk density between 11 samples of pultusk finally examination of two friable ordinary chondrites bjurbole and allegan reveal variability in friability and porosity among pieces of the same fall we suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

precursor and metamorphic condition effects on raman spectra of poorly ordered carbonaceous matter in chondrites and coals abstract geothermometers based on raman spectrometry of carbonaceous matter and covering a wide range of temperatures 100 650 c have been developed over recent years while raman data have been largely interpreted in terms of temperature they are also the fingerprint of certain metamorphic conditions especially in the low temperature range relevant to poorly ordered carbonaceous matter this study investigates the raman spectra of two series of chondritic carbonaceous matter and coal samples formed from different precursors and under different metamorphic conditions the raman spectra of polyaromatic carbonaceous matter pcm from 42 chondrites and 27 coal samples measured with visible 514 nm and ultra violet 244 nm excitation wavelengths are analyzed the raman spectra of low rank coals and chondrites of petrologic types 1 and 2 which contain the more disordered pcm reflect the distinct carbon structures of their precursors the 514 nm raman spectra of high rank coals and chondrites of petrologic type 3 exhibit continuous and systematic spectral differences reflecting different carbon structures present during the metamorphism event they result from differences in the chemical structures of the precursors concerning for instance the reticulation of polyaromatic units or an abundance of ether functional groups or possibly from a lack of carbonization processes to efficiently expel oxygen heteroatoms due to weak lithostatic pressure and confinement these results suggest that the use of low temperature carbon thermometers should be restricted to a given geological context at the same time the sensitivity of raman spectra to precursors and certain metamorphic conditions could be used to obtain information other than temperature the analysis also provides evidence of the accretion of relatively homogeneous pcm precursors among ordinary co and cv carbonaceous chondrite parent bodies given that the 514 nm raman spectra of pcm efficiently trace the metamorphism grades looking closer however the 514 nm raman data are more scattered in chondrites than in coals and the maturity tracers are less sensitive and miscorrelate with the atomic h c ratio suggesting slight compositional and structural differences among the pcm precursors accreted

mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates abstract petrographic mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from hot deserts and antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of fe ni metal producing fe oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins troilite weathers readily in hot desert environments but undergoes very little alteration under antarctic conditions most of the primary porosity of ordinary chondrites has been occluded by the time that 15 to 25 of the initial fe 0 and fe 2 has been oxidised to fe 3 in both environments results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products dilation of primary pores and brecciation which has been observed in parts of some meteorites will only occur if the meteorite is especially metal rich or has a low primary porosity these weathering products are absent from recent falls but have formed in a fall after 100 yr of museum storage cl bearing akaganeite and hibbingite are common weathering products in antarctic finds but occur in abundance in only one hot desert meteorite daraj 014 the majority of fe rich weathering products in meteorites from both environments contain low but variable concentrations of si mg and ca in most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a 1 0 nm lattice fringe spacing where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene in the heavily weathered antarctic finds alha 78045 and 77002 si is concentrated in cronstedtite a fe rich phyllosilicate an unidentified hydrous si fe ni mg mineral or gel has also partially replaced taenite in alha 78045 in addition to fe rich weathering products hot desert meteorites contain sulphates ca carbonate and silica whereas such minerals are largely absent from antarctic finds the abundance of silicate weathering products in antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments as preterrestrial cronstedtite is abundant in cm2 carbonaceous chondrites the antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites

destruction of rocks by low velocity impact and its implications for accretion and fragmentation processes of planetesimals recent experimental results on impact destruction of rocks basalt and tuff are summarized the mode of destruction and some empirical relations between the mass of the largest fragment ml the released kinetic energy divided by target mass ei and the work done during impact w are presented the mass ml can be related to both ei and w log ml mt a log ei and log ml mt b log w an effect due to the difference in impact velocity on the ml versus ei relation can be observed but there seems to be no effect due to impact velocity on the ml versus w relation thus it may be suggested that w is a useful means of describing phenomena associated with the impact destruction of rocks target shape also affects the ml versusei relation but no large differences have been observed between various rocks the size distribution of fragments can be adequately expressed in terms of an inverse power law relation in which slope of the size distribution i e exponent increases with increasing ei 0 90 0 36 log ei for basalt and 1 94 0 24 log ei for tuff based on the empirical ei relations a fragmentation model of the parent bodies of eos and koronis families is presented it is suggested that the parent bodies of eos and koronis families had been differentiated prior to fragmentation and were completely fragmented at ei 2 104 joule kg and 2 103 joule kg

