a new martian meteorite from oman mineralogy petrology and shock metamorphism of olivine phyric basaltic shergottite sayh al uhaymir 150 the sayh al uhaymir sau 150 meteorite was found on a gravel plateau 43 3 km south of ghaba oman on october 8 2002 oxygen isotope 17o 2 78 18o 4 74 cre age 1 3 ma and noble gas studies confirm its martian origin sau 150 is classified as an olivine phyric basalt having a porphyritic texture with olivine macrocrysts set in a finer grained matrix of pigeonite and interstitial maskelynite with minor augite spinel ilmenite merrillite pyrrhotite pentlandite and secondary terrestrial calcite and iron oxides the bulk rock composition in particular mg 68 molar mg mg fe 100 fe mn 37 9 and na al 0 22 are characteristic of martian meteorites based on mineral compositions cooling rates determined from crystal morphology and crystal size distribution it is deduced that the parent magma formed in a steady state growth regime magma chamber that cooled at 70 365 c sau 150 is essentially identical to sau 005 094 all representing samples of the same fall that are similar to but distinct from the dag shergottites

optically stimulated luminescence age controls on late pleistocene and holocene coastal lithosomes north carolina usa luminescence ages from a variety of coastal features on the north carolina coastal plain provide age control for shoreline formation and relative sea level position during the late pleistocene a series of paleoshoreline ridges dating to marine isotope stage mis 5a and mis 3 have been defined the kitty hawk beach ridges on the modern outer banks yield ages of 3 to 2 ka oxygen isotope data are used to place these deposits in the context of global climate and sea level change the occurrence of mis 5a and mis 3 shorelines suggests that glacio isostatic adjustment gia of the study area is large ca 22 to 26 m as suggested and modeled by other workers and or mis 3 sea level was briefly higher than suggested by some coral reef studies correcting the shoreline elevations for gia brings their elevation in line with other sea level indicators the age of the kitty hawk beach ridges places the holocene shoreline well west of its present location at ca 3 to 2 ka the age of shoreline progradation is consistent with the ages of other beach ridge complexes in the southeast usa suggesting some regionally contemporaneous forcing mechanism

petrogenesis of the elephant moraine a79001 meteorite multiple magma pulses on the shergottite parent body abstract the eeta 79001 achondrite consists of two distinct igneous lithologies joined along a planar non brecciated contact both are basaltic rocks composed primarily of pigeonite augite and maskelynite but one contains zoned megacrysts of olivine orthopyroxene and chromite that represent disaggregated xenoliths of harzburzite both lithologies probably formed from successive volcanic flows or multiple injections of magma into a small shallow chamber many similarities between the two virtually synchronous magmas suggest that they are related possible mechanisms to explain their differences involve varying degrees of assimilation fractionation from similar parental magmas or partial melting of a similar source peridotite of these assimilation of the observed megacryst assemblage seems most plausible however some isotopic contamination may be required in any of these petrogenetic models the meteorite has suffered extensive shock metamorphism and localized melting during a large impact event that probably excavated and liberated it from its parent body both basaltic lithologies and the inferred ultramafic protolith of the megacryst assemblage are petrologically similar to other members of the shergottite group and all may have been derived from a volcanic plutonic complex on a planetary body

impact induced frictional melting in ordinary chondrites a mechanism for deformation darkening and vein formation high speed friction experiments have been performed on the ordinary chondrites el hammami h5 s2 and sahara 97001 l6 s3 using an axial friction welding apparatus each sample was subjected to a strain rate of 103 to 104 s 1 which generated 250 to 500 m deep darkened zones on each sample cube thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron size feni and fes blebs fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone these features are not present in the original meteorites the shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites the experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites therefore the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation in fact high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures

developments in the characterization of complex rock slope deformation and failure using numerical modelling techniques recent advances in the characterization of complex rock slope deformation and failure using numerical techniques have demonstrated significant potential for furthering our understanding of both the mechanisms processes involved and the associated risk this paper illustrates how rock slope analyses may be undertaken using three levels of sophistication level i analyses include the conventional application of kinematic and limit equilibrium techniques with modifications to include probabilistic techniques coupling of groundwater simulations and simplistic treatment of intact fracture and plastic yield such analyses are primarily suited to simple translational failures involving release on smooth basal rear and lateral surfaces where the principle active damage mechanisms are progressive failure and or asperity breakdown level ii analyses involve the use of continuum and discontinuum numerical methods in addition to simple translation level ii techniques can be applied to complex translational rock slope deformations where step path failure necessitates degradation and failure of intact rock bridges along basal rear and lateral release surfaces active damage processes in this case comprise not only strength degradation along the release surface e g asperity breakdown but also a significant component of brittle intact rock fracture level iii analyses involve the use of hybrid continuum discontinuum codes with fracture simulation capabilities these codes are applicable to a wide spectrum of rock slope failure modes but are particularly well suited to complex translation rotational instabilities where failure requires internal yielding brittle fracturing and shearing in addition to strength degradation along release surfaces through a series of rock slope analyses the application of varied numerical methods are discussed particular emphasis is given to state of the art developments and potential use of level iii hybrid techniques d 2005 elsevier b v all rights reserved

petrology and chemistry of the new shergottite dar al gani 476 in 1998 dar al gani dag 476 was found in the libyan desert the meteorite is classified as a basaltic shergottite and is only the 13th martian meteorite known to date it has a porphyritic texture consisting of a fine grained groundmass and larger olivines the groundmass consists of pyroxene and feldspathic glass minor phases are oxides and sulfides as well as phosphates the presence of olivine orthopyroxene and chromite is a feature that dag 476 has in common with lithology a of elephant moraine eet a79001 however in dag 476 these phases appear to be early phenocrysts rather than xenocrysts shock features such as twinning mosaicism and impact melt pockets are ubiquitous terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks with a molar mgo mgo feo of 0 68 dag 476 is the most magnesian member among the basaltic shergottites compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites however major elements such as fe and ti as well as lree concentrations are considerably lower than in other shergottites noble gas concentrations are low and dominated by the mantle component previously found in chassigny a component similar to that representing martian atmosphere is virtually absent the ejection age of 1 35 0 10 ma is older than that of eeta79001 and could possibly mark a distinct ejection dar al gani 476 is classified as a basaltic shergottite based on its mineralogy it has a fine grained groundmass consisting of clinopyroxene pigeonite and augite feldspathic glass and chromite ti chromite ilmenite sulfides and whitlockite isolated olivine and single chromite grains occur in the groundmass orthopyroxene forms cores of some pigeonite grains shock features such as shock twinning mosaicism cracks and impact melt pockets are abundant severe weathering in the sahara led to significant formation of carbonate veins crosscutting the entire meteorite dar al gani 476 is distinct from other known shergottites chemically it is the most magnesian member among known basaltic shergottites and intermediate in composition for most trace and major elements between iherzolitic and basaltic shergottites unique are the very low bulk ree element abundances the ci normalized abundances of lrees are even lower than those of iherzolitic shergottites the overall abundance pattern however is similar to that of que 94201 textural evidence indicates that orthopyroxene as well as olivine and chromite crystallized as phenocrysts from a magma similar in composition to that of bulk dag 476 whether such a magma composition can be a shergottite parent melt or was formed by impact melting needs to be explored further at this time it cannot entirely be ruled out that these phases represent relics of disaggregated xenoliths that were incorporated and partially assimilated by a basaltic melt although the texture does not support this possibility trapped noble gas concentrations are low and dominated by a chassigny like mantle component virtually no martian atmosphere was trapped in dag 476 whole rock splits the exposure age of 1 26 0 09 ma is younger than that of most shergottites and closer to that of eeta79001 the ejection age of 1 35 0 1 ma could mark another distinct impact event

39ar 40ar ages of martian nakhlites abstract we report 39ar 40ar ages of whole rock wr and plagioclase and pyroxene mineral separates of nakhlites mil 03346 and y 000593 and of wr samples of nakhlites nwa 998 and nakhla all age spectra are complex and indicate variable degrees of 39ar recoil and variable amounts of trapped 40ar in the samples thus we examine possible ar ar ages in several ways from consideration of both limited plateau ages and isochron ages we prefer ar ar ages of nwa 998 1334 11 ma mil 03346 1368 83 ma mesostasis and 1334 54 ma pyroxene y 000593 1367 7 ma and nakhla 1357 11 ma 2 errors for nwa 998 and mil 03346 the ar ar ages are within uncertainties of preliminary rb sr isochron ages reported in the literature these ar ar ages for y 000593 and nakhla are several ma older than sm nd ages reported in the literature we conclude that the major factor in producing ar ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40ar on weathered grain surfaces that was degassed along with the first several percent of 39ar a total k 40ar isochron for wr and mineral data from five nakhlites analyzed by us plus lafayette data in the literature gives an isochron age of 1325 18 ma 2 we emphasize the precision of this isochron over the value of the isochron age our ar ar data are consistent with a common formation age for nakhlites the cosmic ray exposure cre age for nwa 998 of 12 ma is also similar to cre ages for other nakhlites

stability analysis of cut slopes using empirical kinematical numerical and limit equilibrium methods case of old jeddah mecca road saudi arabia the stability of natural and man made rock slopes has always been of great concern for engineering geology studies to ensure a safe and functional design of excavated slopes the first and most crucial step is to use reliable input parameters while searching for possible failure modes at the preliminary design stage it is highly important for practicing engineers to determine the best material property for a chosen failure criterion this study aims to analyze the stability condition of a case road cut from different aspects for this purpose two well known failure criteria proposed for rock masses and rock joints were applied in the study depending on the scale of the problem in the slope face the hoek brown failure criterion was used to obtain input data required for modeling intact rock and rock mass behavior and the barton bandis shear strength criterion to determine the input data necessary for the modeling of rock joint behavior smrtool and dips 7 0 programs were utilized to analyze the structurally controlled instabilities due to discontinuities in the cut slope slide 2018 and rs2 9 0 programs were executed to examine the stability condition of cut slope subjected to circular and non circular failures respectively according to the results of numerical and limit equilibrium methods no stability problem occurred in the case of slope due to non sliding circular or non circular slip failures however according to the results of empirical and kinematical analyses some structurally controlled sliding failure modes of planar wedge and toppling were expected on the slope face the result of this study clearly shows that to reach a final decision different analysis methods should be used to evaluate the stability of cut slopes by considering various conditions of jointed rock masses

40ar 39ar dating of microgram feldspar grains from the paired feldspathic achondrites gra 06128 and 06129 abstract 40 ar 39 ar ages of single feldspar grains from the paired meteorites graves nunatak 06128 gra8 8 grains and 06129 gra9 26 grains are presented plateau ages 70 of the 39 ar released ranged from 4000 to 4600 ma with an average 1 uncertainty of 90 ma the most precise ages obtained were 4267 17 ma for a grain from gra8 and 4437 19 ma and 4321 18 ma for two grains from gra9 isotope correlation diagrams yield less precise ages ranging from 3800 to 5200 ma with an average 1 uncertainty of 250 ma they indicate a negligible trapped component plateau ages integrated total fusion ages and isochron ages are internally concordant at the 95 confidence level the distribution of the plateau ages for gra9 is bimodal with peaks at 4400 and 4300 ma in contrast the plateau age distribution for gra8 peaks at about 4260 ma with broad wings extending toward younger and older ages to explain the distributions of grain ages we prefer a scenario that includes a major post formation event about 4400 ma ago and a later melt intrusion event that heated gra8 more than some parts of gra9

an empirical failure criterion for rocks and jointed rock masses abstract a new triaxial strength equation is proposed as fitting the available laboratory triaxial test data it is shown how the five triaxial parameters unconfined compressive tensile and shear strengths coefficient of friction and exponent in the triaxial equation can be inter related by three equations a relation between intact rock strength rock mass strength ratio and barton s rock quality 1974 is next developed based on geomechanical data for four longwall panels and in situ coal strength data this relation and the coefficient of internal friction from barton s classification are used in the three inter relations to evaluate the rest of the triaxial parameters for rock masses performance of the failure criterion is studied using practical cases of rock structures in coal and coal measure strata viz five longwall caving panels and four coal pillars

numerical analysis of initiation and progressive failure in natural rock slopes the 1991 randa rockslide the 1991 randa rockslide in the swiss alps involved several complex mechanisms relating to geological mechanical and hydrological processes for which no clear trigger can be asserted this paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the randa rockslide as a working example results from continuum i e finite element modelling are presented to illustrate a hypothesis suggesting that initiation of a progressive rock mass degradation process ultimately leading to failure began following deglaciation of the valley below discontinuum distinct element modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide finally the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure

chronology of a sedimentary sequence from the land ocean interaction zone in the north jiangsu basin abstract the north jiangsu basin njb located in the western part of the north jiangsu south yellow sea basin is a critical region providing invaluable archives about the evolution processes of the land ocean interchange and climatic environments in a coastal zone chronology is of crucial importance to reconstruct the timing of sea level eustacy and shoreline changes as well as the evolution of the regional environment by means of numerical dating to determine the age of sedimentary strata in the sea land interaction zone of the njb however sediment dating in the njb remains controversial due to limitations in the quantity and quality of chronological data therefore the sediments in the jianhu gangxi gx profile of the njb were analyzed in this study by applying both fine grained quartz optically stimulated luminescence osl dating and 14c dating the results were used to discuss the processes of regional environment evolution the osl dating of fine grained quartz components in sediments was found to be a reliable method to determine deposition ages of sediments in the land sea interaction zone in the njb the combination of osl and accelerator mass spectrometry 14c dating techniques allowed a reliable and high resolution chronological framework for the gx profile since the last glaciation the sedimentary environment in the eastern portion of the njb changed significantly in particular 1 between 13 and 7 25 ka b p the area of interest was dominated by lacustrine deposition within a stable sedimentary environment with low deposition rates 2 from 7 25 to 6 45 ka b p marine transgression events occurred and the deposition rate increased rapidly in an area of land sea interaction 3 after 6 45 ka b p a regression event occurred and the study area transformed into a lowland lake environment

39ar 40ar dating of the zagami martian shergottite and implications for magma origin of excess 40ar the zagami shergottite experienced a complex petrogenetic formation history mccoy et al 1992 1999 like several shergottites zagami contains excess 40ar relative to its formation age to understand the origin of this excess 40ar we made 39ar 40ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution surprisingly all these separates show similar concentrations of excess 40ar 1 x 10 6 cm 3 g we present arguments against this excess 40ar having been introduced from the martian atmosphere as impact glass we also present evidence against excess 40ar being a partially degassed residue from a basalt that actually formed 4 gyr ago we utilize our experimental data on ar diffusion in zagami and evidence that it was shock heated to only 70 c and we assume this heating occurred during an ejection from mars 3 myr ago with these constraints thermal considerations necessitates either that its ejected mass was impossibly large or that its shock heating temperature was an order of magnitude higher than that measured we suggest that this excess 40ar was inherited from the zagami magma and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old k rich crustal material similar concentrations of excess 40ar in the analyzed separates imply that this magma maintained a relatively constant 40ar concentration throughout its crystallization this likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released these concepts have implications for excess 40ar in other shergottites

martian gases in an antarctic meteorite significant abundances of trapped argon krypton and xenon have been measured in shock altered phases of the achondritic meteorite elephant moraine 79001 from antarctica the relative elemental abundances the high ratios of argon 40 to argon 36 2000 and the high ratios of xenon 129 to xenon 132 2 0 of the trapped gas more closely resemble viking data for the martian atmosphere than data for noble gas components typically found in meteorites these findings support earlier suggestions made on the basis of geochemical evidence that shergottites and related rare meteorites may have originated from the planet mars

modification of slope mass rating smr by continuous functions this study was partially funded by the spanish ministry of science and technology and feder project tec 2005 06863 by the valencia regional government project gv06 179 and grupos03 085 and by the university of alicante project vigrob 157

assessment of rock slope stability for a segment of the ankara pozanti motorway turkey abstract the ciftehan pozanti segment of the ankara pozanti motorway is problematic due to the existence of lithological units with variable characters ten cut slopes are planned along km 352 350 km 358 000 of the proposed motorway the purpose of this study is to determine engineering geological properties of the rocks exposed along the motorway and to assess excavatability and stability of the cut slopes both field and laboratory studies were carried out during this research field studies involved detailed discontinuity surveys laboratory tests were carried out to determine unit weight point load strength index and shear strength parameters of the discontinuities in the study area recrystallized limestone dolomite limestone microgabbro reefal limestone conglomerate and quaternary deposits are exposed however the cut slopes are located within microgabbro reefal limestone dolomite limestone and recrystallized limestone hard to extremely hard ripping with local blasting for the fresh inner part of the rocks is recommended for the excavatability in eight cut slopes wedge failure is expected based on the field observations and stability analyses of the cut slopes slope flattening with various angles wire mesh and drainage ditches are suggested

structurally controlled instability damage and slope failure in a porphyry rock mass abstract rock slopes fail through structurally controlled mechanisms global circular failures or complex mechanisms depending on structural patterns and rock mass damage here structural geology rock mass characterisation terrestrial laser scanning tls ground based radar interferometry gb insar and finite element modelling are integrated to explore relationships between structure damage and global slope failure at mt gorsa trentino italy there a porphyry quarry has been excavated in complex strongly anisotropic rhyolitic ignimbrite rock masses the slope was affected by a major rockslide in 2003 and undergoes continuing instability site investigations and gb insar monitoring revealed that the 2003 failure was a roto translational rockslide involving about 400 000 m3 of disrupted rock structural analysis of tls and field data shows that the slope is affected by widespread structurally controlled mechanisms sliding toppling strain localization in kink bands the non obvious relationships between structurally controlled and global roto translational slope failure mechanisms are investigated by characterising rock mass damage in different slope sectors a new approach to quantify rock mass damage by mapping the geological strength index and interpreting its topographic signatures in tls point clouds is presented a persistent geological marker is systematically mapped in tls point clouds and correlations between attitude variability statistics and rock mass damage are established providing an efficient assessment tool rock mass damage increases in kinematic domains affected by structurally controlled instability gsi 35 40 and is maximum in areas of ongoing global instability gsi 15 20 the 2003 rockslide occurred inside a damaged rock mass zone with gsi

shock induced compaction melting and entrapment of atmospheric gases in martian meteorites 1 the strongest evidence for a martian origin of the snc meteorites is the strong correlation between the rare gas abundances and isotopic compositions in shock induced melt pockets and those measured for the martian atmosphere however the formation of melt pockets and the entrapment of atmospheric gases remain poorly understood here we report the presence in the melt pockets of three martian meteorites of the hollandite structured high pressure polymorph of feldspar these occurrences set constraints on the continuum pressure 21 25 gpa the local temperature increase 2000 2500 k and the energy delivered during shock we then test a mechanism for melt pocket formation by compaction of pre existing porous precursors the model explains the local temperature increase required for melting and the presence of an atmospheric component in pores that were connected with the martian atmosphere prior to the shock event

mineralogy microtexture and composition of shock induced melt pockets in the los angeles basaltic shergottite analytical electron microscopy of shock features in the basaltic shergottite los angeles stone 1 reveals 1 shock recorded in the bulk sample and 2 localized pressure and temperature excursions that have generated melt pockets up to 4 mm in diameter bulk shock effects include microfaulting offsets 1 200 mm mosaicism deformed exsolution lamellae and planar fracturing in pyroxene undulose extinction in whitlockite mechanical twinning in titanomagnetite and ilmenite and the transformation of plagioclase to maskelynite 4 remnant reduced birefringence the pressure estimates for bulk shock are 35 40 gpa localized shock excursions have generated three types of discrete melt zones 0 07 1 3 mm to 3 0 3 5 mm apparent diameter possessing glassy to microcrystalline groundmasses these melt pockets are differentiated on the basis of size clast volume and degree of crystallization and vesiculation melt veins and melt dikelets emanate from the melt pockets up to 3 mm into the host rock but do not necessarily connect with other melt pockets the melt pockets were generated by pressure temperature excursions of 60 80 gpa and 1600 2000 c resulting in discrete melting of adjacent host rock minerals at grain boundary margins concentric zoning in the margins of clinopyroxenes coincides with a progressive reduction in birefringence as melt pockets are approached this suggests that the shock excursions were focused as point sources in the wake of the shock front that induced bulk damage

ar ar ages and trapped ar components in martian shergottites rbt 04262 and lar 06319 abstract we made 39ar 40ar ar ar analyses of whole rock wr and mineral samples of two martian shergottites rbt 04262 rbt and lar 06319 lar in order to determine their ar ar ages and the 40ar 36ar ratios of the trapped martian ar they contain all samples released trapped excess 40ar and 36ar and suggested ar ar ages older than their formation ages because trapped ar components having different 40ar 36ar were released at different extraction temperatures we utilized only a portion of the data to derive preferred ar ar ages we obtain ar ar ages of 171 8 ma for rbt plagioclase and 163 13 ma for lar whole rock we identify two trapped ar components at low temperatures particularly for plagioclase trapped a with 40ar 36ar 285 3 was released and we believe this is most likely absorbed terrestrial air at high extraction temperatures particularly for pyroxene trapped b with 40ar 36ar 1813 127 was released the poikilitic non poikilitic texture of rbt and the presence of large pyroxene oikocrysts allowed a clear definition of trapped b this ar component is martian and its isotopic similarity to the martian atmospheric composition suggests that it may represent martian atmospheric ar incorporated into the shergottite melt via crustal rocks trapped b partitioned into pyroxene at a constant molar ratio of k 36artr 33 2 9 5 106 for rbt 04262 and 80 21 106 for lar 06319 trapped a mixed in different proportions with trapped b could give apparently intermediate trapped 40ar 36ar compositions commonly observed in shergottites

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

shocking rocks by cavitation and bubble implosion cavitation is proposed as a mechanism for the discrete generation of high pressure high temperature polymorphs in veins within meteorites and hypervelocity impact structures cavitation involves the inception isothermal growth and adiabatic implosion of bubbles their collapse has the potential of generating ultrahigh temperatures and pressures the formation of pseudotachylytic melt veins within the shocked target is critical to the cavitation process these veins facilitate bubble growth on decompression and subsequent bubble collapse on relaxation within the liquid asymmetric bubble implosion can lead to the rapid inrush of liquid to form a supersonic jet if the liquid jet strikes a solid surface i e melt vein walls or suspended clasts at high velocities then temperatures of several thousand kelvin and pressures of 10 gpa or more can be realized in the area of jet impact these conditions are capable of generating high pressure phases such as ringwoodite the spinel structured polymorph of olivine which requires formation pressures of 50 gpa these extreme effects are transient and localized being restricted to microscopic regions of liquid solid impact within the cavitating melt vein the presence of high pressure polymorphs in meteorites and impact structures does not necessarily imply that the primary shock event attained the conditions necessary for polymorph formation instead the presence of polymorphs may be the result of extreme pressure temperature excursions due to cavitation

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

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

the case for a martian origin of the shergottites nitrogen and noble gases in eeta 79001 abstract nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite eeta 79001 a nitrogen component trapped in the glass but not present in the matrix has a 15n value at least as high as 190 ratios of40ar 14n and15n 14n in the glass are consistent with dilution of a martian atmospheric component 15n 620 160 40ar 14n 0 33 0 03 by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock trapped noble gases in the glass reproduce within error the elemental and isotopic compositions measured in mars atmosphere by viking and are in general agreement with previous measurements except for much lower abundances of neutron generated krypton and xenon isotopes the most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the eeta 79001 glass and that this meteorite and thus the shergottites and probably the nakhlites and chassignites as well originated on mars nitrogen in the non glassy matrix of eeta 79001 amounts to less than 0 5 ppm and has a spallation corrected 15n value in the range 0 to 20 it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen spallogenic noble gases yield single stage exposure ages between 400 000 and 900 000 years depending on irradiation geometry trapped argon may have an unusually low36ar 38ar ratio trapped krypton except for a small excess at80kr is smoothly mass fractionated with respect to either terrestrial or chondritic kr the trapped xenon composition is consistent with addition of neutron capture radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric xe the elemental84kr 132xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio

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

petrology and chemistry of the picritic shergottite north west africa 1068 nwa 1068 we report on the petrology and chemistry of north west africa 1068 nwa 1068 a shergottite recently recovered in morocco this meteorite has a total known mass of about 577 g and comprises 23 fragments the largest fragment is a greenish brown rock devoid of fusion crust it displays a porphyritic texture consisting of a fine grained groundmass and olivine grains excluding the impact melt pockets and the minor carbonate veins produced by terrestrial weathering modal analyses indicate the following mineral proportions 52 vol pyroxenes 22 maskelynite 21 olivine 2 phosphates merrillite and chlorapatite 2 opaque oxides mainly ilmenite and chromite and sulfides and 1 k rich mesostasis olivines with various habits occur as clusters often associated with chromite or single crystals ranging in size from 50 mum to 2 millimeters megacrysts these crystals originate probably from disrupted cumulates with strong affinities with peridotitic shergottites

dynamic crystallization of shock melts in allan hills 77005 implications for melt pocket formation in martian meteorites abstract a series of crystallization experiments have been performed on synthetic glasses matching the composition of a melt pocket found in allan hills alh 77005 in order to evaluate the heterogeneous nucleation potential of the melt and the effect of oxygen fugacity on crystallization the starting temperature of the experiments varied from superliquidus liquidus and subliquidus temperatures each run was then cooled at rates of 10 500 and 1000 c h at fmq the results of this study constrain the heating and cooling regime for a microporphyritic melt pocket within the melt pocket strong thermal gradients existed at the onset of crystallization giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite skeletal olivine in the melt pocket center crystallized from a melt containing few nuclei cooled at a fast rate nearer to the melt pocket margin elongate skeletal shapes progress to hopper and equant euhedral reflecting an increase in nuclei in the melt at the initiation of crystallization and growth at low degrees of supercooling cooling from post shock temperatures took place on the order of minutes an additional series of experiments were conducted for a melt temperature of 1510 c and a cooling rate of 500 c h at the fmq buffer as well as 1 and 2 log units above and below fmq variation in chromite stability in these experiments is reflected in crystal shapes and composition and place constraints on the oxygen fugacity of crystallization of the melt pocket we conclude that the oxygen fugacity of the melt pocket was set by the fe 3 fe 2 ratio imparted by melting of the host rock rather than external factors such as incorporation of co 2 rich martian atmosphere or melting and injection of oxidized surface e g regolith material comparison with previous crystallization experiments on melt pockets in martian basalts indicate that the predominance of dendritic crystal shapes reflects the likelihood that those melt pockets with lower liquidus temperatures will be more completely melted destroying most or all nuclei in the melt in this case crystal growth takes place at high degrees of supercooling yielding dendritic shapes it appears as though the melting process is just as important as cooling rate in determining the final texture of the melt pocket as this process controls elimination or preservation of nuclei at the onset of cooling and crystallization

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

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

shock implantation of martian atmospheric argon in four basaltic shergottites a laser probe 40ar 39ar investigation spatially resolved argon isotope measurements have been performed on neutron irradiated samples of two martian basalts los angeles and zagami and two martian olivine phyric basalts dar al gani dag 476 and north west africa nwa 1068 with a similar to 50 mu m diameter focused infrared laser beam it has been possible to distinguish between argon isotopic signatures from host rock matrix minerals and localized shock melt products pockets and veins the concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured j values ar 40 ar 39 ratios and k2o wt in each phase melt pockets contain on average 10 times more gas 7 24 ppb ar 40 than shock veins and matrix minerals 0 3 3 ppb ar 40 the ar 40 ar 36 ratio of the martian atmosphere estimated from melt pocket argon extractions corrected for cosmogenie ar 36 is los angeles similar to 1852 zagami similar to 1744 and nwa 1068 similar to 1403 in addition los angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs 1 martian atmosphere in melt pockets and 2 a trapped component possibly martian interior ar 40 ar 16 480 490 in matrix minerals average apparent ar 40 ar 39 ages determined for matrix minerals in the four analyzed meteorites are 1290 ma los angeles 692 ma zagami 515 ma nwa 1068 and 1427 ma dag 476 these ar 40 ar 39 apparent ages are substantially older than the similar to 170 0474 ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40ar cosmic ray exposure cre ages were measured using spallogenic ar 16 and ar 38 production los angeles 3 1 0 2 ma zagami 2 9 0 4 ma and nwa 1068 2 0 0 5 ma yielded ages within the range of previous determinations dag 476 however yielded a young cre age 0 7 0 25 ma attributed to terrestrial alteration the high spatial variation of argon indicates that the incorporation of martian atmospheric argon into near surface rocks is controlled by localized glass bearing melts produced by shock processes in particular the larger mm size melt pockets contain near end member martian atmospheric argon based on petrography composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection three processes may have led to atmosphere incorporation 1 argon implantation due to atmospheric shock front collision with the martian surface 2 transformation of an atmosphere filled cavity into a localized melt zone and 3 shock implantation of atmosphere trapped in cracks pores and fissures

ejection ages from krypton 81 krypton 83 dating and pre atmospheric sizes of martian meteorites cosmic ray exposure cre ages and mars ejection times were calculated from the radionuclide 81 and stable kr isotopes for seven martian meteorites the following 8lkr kr cre ages were obtained los angeles 3 35 0 70 ma queen alexandra range 94201 2 22 0 35 ma shergotty 3 05 0 50 ma zagami 2 98 0 30 ma nakhla 10 8 4 0 8 ma chassigny 10 6 2 0 ma and allan hills 84001 15 4 5 0 ma comparison of these ages with previously obtained cre ages from the stable noble gas nuclei 3he 21ne and 38ar shows excellent agreement this indicates that the method for the production rate calculation for the stable nuclei is reliable in all martian meteorites we observe effects induced by secondary cosmic ray produced epithennal neutrons epithermal neutron fluxes 30 300 ev are calculated based on the reaction 79br n yp gokr we show that the neutron capture effects were induced in free space during mars earth transfer of the meteoroids and that they are not due to a pre exposure on mars before ejection of the meteoritic material neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre atmospheric sizes are estimated we obtain minimum radii in the range of 22 25 cm and minimum masses of 150 220 kg these results are in good agreement with the mean sizes reported for model calculations using current semiempirical data

modern atmospheric signatures in 4 4 ga martian meteorite nwa 7034 the nwa 7034 martian basaltic breccia dated at 4 4 ga 4 4 ga represents an entirely new type of martian meteorite however due to the unique make up of nwa 7034 compared to other martian meteorite types including its anomalous oxygen isotope ratios noble gas analyses a key tool for martian meteorite identification are important to confirm its martian origin here we report the first noble gas results for nwa 7034 which show the presence of a trapped component that resembles the current martian atmosphere this trapped component is also similar in composition to trapped gases found in the much younger shergottites 150 600 ma 150 600 ma our formation ages for the sample suggest events at 1 6 ga k ar and 170 ma u th he which are considerably younger than those observed by rb sr 2 1 ga and sm nd 4 4 ga zircons 4 4 ga however our k ar age is similar to a disturbance in the u pb zircon data at 1 7 ga which could hint that both chronometers have been subjected to disturbance by a common process or event the u th he age of 170 ma could relate to complete loss of radiogenic 4he at this time and is a similar age to the crystallisation age of most shergottites while this may be coincidental it could indicate that a single event is responsible for both shergottite formation and nwa 7034 thermal metamorphism as for cosmic ray exposure ages our favoured age is 5 ma which is outside the ranges for other martian meteorite groups and may suggest a distinct ejection event nwa 7034 shows evidence for neutron capture on br which has caused elevations in kr isotopes 80 kr and 82 kr these elevated abundances indicate significant shielding and could relate to either a large meteoroid size and or shielding in relation to a regolithic origin we have also applied similar neutron capture corrections to ar and xe data which further refine the likelihood of a modern atmospheric component though such corrections remain speculative cosmogenic production rates and noble gas data are consistent with a meteoroid radius of 50 cm fission contributions are clear in the xe data with evidence to suggest that nwa 7034 contains both 238 u and 244 pu derived fission xe components if the gas in nwa 7034 was trapped at its ancient igneous formation this would suggest little evolution of the martian atmosphere between 4 4 ga and present day however as nwa 7034 is a regolith breccia with multiple lithologies and a strong compositional similarity to gusev soils the timing and incorporation of trapped atmospheric gases is unclear with hints of resetting events at 1 5 2 1 ga 1 5 2 1 ga the atmospheric component may have been incorporated during breccia formation possibly in the amazonian though it could also have been incorporated on ejection from the surface

crystallization conditions of los angeles a basaltic martian meteorite the texture of los angeles stone 1 is dominated by relatively large 0 5 2 0 mm anhedral to subhedral grains of pyroxene and generally subhedral to euhedral shocked plagioclase feldspar maskelynite minor phases include subhedral titanomagnetite and ilmenite fe rich olivine olivine augite dominated symplectites some of which include a si rich phase and some which do not pyrrhotite phosphate s and an impact shock related alkali and silica rich glass closely associated with anhedral to euhedral silica grains observations and model calculations indicate that the initial crystallization of mg rich pigeonitic pyroxenes at 1150 c probably concomitantly with plagioclase was followed by pigeonitic and augitic compositions between 1100 and 1050 c whereas between 1050 and 920 to 905 c pyroxene of single composition crystallized below 920 to 905 c single composition fe rich clinopyroxene exsolved to augite and pigeonite initial appearance of titanomagnetite probably occurred near 990 c and fmq 1 5 whereas at and below 990 c and fmq 1 5 titanomagnetite and single composition fe rich clinopyroxene may have started to react producing ilmenite and olivine however judging from the most common titanomagnetite compositions we infer that most of this reaction likely occurred between 950 and 900 c at fmq 1 0 0 2 and nearly simultaneously with pyroxene exsolution thus producing assemblages of pigeonite titanomagnetite olivine ilmenite and augite we deem this reaction as the most plausible explanation for the formation of the olivine augite dominated symplectites in los angeles but we cannot preclude possible contributions to the symplectites from the shock related alkali and silica rich glass or shocked plagioclase and the breakdown of fe rich pigeonite compositions to olivine augite silica below 900 c reactions between fe ti oxides and silicate minerals in los angeles and other similar basaltic martian meteorites can control the t fo2 equilibration path during cooling which may better explain the relative differences in fo2 among the basaltic martian meteorites

39ar 40ar ages and origin of excess 40ar in martian shergottites we report new 39ar 40ar measurements on 15 plagioclase pyroxene and or whole rock samples of 8 martian shergottites all age spectra suggest ages older than the meteorite formation ages as defined by sm nd and rb sr isochrons employing isochron plots only los angeles plagioclase and possibly northwest africa nwa 3171 plagioclase give ages in agreement with their formation ages isochrons for all shergottite samples reveal the presence of trapped martian 40ar 40arxs which exists in variable amounts in different lattice locations some 40arxs is uniformly distributed throughout the lattice resulting in a positive isochron intercept and other 40arxs occurs in association with k bearing minerals and increases the isochron slope these samples demonstrate situations where linear ar isochrons give false ages that are too old after subtracting 40ar that would accumulate by 40k decay since meteorite formation and small amounts of terrestrial 40ar all young age samples give similar 40arxs concentrations of 1 2 x 10 6 cm 3 g but a variation in k content by a factor of 80 previously reported nasa johnson space center data for zagami shergotty yamato y 000097 y 793605 and queen alexandra range que 94201 shergottites show similar concentrations of 40arxs to the new meteorite data reported here similar 40arxs in different minerals and meteorites cannot be explained as arising from martian atmosphere carried in strongly shocked phases such as melt veins we invoke the explanation given by bogard and park 2008 for zagami that this 40arxs in shergottites was acquired from the magma similarity in 40arxs among shergottites may reveal common magma sources and or similar magma generation and emplacement processes

terrestrial and martian weathering signatures of xenon components in shergottite mineral separates abstract xenon isotopic ratios step heating release patterns and gas concentrations of mineral separates from martian shergottites roberts massif rbt 04262 dar al gani dag 489 shergotty and elephant moraine eet 79001 lithology b are reported concentrations of martian atmospheric xenon are similar in mineral separates from all meteorites but more weathered samples contain more terrestrial atmospheric xenon the distributions of xenon from the martian and terrestrial atmospheres among minerals in any one sample are similar suggesting similarities in the processes by which they were acquired however in opaque and maskelynite fractions martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon it is suggested that both martian and terrestrial atmospheric xenon were initially introduced by weathering low temperature alteration processes however the martian component was redistributed by shock accounting for its current residence in more retentive sites the presence or absence of detectable 129xe from the martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock s parent melt variable contents of excess 129xe contrast with previously reported consistent concentrations of excess 40ar suggesting distinct sources contributed these gases to the parent magma

los angeles the most differentiated basaltic martian meteorite los angeles is a new martian meteorite that expands the compositional range of basaltic shergottites compared to shergotty zagami que94201 and eet79001 b los angeles is more differentiated with higher concentrations of incompatible elements e g la and a higher abundance of late stage phases such as phosphates and k rich feldspathic glass the pyroxene crystallization trend starts at compositions more ferroan than in other martian basalts trace elements indicate a greater similarity to shergotty and zagami than to que94201 or eet79001 b but the mg fe ratio is low even compared to postulated parent melts of shergotty and zagami pyroxene in los angeles has 0 7 4 m thick exsolution lamellae 10 times thicker than those in shergotty and zagami opaque oxide compositions suggest a low equilibration temperature at an oxygen fugacity near the fayalite magnetite quartz buffer los angeles cooled more slowly than shergotty and zagami slow cooling coupled with the ferroan bulk composition produced abundant fine grained intergrowths of fayalite hedenbergite and silica by the breakdown of pyroxferroite shock effects in los angeles include maskelynitized plagioclase pyroxene with mosaic extinction and rare fault zones one such fault ruptured a previously decomposed zone of pyroxferroite although highly differentiated the bulk composition of los angeles is not close to the low ca si composition of the globally wind stirred soil of mars

noble gases and chemical composition of shergotty mineral fractions chassigny and yamato 793605 the trapped argon 40 argon 36 ratio and ejection times of martian meteorites we report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene maskelynite mesostasis glass and bulk material of shergotty and of bulk samples from chassigny and yamato 793605 the 40k 40ar isochron for the shergotty minerals yields a gas retention age of 196 ma which is within errors in agreement with previously determined rb sr internal isochron ages argon that was trapped at this time has a 40ar 36ar ratio of 1100 for chassigny and y 793605 we obtain trapped 40ar 36ar ratios of 1380 and 950 respectively using these results and literature data we show that the three shergottites shergotty zagami and que 94001 the lherzolites alh 77005 lew 88516 and y 793605 as well as chassigny and alh 84001 contain a mixture of martian mantle and atmospheric ar whereas the trapped 40ar 36ar ratio of the nakhlites nakhla lafayette and governador valadares cannot be determined with the present data we show that martian atmospheric trapped ar in martian meteorites is correlated with the shock pressure that they experienced hence we conclude that the martian atmospheric gases were introduced by shock into the meteoritic material for the shergotty minerals we obtain 3he 21ne and 38ar based cosmic ray exposure ages of 3 0 ma and for the lherzolite y 793605 4 0 ma which confirms our earlier conclusion that the lherzolites were ejected from mars 1 ma before the shergottites chassigny yields the previously known ejection age of 11 6 ma

helium loss from martian meteorites mainly induced by shock metamorphism evidence from new data and a literature compilation noble gas data from martian meteorites have provided key constraints about their origin and evolution and their parent body these meteorites have witnessed varying shock metamorphic overprinting at least 5 to 14 gpa for the nakhlites and up to 45 55 gpa e g the lherzolitic shergottite allan hills alh a77005 solar heating cosmic ray exposure and weathering both on mars and earth influences on the helium budgets of martian meteorites were evaluated by using a new data set and literature data concentrations of 3he 4he u and th are measured and shock pressures for same sample aliquots of 13 martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration partitioning of 4he into cosmogenic and radiogenic components was performed using the lowest 4he 3he ratio we measured on mineral separates 4he 3he 4 1 pyroxene of alha77005 our study revealed significant losses of radiogenic 4he systematics of cosmogenic 3he and neon led to the conclusion that solar radiation heating during transfer from mars to earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites for bulk rock we observed a correlation of shock pressure and radiogenic 4he loss ranging between 20 for chassigny and other moderately shocked martian meteorites up to total loss for meteorites shocked above 40 gpa a steep increase of loss occurs around 30 gpa the pressure at which plagioclase transforms to maskelynite this correlation suggests significant 4he loss induced by shock metamorphism noble gas loss in rocks is seen as diffusion due to 1 the temperature increase during shock loading shock temperature and 2 the remaining waste heat after adiabatic unloading post shock temperature modeling of 4he diffusion in the main u th carrier phase apatite showed that post shock temperatures of 300 c are necessary to explain observed losses this temperature corresponds to the post shock temperature calculated for bulk rocks shocked at about 40 gpa from our investigation data survey and modeling we conclude that the shock event during launch of the meteorites is the principal cause for 4he loss

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

martian low temperature alteration materials in shock melt pockets in tissint constraints on their preservation in shergottite meteorites we apply an array of in situ analytical techniques including electron and raman microscopy electron and ion probe microanalysis and laser ablation mass spectrometry to the tissint martian meteorite in order to find and elucidate a geochemical signature characteristic of low temperature alteration at or near the martian surface tissint contains abundant shock produced quench crystallized melt pockets containing water in concentrations ranging from 73 to 1730 ppm water content is positively correlated with cl content the isotopic composition of hydrogen in the shock produced glass ranges from d 2559 to 4422 water is derived from two distinct hydrogen reservoirs the martian near surface 500 and the martian mantle 100 in one shock melt pocket comprising texturally homogeneous vesiculated glass the concentration of h in the shock melt decreases while simultaneously becoming enriched in d attributable to the preferential loss of h over d to the vesicle while the pocket was still molten while igneous sulfides are pyrrhotite in composition fe 0 88 0 90 s the iron to sulfur ratios of spherules in shock melt pockets are elevated up to fe 1 70 s which we attribute to shock oxidation of igneous pyrrhotite and the formation of hematite at high temperature the d and cl enrichment and higher oxidation of the pockets as indicated by hematite support a scenario in which alteration products formed within fractures or void spaces within the rock the signature of these alteration products is preserved within shock melt now glass which formed upon collapse of these fractures and voids during impact shock thermal modeling of tissint shock melt pockets using the heat program demonstrates that the shock melt pockets with the greatest potential to preserve a signature of aqueous alteration are small isolated from other regions of shock melt vesicle free and glassy

the case for old basaltic shergottites abstract the crystallization age of shergottites is currently not agreed upon although mineral 87 rb 87 sr 147 sm 143 nd 176 lu 176 hf and u pb isochrons all give very young ages typically in the range of 160 180 ma 207 pb 206 pb data support a much older crystallization age at 4 1 ga which is consistent with published whole rock 87 rb 87 sr data on basaltic shergottites different isotopic systems present different complexities but crater counting chronology which shows that a substantial fraction of the martian surface was resurfaced during the late heavy bombardment is in favor of an old martian lithosphere with ages in accordance with pb pb and rb sr isotopic data a 4 1 ga pb pb age of shergottites also agrees with the 142 nd and 182 w anomalies found in these rocks and concur with the presence of an actively convecting mantle during the first 500 myr of the planet s history we here present new sm nd lu hf and pb pb mineral isochrons for the basaltic shergottites shergotty and los angeles complementing our previous results on zagami bouvier a blichert toft j vervoort j d and albarede f 2005 the age of snc meteorites and the antiquity of the martian surface earth planet sci lett 240 221 233 the internal 147 sm 143 nd and 176 lu 176 hf isochrons give young ages of respectively 172 40 mswd 2 0 and 188 91 mswd 3 1 for shergotty and 181 13 mswd 0 14 and 159 42 mswd 0 01 for los angeles in contrast the pb isotope compositions of the leached whole rock fragments and maskelynite separates of shergotty and los angeles fall on the whole rock isochron previously established for zagami and other shergottite samples and collectively yield a pb pb age of 4050 70 ma for the crystallization of the basaltic shergottite suite the contrast between the 170 ma ages of internal isochrons and the 4 1 ga age supported by pb pb and 87 rb 87 sr on whole rocks simply reflects that the younger age dates the perturbation of a suite of rocks of noachian age the internal rb sr sm nd lu hf and u pb errorchrons are heavily biased by the presence of disseminated phosphate minerals and inclusions for which d h ratios d 4600 indicate strong interaction with martian subsurface waters in contrast baddeleyite occasionally present in sncs and also having extremely young u pb ages reflect resetting under the shock conditions that prevailed either during shergottite extraction from the planet or from impacts associated with a major break up event of planetesimals in the main asteroid belt we finally also re examine 39 ar 40 ar data on snc meteorites and suggest that they as well support old crystallization ages

k ar dating of rocks on mars requirements from martian meteorite analyses and isochron modeling radiometric age dating of martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand martian history performing k ar age dating of igneous rocks on mars by robots however presents technical challenges some of these challenges can be defined by examining ar ar data acquired on martian meteorites and others can be evaluated through numerical modeling of simulated k ar isochrons like those that would be acquired robotically on martian rocks excess 40ar is present in all shergottites thus for martian rocks the slopes of k ar isochrons must be determined to reasonable precision in order to calculate reliable ages model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age issues addressed here are how many k ar analyses should be made of rocks thought to have the same age what range of k concentrations should these analyzed samples have and what analytical uncertainty in k ar measurements is desirable meteorite data also are used to determine the d a 2 diffusion parameters for ar in plagioclase and pyroxene separates of several shergottites and nakhlites these data indicate the required temperatures and times for heating similar martian rocks in order to extract ar quantitatively extracting radiogenic 40ar could be difficult and degassing cosmogenic ar from mafic phases even more so considering all these factors robotic k ar dating of martian rocks may be achievable but will be challenging

context matters ar ar results from in and around the manicouagan impact structure canada implications for martian meteorite chronology abstract as an analog for interpretations of the ages of martian shergottite meteorites we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the manicouagan impact structure canada plagioclase from the impact melt sheet at manicouagan yields an age of 215 40 0 16 ma which indicates the time of impact plagioclase from a clast within melt bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 3 ma and 218 7 ma respectively which are interpreted to have been reset by contact metamorphism from the impact melt country rocks that were unaffected by the impact gives 849 ma ages consistent with the known grenvillian target rock history maskelynite amorphous plagioclase which has been transformed in the solid state yields an age of 567 6 ma this age is geologically meaningless because it is not consistent with the target age the impact age or regional metamorphic ages at manicouagan our results show that maskelynite argon ages are not meaningful and that context is critical for proper interpretation of impact affected argon ages

geology mineralization and emplacement of the foy offset dike sudbury impact structure the foy offset dike is the longest of the known radial dikes associated with the 1 85 ga sudbury igneous complex the foy dike extends for at least 30 km from the complex into the north range archean and proterozoic footwall rocks of the sudbury impact structure the dike ranges in width from 400 m where it connects to the sudbury igneous complex to 50 m at its most distal known exposure we divide the dike into a proximal segment and a distal segment based on where it intersects the concentric hess offset dike three main dike lithologies are distinguished all referred to as varieties of quartz diorite inclusion rich pyroxene quartz diorite occurs in the embayment adjacent to the base of the main mass of the sudbury igneous complex this constitutes the sublayer northward the sublayer gives way to amphibole biotite inclusion bearing quartz diorite and amphibole biotite inclusion poor quartz diorite the dike is mineralized and deposits in the proximal segment have been mined mineralization is associated with the inclusion rich dike phase at the nickel offset mine the dike contains inclusion bearing massive ni cu sulfides that also host platinum group minerals the orebodies occurred as strike parallel sheets within the central parts of the dike in association with inclusion bearing quartz diorite the geochemistry of the dike lithologies indicates that both the inclusion poor and inclusion bearing quartz diorite varieties are close in composition to the postulated bulk composition of the sudbury igneous complex this implies that the dike was emplaced prior to any significant fractionation of the main melt body and hence very early in the evolution of the sudbury complex we envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion poor quartz diorite phases were still unconsolidated uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts primarily via gravitationally driven intrusion to the primitive polymict impact melt breccias and sulfide ores the chilled margin compositions of the foy inclusion poor quartz diorite are considered good indicators of the original bulk composition of the sudbury igneous complex this idea is supported by comparison with calculated estimations of the bulk composition a shift in geochemical trends a grain size increase and a decrease in inclusion content occurs 22 km along the length of the dike which we believe corresponds to a fault system that downfaults to the north given that the dense ore would have preferentially settled toward the base of the foy dike fracture system postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment

a new natural high pressure na ca hexaluminosilicate caxna1 x al3 xsi3 xo11 in shocked martian meteorites a ca na hexaluminosilicate whose ca end member was previously synthesized in numerous high pressure experiments has been identified by raman spectroscopy in heavily shocked martian meteorites this mineral has a structural formula close to caxna1 x al3 xsi3 xo11 and is similar to the calcium aluminum silicate phase previously synthesized in high pressure experiments performed on anorthite and rocks of basaltic composition this new mineral occurs in shock melt pockets in two distinct settings and is intimately intergrown with sio2 stishovite the first setting encountered in zagami consists of idiomorphic equant crystals overgrown by acicular stishovite that crystallized from a melt of labradorite composition the second setting contains the na ca hexaluminosilicate phase intergrown with stishovite and hollandite and was formed during partial melting at high pressures the mineralogical association na ca hexaluminosilicate stishovite was observed in shock melt pockets which have distinct bulk compositions in seven martian shergottites this new mineral represents after majorite the second natural occurrence of a silicate mineral with silicon in both four and six coordination the assemblage stishovite na ca hexalummosilicate sets constraints on the pressure and temperature conditions that prevailed during shock in some of the studied meteorites the na ca hexaluminosilicate mineral is a potential carrier of al and na during subduction of oceanic crust in the lower mantle of the earth c 2003 elsevier b v all rights reserved

atmospheric effects on cratering on venus a paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock induced modifications surrounding venusian impact craters we apply two dimensional oblique shock dynamics to describe a three dimensional paraboloidal shaped bow shock impinging upon an assumed incompressible venusian surface the effects of the hypersonic atmospheric shock acting on the venusian surface are considered in terms of induced maximum gas pressure density particle velocity and temperature for varying angles and velocities of impact the maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered the present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity and radii of impactors for a 1 km radius stony meteorite impacting normally at 20 km s the radius of the disturbed area extends 10 17 times the 3 5 km crater radius the perturbed surface affects the surface radar properties and the present results can provide an explanation of the wide dark bright halos surrounding some of the venusian impact craters observed via magellan imagery for example a 50 km radius bright halo surrounding a 20 km dark halo is observed around the 3 1 km radius crater located at 16 5 north latitude and 334 4 longitude the average value of the radar backscatter cross section of the 20 km radius dark halo indicates that 50 cm thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface the bright halo indicates that the surface roughness in this region is 30 greater than that of the surrounding original surface these features can be induced by atmospheric shock waves the present model can relate the observed crater halo radii to the impact parameters such as projectile radius and density and the impact velocity and angle

experimental approach to generate shock veins in single crystal olivine by shear melting a shock experiment has been devised to produce large shear in a single crystal sample of olivine the recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine the shear faults appear as thin veins containing small grains of olivine and pockets of glass the microstructure and composition of the material in the veins point to fractional crystallization of a melt an order of magnitude calculation is consistent with the idea that the veins were produced by shear melting these results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities

crystallization of the basaltic shergottites insights from crystal size distribution csd analysis of pyroxenes quantitative petrographic analysis using the crystal size distribution csd method provides a novel approach for examining the crystallization histories of basaltic shergottites grain number densities at different sizes are plotted against grain size and the resulting curve relates to the geologic processes involved with the crystallization of the grain population most basaltic shergottites are dominated by pigeonite and augite and because plagioclase is primarily interstitial and therefore constrained in its growth by the surrounding pyroxenes we limited our size measurements to the pyroxene phases the groundmasses of elephant moraine eet a79001 lithology a and dar al gani dag 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass their simple linear csd plots record a single stage of pyroxene crystallization under steady state conditions of continuous nucleation and growth the textures of queen alexandra range que 94201 and eeta79001 lithology b are quite different from the other shergottites with intergrown pyroxene and plagioclase likewise their csd plots are also distinct with curved trends that suggest a lack of large grains most likely because of interference between simultaneously growing silicate phases however the csd plot shapes are smooth also implying a single stage of growth shergotty and zagami with coarser cumulus textures display csd plots that are generally linear over most grain sizes this implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations both plots however also display kinks implying multiple stages of growth a similar kink is also visible in a csd plot of only the mg rich cores of shergotty pyroxenes which suggests the feature represents changes in conditions during core crystallization rather than an event coincident with the change in composition to the fe rich rims the plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation with continued crystallization associated with ascent of the magma eruption onto the surface probably triggered the compositional change to fe rich rims the csd analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for zagami were on the order of a few tenths of a degree per hour growth rates derived from these cooling rates suggest crystallization of shergotty and zagami pyroxenes occurred over a period of a few weeks to months

a laser probe 40ar 39ar investigation of poikilitic shergottite nwa 4797 implications for the timing of shock metamorphism spatially resolved argon isotope measurements have been performed on neutron irradiated samples of nw africa nwa 4797 shock heating of nwa 4797 completely melted and vesiculated precursor igneous plagioclase which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition ca k ratios using a focused ultraviolet laser beam is has been possible to distinguish between argon isotopic signatures from groundmass minerals igneous olivine pyroxene plagioclase and a shock vein this study focuses on the potential for this meteorite to shed light on shock ages of shergottites apparent 40 ar 39 ar ages of groundmass minerals show that there are large amounts of excess argon in this phase yielding a wide range of calculated ages from 690 30 ma to several apparent ages older than 4 5 ga a traverse of laser probe extractions across the 1 mm diameter shock vein in nwa 4797 yielded apparent 40 ar 39 ar ages younger than the groundmass a signature of the martian atmosphere identified by 40 ar 36 ar ratios of 1600 1900 was not found in the nwa 4797 shock vein this is distinct from other shergottites where the products of shock melting contain a nearly pure sample of martian atmosphere we attribute this to a distinct formation mechanism and hence gas trapping mechanism of the nwa 4797 shock vein we undertook 44 analyses of plagioclase areas identified by sem analysis ages ranged from 45 27 to 3771 109 ma and yield an average age of 375 77 ma considerably younger than ages obtained in this study from either the groundmass or the shock vein a plot of age v 37 ar 39 ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured older ages are correlated with higher ca k ratios of plagioclase indicating contamination from groundmass minerals rich in excess argon the youngest ages correlate to plagioclase extractions with the lowest ca k ratios interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a k rich mesostasis we see no evidence for multiple shock events in nwa 4797 rather we favour the interpretation that the cosmic ray exposure cre age of 3 0 0 5 ma obtained on nwa 4797 in this study using cosmogenic 38 ar approximates the timing of shock melting in this meteorite

localized shock melting in lherzolitic shergottite northwest africa 1950 comparison with allan hills 77005 the lherzolitic martian meteorite northwest africa nwa 1950 consists of two distinct zones 1 low ca pyroxene poikilically enclosing cumulate olivine fo70 75 and chromite and 2 areas interstitial to the oikocrysts comprised of maskelynite low and high ca pyroxene cumulate olivine fo68 71 and chromite shock metamorphic effects most likely associated with ejection from the martian subsurface by large scale impact include mechanical deformation of host rock olivine and pyroxene transformation of plagioclase to maskelynite and localized melting pockets and veins these shock effects indicate that nwa 1950 experienced an equilibration shock pressure of 35 45 gpa large millimeter size melt pockets have crystallized magnesian olivine fo78 87 and chromite embedded in an fe rich al poor basaltic to picro basaltic glass within the melt pockets strong thermal gradients minimum 1 c m existed at the onset of crystallization giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite dendritic and skeletal olivine crystallized in the melt pocket center has a nucleation density 1 0 x 10 3 crystals mm 2 that is two orders of magnitude lower than olivine euhedra near the melt margin 1 6 x 10 5 crystals mm 2 based on petrography and minor element abundances melt pocket formation occurred by in situ melting of host rock constituents by shock as opposed to melt injected into the lherzolitic target despite a common origin nwa 1950 is shocked to a lesser extent compared to allan hills alh 77005 45 55 gpa assuming ejection in a single shock event by spallation this places nwa 1950 near to alh 77005 but at a shallower depth within the martian subsurface extensive shock melt networks the interconnectivity between melt pockets and the ubiquitous presence of highly vesiculated plagioclase glass in alh 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting

on the siting of gases shock emplaced from internal cavities in basalt abstract noble gases were extracted by stepped combustion and crushing from basalts which contained gas filled cavities of controlled sizes prior to shock at 40 gpa analysis of fractions enriched and depleted in shock glass from a single sample gave a factor of 2 higher gas abundances in the glass rich separate release patterns were nearly identical suggesting similar siting in glass in both fractions crushing of a sample released 45 of implanted noble gases but only 17 of n 2 indicating that most or all of the noble gas was trapped in vesicles analysis by sem eds confirmed the presence of vesicles in glassy areas with an average diameter of 10 m samples with relatively large pre shock cavities were found to consist of up to 70 80 glass locally and generally exhibit greater local shock effects than solid and densely packed paniculate targets at the same shock pressure though the latter give higher gas emplacement efficiencies the petrographic results indicate that in situ production of glassy pockets grossly similar to those in the shergottite eeta 79001 is possible from shock reverberations in the vicinity of a vug however the siting of the gases points to a more complex scenario in which spb gas and melt material were probably injected into eeta 79001

martian regolith in elephant moraine 79001 shock melts evidence from major element composition and sulfur speciation abstract shock veins and melt pockets in lithology a of martian meteorite elephant moraine eeta 79001 have been investigated using electron microprobe em analysis petrography and x ray absorption near edge structure xanes spectroscopy to determine elemental abundances and sulfur speciation s 2 versus s 6 the results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances the xanes spectra for eeta79001 glasses show a sharp peak at 2 471 kev characteristic of crystalline sulfides and a broad peak centered at 2 477 kev similar to that obtained for sulfide saturated glass standards analyzed in this study sulfate peaks at 2 482 kev were not observed bulk compositions of eeta79001 shock melts were estimated by averaging defocused em analyses vein and melt pocket glasses are enriched in al ca na and s and depleted in fe mg and cr compared to the whole rock petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins contributing to the enrichments estimates of shock melt bulk compositions obtained from glass analyses are biased towards fe and mg depletions because in general basaltic melts produced from groundmass minerals plagioclase and clinopyroxene will quench to a glass whereas ultramafic melts produced from olivine and low ca pyroxene megacrysts crystallize during the quench we also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt pyroxene en 72 75 fs 20 21 wo 5 7 and olivine fo 75 80 reconstruction of glass crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology a of the meteorite reflecting bulk melting of everything except xenolith chromite our results show that eeta79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts which can account for the observed compositional anomalies compared to the whole rock sample the observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites rocks from terrestrial impact structures and other martian meteorites the bulk composition of shock melts reflects the proportions of minerals melted large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy in the latter the modal abundance of sulfide globules may reach up to 15 vol we conclude the shock melt pockets in eeta79001 lithology a contain no significant proportion of martian regolith

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

isotopic and petrographic evidence for young martian basalts radiometric age data for shergottites yield ages of 4 0 ga and 180 575 ma the interpretation of these ages has been and remains a subject of debate here we present new 39 ar 40 ar laser probe data on lherzolitic shergottites allan hills alh 77005 and northwest africa nwa 1950 these two meteorites are genetically related but display very different degrees of shock damage on a plot of 40 ar 36 ar versus 39 ar 36 ar the more strongly shocked alh 77005 45 55 gpa does not yield an array of values indicating an isochron but the data are highly scattered with the shock melts yielding 40 ar 36 ar ratios of 1600 2026 apparent ages calculated from these extractions range from 374 8183 ma with 50 of the data particularly from the shock melts yielding impossibly old ages 4 567 ga on the same plot extractions from igneous minerals in the less shocked nwa 1950 30 44 gpa yield a fitted age of 382 36 ma argon extractions from the shock melts are well distinguished from minerals with the melts exhibiting the highest 40 ar 36 ar ratios 1260 1488 and the oldest apparent ages laser step heating was also performed on maskelynite separates from nwa 1950 yielding ages of 1000 ma at the lowest release temperatures and ages of 360 and 362 ma at higher temperature steps stepped heating data from previous studies have yielded ages of 500 and 700 ma to 1 7 ga for alh 77005 maskelynite separates if the ages obtained from igneous minerals represent undegassed argon from an ancient 4 0 ga rock then the ages are expected to anticorrelate with the degree of shock heating the data do not support this inference our data support young crystallization ages for minerals and martian atmosphere as the origin of excess 40 ar in the shock melts the shock features of shergottites are also reviewed in the context of what is known of the geologic history of the martian surface through remote observation the oldest most heavily cratered surfaces of mars are thought to be 4 0 ga we contend that ancient rocks from mars noachian 3 5 ga are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era young rocks late amazonian 0 6 ga should record a relatively simple history of emplacement and ejection from the near surface we show that although shergottites are strongly shocked they are relatively pristine crystalline igneous rocks and not pervasively altered breccias the petrography of shergottites is at odds with an ancient age interpretation a model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible

understanding the textures and origin of shock melt pockets in martian meteorites from petrographic studies comparisons with terrestrial mantle xenoliths and experimental studies we present a textural comparison of localized shock melt pockets in martian meteorites and glass pockets in terrestrial mantle derived peridotites specific textures such as the development of sieve texture on spinel and pyroxene and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets based on petrographic and compositional observations it is concluded that void collapse variable shock impedance is able to account for the occurrence of pre terrestrial sulfate bearing secondary minerals in the melts high gas emplacement efficiencies and s al ca and na enrichments and fe and mg depletion of shock melt compositions compared to the host rock previously used as arguments against such a formation mechanism recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in martian meteorites

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

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

zagami product of a two stage magmatic history abstract large specimens of the zagami shergottite show highly varied grain sizes and mineral abundances on a cm scale and preferred alignment of pyroxene laths we document the presence of whitlockite and of melt veins and pockets of shock origin pyroxene crystals have homogeneous mg rich pigeonite and augite cores overgrown by fe rich zoned pyroxene mostly pigeonite rims amphibole bearing magmatic melt inclusions occur exclusively in the cores we conclude that zagami experienced a twostage crystallization history the first stage occurred in a deep seated slowly cooling magma chamber there the homogeneous mg rich cores of the pyroxenes crystallized during relatively slow cooling the deep seated origin of the cores is also indicated by the presence of amphibole within them which requires pressures of formation equivalent to crystallization at depths 7 5 km on mars modest abundance of homogeneous mg rich cores 15 20 indicates that crystal settling did not play a significant role in this part of the magma chamber during the second stage the mg rich pyroxenes were entrained into a magma that either intruded to the near surface and cooled in a relatively thin dike or sill or extruded to the surface and crystallized in a lava flow 10 m thick again without indications for crystal settling this scenario is suggested by estimates of cooling rates of 0 1 0 5 c h based on sizes of the plagioclase maskelynite crystals and of 0 02 c h based on the width of pyroxene exsolution lamellae brearley 1991 irregular shapes of the mg rich cores result from resorption and possible solid state diffusion and the shapes and sizes of the pyroxenes after crystallization of fe rich pigeonite rims onto the cores were strongly controlled by the shapes and sizes of the cores the finer grained areas of the rock inherited smaller and more numerous mg rich pyroxene cores from the first stage than did the coarser grained areas crystallization of both augite and pigeonite cores at depth but mostly pigeonite rims in the nearsurface environment may be the result of a phase boundary shift and expansion of the pigeonite stability field at lower pressures the estimated depth of the magma chamber for zagami of 7 5 km and thickness of the putative lava flow of 10 m are consistent with calculations and observations of volcanic constructs and flows in the tharsis region of mars

fe3 in shocked olivine crystals of the alha 77005 meteorite abstract alha 77005 a shocked achondrite of the shergottite group is unique in containing cumulus olivine crystals which display a brownish color in thin section mossbauer epr and optical spectroscopic studies show the presence of fe 3 in the olivine approximately 4 5 wt of the total iron in the olivine 24 wt as feo is in the trivalent state the fe 3 ions preferentially occupy m2 lattice positions charge transfer between fe 3 and fe 2 as well as between fe 3 and oxygen ligands causes a broad absorption band in the optical spectrum at high energies which is the reason for the olivine color it is concluded that the color of the alha 77005 olivine is preterrestrial and may be the result of shock induced oxidation of the olivine crystals on the meteorite parent body

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

stability assessment of himalayan road cut slopes along national highway 58 india himalaya is one of the most tectonically and seismically active mountain chains in the world having complex geological and geotechnical conditions the himalayan region experiences frequent slope failure posed due to various natural and anthropogenic causes slope instability issues have consequent effects on the socio economic development of the people and the region in a large scale in the present study stability analysis of vulnerable road cut slopes along nh 58 from rishikesh to devprayag in the lesser himalayas has been conducted critical slopes were identified by considering the geological and the geotechnical complexities within the region rock mass characterisation techniques have been employed for slope stability assessment rock mass rating rmr slope mass rating smr and continuous slope mass rating csmr methods have been applied to evaluate different stability levels of rock mass along the highway spatial variation of stability classes using rmr smr and csmr techniques has been analysed on geographic information system gis tool kinematic analysis technique was also employed to identify the different modes of structurally controlled failures in jointed rock mass accordingly remedial measures have been suggested to improve slope stability

volatiles nitrogen noble gases in recently discovered snc meteorites extinct radioactivities and evolution abstract we report noble gas and nitrogen analyses of newly discovered snc meteorites one nakhlite nwa817 and four shergottites nwa480 nwa856 nwa1068 and sau 005 the k ar age 1 3 ga as well as the cosmic ray exposure cre age 10 0 1 3 ma of nakhlite nwa817 agree with data of nakhla the cre ages of nwa480 nwa856 and nwa1068 2 35 0 20 2 60 0 21 and 2 01 0 65 ma respectively are consistent within uncertainties with other basaltic shergottites but the cre age of sau 005 1 25 0 07 ma is distinct and indicates a different ejection event bulk k ar ages of all shergottites exceed the reported radiometric ages and reveal the presence of inherited radiogenic 40 ar in basaltic lavas the isotopic composition of nitrogen trapped in these snc meteorites is not homogeneous since 15 n values of either 15 to 20 or 45 indicate different nitrogen reservoirs all shergottites contain fission xenon from 238 u and fission xe of extinct t 1 2 82 ma 244 pu previously identified in alh84001 in chassigny and in nakhla is also present in at least one shergottite nwa856 the shergottites contain less fissiogenic xe than other snc suggesting that either their source was more degassed or that the magma source region closed at a later time in nakhlites fission xenon from 244 pu correlates with uranium a geochemical proxy of plutonium thus it is possible that fissiogenic xe was not inherited during magma differentiation but rather was produced in situ and retained in refractory mineral assemblages in this interpretation the magma evolution that settled the mineralogy and geochemistry of nakhlites took place at a time when 244 pu was alive and pre dated the late events recorded in their radiometric ages alternatively fissiogenic xenon was trapped from a mantle source during closed system evolution of the parent magmas in which case such evolution might have taken place at considerable depth pressure in order to inhibit magma degassing during the course of differentiation

mineralogy petrology and shock history of lunar meteorite sayh al uhaymir 300 a crystalline impact melt breccia sayh al uhaymir sau 300 comprises a microcrystalline igneous matrix grain size s4 the association of maskelynite with melt pockets and shock veins indicates a subsequent local 28 45 gpa shock stage s2 s3 excursion which was probably responsible for lofting the sample from the lunar surface subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor

pseudotachylyte petrogenesis constraints from the sudbury impact structure pseudotachylytes and their host rocks from the north range of the 1 85 ga sudbury impact structure have been investigated using analytical scanning electron microscopy electron microprobe analysis and xrf spectrometry the results show that the pseudotachylytes were produced in high speed slip zones by the frictional comminution and selective melting of wall rock lithologies the preferential assimilation of hydrous ferromagnesian phases during frictional melting produced relatively basic melts leaving the more mechanically resistant quartz and to a lesser extent plagioclase as included mineral clasts three distinct assemblages are identified within the pseudotachylytes a pre impact 1 85 ga rock and mineral clasts derived from host lithologies b a syn to immediately post impact 1 85 ga rapidly cooled quartz sanidine labradorite phlogopitic biotite matrix assemblage formed due to crystallization from a melt at 800 900 c and c a post impact 1 85 ga retrograde assemblage which overprints both clasts and matrices field evidence indicates that most pseudotachylyte formed in large displacement fault systems during gravitational collapse of the impact generated transient cavity the sudbury pseudotachylytes like endogenic pseudotachylytes were generated by frictional melting on fault surfaces the difference is primarily one of scale large km displacements occurring on impact induced ring faults can generate immense volumes of friction melt resulting in spectacular pseudotachylyte bodies up to 0 5 km thick and more than 10 km long

pleistocene glass in the australian desert the case for an impact origin irregular masses and flat slabs of vesicular slaglike and glassy melt referred to herein as edeowie glass are locally abundant on a desert plain in central south australia where the material appears to be associated with an old land surface being exhumed by deflation and water erosion the slabs of melt are associated with outcrops of baked sediment having very similar geochemistry suggesting an origin by in situ surface fusion embedded clasts displaying shock metamorphism in quartz suggest that the thermal source may have been in some way associated with an impact event although an obvious crater is lacking if edeowie glass is related to impact a different thermal mechanism from that generally ascribed to the production of impact melt is required because of evidence for in situ generation of melt distal from any crater ar 40 ar 39 laser probe dating of two samples has produced overlapping dates of 0 67 0 07 and 0 78 0 33 ma

a reappraisal of the martian 36ar 38ar ratio the currently used value of 4 1 0 2 for the 36 ar 38 ar isotopic ratio in the martian atmosphere is reappraised and a significantly lower ratio is suggested previous analyses of noble gases in impact glass from some martian meteorites demonstrate that large quantities of martian atmospheric gases were shock emplaced into these samples however several observations indicate that this trapped martian gas is composed of two components one atmospheric and one probably from the martian mantle these observations include large variations in 36 ar 38 ar during stepwise gas release variations in the ar kr xe elemental ratios among martian meteorites and variations in the cosmogenic corrected trapped 36 ar 38 ar ratios over 3 5 4 3 among shock glass samples uncertainties in applied corrections for cosmogenic ar cannot explain these variations by assuming a range of reasonable values for martian atmospheric and mantle 40 ar 36 ar the observed 40 ar 36 ar ratios in shock glass are used to correct for the mantle component and to derive a more precise ratio for the atmospheric component using this procedure one obtains an upper limit for martian atmospheric 36 ar 38 ar of 3 9 and a probable but poorly defined range for martian atmospheric 36 ar 38 ar of 3 0 3 6 these lower ratios imply that the degree of mass fractionation during ar loss from mars upper atmosphere is considerably larger than that previously calculated

laboratory shock emplacement of noble gases nitrogen and carbon dioxide into basalt and implications for trapped gases in shergottite eeta 79001 abstract basalt samples have been analyzed mass spectrometrically for shock implanted noble gases n2 and co2 after exposure to 20 60 gpa 200 600 kbar shock in the presence of 0045 3 0 atm of ambient gas isotopically labelled gases were used in some samples to distinguish implanted atmospheric gases from contaminant or indigenous gases abundances of emplaced gases varied linearly with ambient gas pressure for constant shock pressure and gas emplacement was most efficient in the range of 35 50 gpa shock pressure uncrushed samples shocked in this range gave emplacement efficiencies number densities of gases in shocked samples number densities in ambient gas of 2 0 6 7 for heavy noble gases and nitrogen while powdered samples with higher effective porosity yielded 40 50 efficiencies indicating that 50 of noble gases and nitrogen available in porespaces was emplaced no elemental or isotopic fractionation was detected with ar kr xe or n2 helium and in some samples ne were lost by diffusion subsequent to shock emplacement efficiencies for co2 averaged a factor of 1 8 0 2 greater than those of n2 and noble gases in 4 samples with 20 50 gpa shock and yielded 3 2 times greater efficiency in a sample shocked to 60 gpa enhanced co2 emplacement is thought to be due to reaction with silicate materials data from shergottite eeta 79001 glass may be consistent with co2 efficiencies a factor of 2 3 greater than noble gas efficiencies trapped gases in eeta 79001 glass were probably emplaced by shock however apparent emplacement efficiencies are somewhat higher than even shocked powder samples possible explanations for the difference include atmospheric overpressure at the time of shock trapping of gas already in vugs by intruding melt material or collapse of gas filled vugs to form gas laden glass inclusions not enough is yet known about the behavior of vugs under shock to discount the latter possibility

new lithologies in the zagami meteorite evidence for fractional crystallization of a single magma unit on mars abstract zagami consists of a series of increasingly evolved magmatic lithologies the bulk of the rock is a basaltic lithology dominated by pigeonite fs 28 7 54 3 augite fs 19 5 35 0 and maskelynite ab 42 53 approximately 20 vol of zagami is a basaltic lithology containing feo enriched pyroxene pigeonite fs 27 0 80 8 and mm to cm sized late stage melt pockets the melt pockets are highly enriched in olivine bearing intergrowths mesostases phosphates both whitlockite and water bearing apatite fe ti oxides and sulfides the systematic increases in abundances of late stage phases fs and incompatible element e g y and the rees contents of pigeonite ab contents of maskelynite and feo concentrations of whitlockite all point to a fractional crystallization sequence the crystallization order in zagami and the formation of these various lithologies was controlled by the abundances of iron phosphorus and calcium during fractional crystallization iron and phosphorus enrichment occurred ultimately forcing the crystallization of calcium phosphates and olivine bearing intergrowths the limited amount of calcium in the melt and its partitioning between phosphates and silicates controlled the crystallization of phosphates plagioclase pigeonite and augite the presence of these feo enriched water poor late stage lithologies has important implications discrepancies between experimental and petrologic studies to infer the history of basaltic shergottites may be partially explained by the use of starting compositions which are too feo poor in the experimental studies the water poor nature of the late stage melt pockets suggests crystallization from a very dry magma although whether this magma was always dry or experienced significant near surface degassing remains an open question finally the presence of fractional crystallization products within zagami suggests that this may be a relatively common process on mars