physical properties of ordinary chondrites physical properties intrinsic and bulk densities porosity compressional and shear wave velocities thermal diffusivity and conductivity are measured on 11 ordinary chondrites one carbonaceous chondrite and two achondritic clasts of a mesosiderite with the previously reported results from antarctic ordinary chondrites the data are useful in clarifying the difference between h and l chondrites for example the intrinsic density of h chondrites 3800 kg m3 is generally higher than that of l chondrites 3600 kg m3 the sample porosity less than 20 strongly controls its elastic wave velocities thermal diffusivity and conductivity the variations of elastic wave velocities and thermal properties with porosity show that as with lunar rocks chondrites contain many cracks these cracks are thought to be created on their parent bodies by the cumulative effects of many mutual collisions and impacts thermal properties are linear functions of elastic wave velocities for both h and l chondrites the difference between h and l chondrites can be attributed to the content of metallic fe ni correlations between petrologic types which roughly represent metamorphic temperature and porosity are less obvious for l chondrites than for h chondrites since porosity is not changed significantly by impact events it appears that the sintering process for l chondrites is independent of the metamorphic events represented by petrologic type

density magnetic susceptibility and the characterization of ordinary chondrite falls and showers bulk and grain densities of 132 ordinary chondrites from the vatican observatory collection were measured and compared with their magnetic susceptibility for the most part using previously measured values ten new susceptibility measures were taken for this study grain density and magnetic susceptibility combined provide a reliable method of classifying un weathered ordinary chondrites unlike traditional chemical tests this method is fast nondestructive and characterizes the whole rock making it especially appropriate for surveying large collections the system is less viable for finds extensive weathering of metallic iron in an h chondrite can cause it to plot among l chondrites while heavily weathered l chondrites plot among the ll group this system has revealed outlier stones that may be misclassified meteorites or mislabeled samples in every case where the magnetic susceptibility of a meteorite does not fit its putative classification the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification an analysis of stones from five showers shows that excluding outliers these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility

application of numerical modelling to a case of compaction driven dolomitization a jurassic palaeohigh in the po plain italy dolomitization of a carbonate platform can occur at different times and in different diagenetic environments from synsedimentary to deep burial settings numerical simulations are valuable tools to test and select the model that among different hypotheses compatible with field and geochemical data best honour mass balance kinetic and thermodynamic constraints moreover the simulation can predict the distribution of the dolomitized bodies in the subsurface and evaluate porosity changes valuable information for the oil industry this study is the first attempt to reproduce and investigate the compaction dolomitization model the diagenetic study of the jurassic carbonate basin and palaeohigh system of the po plain indicates that the carbonates of the palaeohighs were dolomitized by basin compaction fluids the main goal of the simulations is to evaluate the origin and evolution of the dolomitizing fluids and to provide insights regarding the distribution of the potential reservoir dolomitized bodies in the po plain the modelling process is subdivided into two steps basin modelling and reactive transport modelling the sebe3 basin simulator eni proprietary was used to create a three dimensional model of the compacting system the results include compaction fluid flow rate from the basin to the palaeohigh compaction duration and a determination of the total amount of fluid introduced into the palaeohigh these data are then used to perform reactive transport modelling with the toughreact code sensitivities on dolomite kinetic parameters suggest that dolomitization was an efficient process even at low temperatures with differences mainly related to the dynamics of the process fluid composition is one of the main constraints the sea water derived compaction fluid is proven to be efficient for dolomitization due to its relatively high mg content simulations also confirmed that permeability is the most important factor influencing fluid flow and consequently the dolomite distribution in the formation permeable fractured zones have a strong influence diverting the dolomitizing fluids from their normal path towards overlying or lateral zones moreover the simulations showed that after dolomite replacement is complete the dolomitizing fluids can precipitate dolomite cement causing over dolomitization with related localized plugging effects in the zone of influx mass balance calculations indicate that in the dolomitization compaction model the amount of compaction water fluxed from the basin to the carbonate is the main constraint on dolomitization efficiency this observation implies that the ratio between the volume of the basin undergoing compaction and the volume of the palaeohigh is a limiting factor on the final size of the dolomitized bodies an isolated palaeohigh could be an ideal site for pervasive replacement dolomitization due to the large volume of compaction fluids available compared with the carbonate rock volume in the case of large platforms the more permeable margin lithofacies are the most likely sites for compaction model dolomitization the combined use of a basin simulator and reactive transport modelling has proved to be a successful method to verify model reliability and it provides insights into the volumetric distribution of diagenetic products

reactive transport modelling of hydrothermal dolomitisation using the csmp gem coupled code effects of temperature and geological heterogeneity abstract reactive transport simulations using our csmp gem coupled code were applied to study the major controls on replacement dolomitisation and the development of dolomite geobodies in a hydrothermal setting a series of 2d simulations show how elevated temperature and reactive surface area increase the rate of dolomitisation and result in a dolomite replacement front that is both sharper and inclined at a higher angle from vertical this inclination an effect of gravity segregation is apparent in thick homogeneous units but in layered systems the lithological contrast determines the shape of the dolomite front the increase in permeability resulting from porosity generation upon replacement of calcite by dolomite has a major effect on accelerating the overall progress of dolomitisation in contrast the changes in fluid density due to chemical reactions and the pressure dependence of thermodynamic data have a minor influence under simulated conditions primary dolomite forms slowly after complete replacement of host calcite leading to porosity decrease and is only locally important around the source of the hydrothermal fluid for a simple layered system our model results are in excellent agreement with those obtained using toughreact code they do however show the advantage of unstructured triangular over structured rectangular meshes for resolving complex curved inclined front shapes such meshes also offer benefits in simulating fault controlled hydrothermal dolomitisation our simulations predict dolomite geobodies comparable in scale and morphology to natural examples documented at outcrops and underline the importance of understanding the permeability structure within and around the fault zone

radar imaging of asteroid 7 iris arecibo radar images of iris obtained in november 2006 reveal a topographically complex object whose gross shape is approximately ellipsoidal with equatorial dimensions within 15 of 253 228 km the radar view of iris was restricted to high southern latitudes precluding reliable estimation of iris entire 3d shape but permitting accurate reconstruction of southern hemisphere topography the most prominent features three roughly 50 km diameter concavities almost equally spaced in longitude around the south pole are probably impact craters in terms of shape regularity and fractional relief iris represents a plausible transition between 50 km diameter asteroids with extremely irregular overall shapes and very large concavities and very much larger asteroids ceres and vesta with very regular nearly convex shapes and generally lacking monumental concavities

a laboratory impact study of simulated edgeworth kuiper belt objects abstract this paper reports on a series of laboratory impact experiments designed to provide basic data on how simulated edgeworth kuiper belt objects ekos fragment in an impact event in september october 1997 we carried out 20 low velocity airgun shots at the ames vertical gun range into porous and homogeneous ice spheres using aluminum fractured ice and solid ice projectiles we found that the porous ice targets behaved as strongly as solid ice in collision energy is apparently well dissipated by the void spaces within the target such that these fragile ice structures respond as if they were strong in impacts therefore it would appear that if ekos are porous they are not collisionally weak also our data show that collisional outcomes for low velocity impacts into ice targets depend on the type of projectile used as well as the properties of the target we observed that the degree of fragmentation for a given type of target increases as the strength of the projectile increases aluminum projectiles are far more damaging to the target at the same collisional energy than are solid ice projectiles which in turn are more damaging than fractured ice projectiles one possible explanation for this behavior is the variable depth of penetration of the projectile for the different cases stronger projectiles penetrate more deeply and couple more energy into the target than do weak projectiles based on this if we assume that there has not been significant heating or differentiation in the edgeworth kuiper e k belt the most applicable impact strength for the low velocity e k belt collisions is likely to be that derived from similar target projectile materials impacting each other the laboratory data from this analysis indicate that a value for impact strength 5 10 5 erg cm 3 is appropriate for porous ice targets impacted with solid porous ice projectiles

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

stony meteorite porosities and densities a review of the data through 2001 in this review we summarize the data published up to december 2001 on the porosity and density of stony meteorites these data were taken from 925 samples of 454 different meteorites by a variety of techniques most meteorites have densities on the order of 3 to 4 g cm 3 with lower densities only for some volatile rich carbonaceous meteorites and higher densities for stony irons for the vast majority of stones porosity data alone cannot distinguish between different meteorite compositions average porosities for most meteorite classes are around 10 though individual samples can range as high as 30 porosity unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present the measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment a theoretical grain density based on composition can model pre weathered porosities the average model porosity for h and ll chondrites is 10 while l chondrite model porosities average only 6 a statistically significant difference

the barringer award address presented 1996 july 25 berlin germany impact experiments related to the evolution of planetary regoliths impact induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales resulting in comminution products that range from fine grained surface soils to massive polymict ejecta deposits to collisionally fragmented objects within this wide range of comminution products we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock s in this paper we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows and we discuss some of their implications for asteroidal surfaces cratering experiments in both rock and noncohesive materials combined with photogeologic observations of the lunar surface demonstrate that craters 500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites the overall excavation rate and thus growth rate of the debris layer decreases with time because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock typical growth rates for a 5 m thick lunar soil layer are initially 3 ga ago a few mm ma and slowed to 1 mm ma at present the coarse grained crater ejecta are efficiently comminuted by collisional fragmentation processes and the mean residence time of a 1 kg rock is typically 10 ma the actual comminution of either lithic or monomineralic detritus is highly mineral specific with feldspar and mesostasis comminuting preferentially over pyroxene and olivine thus resulting in mechanically fractionated fines especially at grain sizes 20 m such fractionated fines also participate preferentially in the shock melting of lunar soils thus giving rise to agglutinate melts as a consequence agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil s compositionally homogeneous impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces reflecting lithic regolith components and associated mineral mixtures cumulatively experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock the addition of foreign exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils regoliths on asteroids are qualitatively different from those of the moon the modest impact velocities in the asteroid belt some 5 km s 1 are barely sufficient to produce impact melts also substantially more crater mass is being displaced on low gravity asteroids compared to the moon collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces these processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the moon impact melting does not seem to be a significant process under these conditions however the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past unfortunately the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of s type asteroids remain poorly understood at present despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

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

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk