feldspathic clasts in yamato 86032 remnants of the lunar crust with implications for its formation and impact history abstract low concentrations of th and fe in the yamato y 86032 bulk meteorite support earlier suggestions that y 86032 comes from a region of the moon far distant from the procellarum kreep terrain pkt probably from the lunar farside 39 ar 40 ar rb sr sm nd and sm isotopic studies characterize the chronology of y 86032 and its precursors in the mega regolith one of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with an 93 i e more sodic than lunar fans but with very low 87 rb 86 sr and 87 sr 86 sr similar to those of fans fan stands for ferroan anorthosite this an93 anorthosite has nd isotopic systematics similar to those of nearside norites a fan like an97 anorthosite is present in a second light colored feldspathic breccia clast and has a more negative e nd value consistent with residence in a lree enriched environment as would be provided by an early plagioclase flotation crust on the lunar magma ocean lmo this result contrasts with generally positive values of e nd for apollo 16 fans suggesting the possibility of assymetric development of the lmo other possible explanations for the dichotomy in e nd values are advanced in the text the y 86032 protolith formed at least 4 43 0 03 ga ago as determined from a sm nd isochron for mineral fragments from the breccia clast composed predominantly of an93 anorthosite and a second clast of more varied composition we interpret the mineral fragments as being predominatly from a cogenetic rock suite an 39 ar 40 ar age of 4 36 4 41 0 035 ga for a third clast composed predominantly of an97 anorthosite supports an old age for the protolith initial 143 nd 144 nd in that clast was 0 64 0 13 e units below 143 nd 144 nd in reservoirs having chondritic sm nd ratios consistent with prior fractionation of mafic cumulates from the lmo a maximum in the 39 ar 40 ar age spectrum of 4 23 0 03 ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive 40 ar loss lack of solar wind and lunar atmosphere implanted ar in the light gray breccia clast allows determination of an 39 ar 40 ar age of 4 10 0 02 ga which is interpreted as the time of initial brecciation of this litholgy after correction for implanted lunar atmosphere 40 ar impact melt and dark regolith clasts give ar ages of 3 8 0 1 ga implying melt formation and final breccia assembly 3 8 ga ago some breccia lithologies were exposed to thermal neutron fluences of 2 10 15 n cm 2 only about 1 of the fluence experienced by some other lunar highlands meteorites other lithologies experienced neutron fluences of 1 10 15 n cm 2 thus y 86032 spent most of the time following final brecciation deeply buried in the megaregolith the neutron fluence data are consistent with cosmogenic 38 ar cos cosmic ray exposure ages of 10 ma variations among differing lithologies in the amount of several regolith exposure indicators including cosmogenic noble gas abundances neutron capture induced variations in sm isotopic abundances and ir contents are consistent with a period of early 3 8 ga ago lunar regolith exposure subsequent deep burial at 5 m depth and ejection from the moon 7 10 ma ago

samarium neodymium and rubidium strontium systematics of nakhlite governador valadares abstract the sm nd systematics of whole rock and mineral separate samples from nakhlite governador valadares define a good 147sm 143nd mineral isochron age of 1 37 0 02 ga this age is in excellent agreement with the 39ar 40ar and rb sr ages obtained previously for this meteorite however the rb sr isotopic data for our sample show that the isotopic system is disturbed the lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases the whole rock and acid washed mineral data yield a rb sr age of 1 20 0 05 ga which probably represents a lower limit to the crystallization age of the rock the petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill mcsween 1994 thus the sm nd isotopic age probably represents the age of such a magmatic event the initial e143nd value determined for the rock at 1 37 ga is 17 1 indicating that the parent magma of the rock came from a light rare earth element depleted source of 147sm 144nd 0 237 based on a simple two stage evolution model results of the same model calculation for the initial 87sr 86sr ratio of the rock suggest that its source material was depleted in 87rb 86sr by 50 relative to the estimated martian value at 1 37 ga both the high sm nd and low rb sr values support a clinopyroxene rich cumulate source for the genesis of the nakhlite governador valadares furthermore our sm nd age and e143nd data and the previously published e142nd datum for the rock harper et al 1995 are consistent with early differentiation of the parent planet formation of cumulate sources 4 56 ga ago and late melting of the sources and formation of the rock 1 37 ga ago the good agreement of isotopic ages and petrographic features among governador valadares nakhla and lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories the nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to 1 37 ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history

constraints on martian differentiation processes from rbsr and smnd isotopic analyses of the basaltic shergottite que 94201 isotopic analyses of mineral leachate and whole rock fractions from the martian shergottite meteorite que 94201 yield rb sr and sm nd crystallization ages of 327 12 and 327 19 ma respectively these ages are concordant although the isochrons are defined by different fractions within the meteorite comparison of isotope dilution sm and nd data for the various que 94201 fractions with in situ ion microprobe data for que 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite this component is likely to have been added to que 94201 by secondary alteration processes on mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions initial 87sr 86sr of 0 701298 14 nd143 of 47 6 1 7 and whole rock nd142 of 0 92 0 11 indicate that que 94201 was derived from a source that was strongly depleted in 87rb 86sr and enriched in 147sm 144nd early in its history modeling demonstrates that the sm nd isotopic compositions of que 94201 can be produced by either four episodes of melting at 327 ma of cumulates crystallized from a magma ocean at 4 525 ga or five episodes of melting of an initially solid mars at 4 525 ga and 327 ma the neodymium isotopic systematics of que 94201 are not consistent with significant melting between 4 525 ga and 327 ma the estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the que 94201 parental magma rb sr based partial melting models are unable to reproduce the composition of que 94201 using the same model parameters employed in the sm nd based models implying a decoupling of rb sr and sm nd isotopic systems the initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate rb from sr compared to sm from nd the fact that all martian meteorites analyzed so far define a rb sr whole rock isochron age of 4 5 ga suggests that virtually all rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time thus the martian mantle cumulates and restites are not expected to evolve past 87sr86sr of 0 700 and could not have been significantly enriched in incompatible elements by crustal recycling processes all martian meteorites have initial 87sr86sr values that are higher than 0 700 and are therefore likely to be produced by mixing between evolved crustal like and depleted mantle reservoirs the absence of crustal recycling processes on mars may preserve the geochemical evidence for decoupling of the rb sr and sm nd isotopic systems underscoring one of the fundamental differences between geologic processes on mars and the earth

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

distribution and structure of hydrocarbons in four organic rich ordovician rocks abstract organic matter in four north american rocks of ordovician age has been analysed using geochemical and optical techniques including reflected light and fluorescence microscopy rock eval pyrolysis elemental analysis of isolated kerogens gas chromatography gc and gas chromatography mass spectrometry gc ms of hydrocarbons in the solvent extracted bitumens the samples analysed were guttenberg oil rock and black shales from the vinini womble and big fork formations in the united states the origin of some of the biomarkers found in the bitumen and the vitrinite particles observed in reflected light are discussed briefly with reference to possible algal and bacterial precursors the results indicate that the four samples are potentially excellent oil source rocks although the guttenberg oil rock is too immature to have generated a substantial quantity of hydrocarbons

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

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

noble gases released by vacuum crushing of eeta 79001 glass abstract gases released by in vacuo crushing of an eeta 79001 glass sample 15 of total gas concentrations were a mixture of a small amount of spb type gas with larger proportions of another more air like component though fractionated air is unlikely crush released 84kr 132xe 31 6 3 1 and36ar 132xe 809 49 higher than spb gas 15ncrush 13 2 1 4 xenon in the second eetv component has 129xe 132xe 1 21 while argon from this component has40ar 36ar 680 and is constrained by ar xe systematics to be 430 the eetv36ar 38ar ratio of 4 9 0 2 is lower than air but not as low as the estimated spb ratio which agrees with earlier measurements of 4 1 less than 5 of the spb gas was released by crushing while 36 40 of the eetv gas was crush released these results are consistent with a siting of the eetv component in 10 100 m vesicles seen in the glass while the spb component must be in vesicles 6 m diameter or more likely is primarily sited elswhere

a new approach to cosmogenic corrections in 40 ar 39 ar chronometry implications for the ages of martian meteorites abstract anomalously old 40ar 39ar ages are commonly obtained from shergottites and are generally attributed to uncertainties regarding the isotopic composition of the trapped component and or the presence of excess 40ar old ages can also be obtained if inaccurate corrections for cosmogenic 36ar are applied current methods for making the cosmogenic correction require simplifying assumptions regarding the spatial homogeneity of target elements for cosmogenic production and the distribution of cosmogenic nuclides relative to trapped and reactor derived ar isotopes to mitigate uncertainties arising from these assumptions a new cosmogenic correction approach utilizing the exposure age determined on an un irradiated aliquot and step wise production rate estimates that account for spatial variations in ca and k is described data obtained from nwa 4468 and an unofficial pairing of nwa 2975 which yield anomalously old ages when corrected for cosmogenic 36ar using conventional techniques are used to illustrate the efficacy of this new approach for these samples anomalous age determinations are rectified solely by the improved cosmogenic correction technique described herein ages of 188 17 and 184 17 ma are obtained for nwa 4468 and nwa 2975 respectively both of which are indistinguishable from ages obtained by other radioisotopic systems for other shergottites that have multiple trapped components have experienced diffusive loss of ar or contain excess ar more accurate cosmogenic corrections may aid in the interpretation of anomalous ages the trapped 40ar 36ar ratios inferred from inverse isochron diagrams obtained from nwa 4468 and nwa 2975 are significantly lower than the martian atmospheric value and may represent upper mantle or crustal components

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

chronology and petrogenesis of young achondrites shergotty zagami and alha77005 late magmatism on a geologically active planet a study was undertaken to determine the chronology petrogenesis and relationships among the shergottites shergotty and zagami and the unique achondrite alha77005 these meteorites are the product of a variety of complex processes petrogenesis chondrite normalized abundance patterns of shergotty and zagami are very similar and show pronounced depletions of both the light ree la nd and heavy ree dy lu relative to sm gd these characteristic depletions are even more pronounced for alha77005 the light ree depletion is qualitatively consistent with the presence of cumulus pyroxene and or olivine in these meteorites but trace element models show that the parental magmas of all three meteorites were probably also light ree depleted both trace element model calculations and combined rb sr and sm nd isotopic systematics show that the meteorites could not have been co magmatic nor can alha77005 be representative of the source material of the shergottites light ree depletion of the parental magmas also implies light ree depletion of the source material the sm nd systematics of the shergottites require a time averaged sub chondritic light ree enriched sm nd ratio since 4 6 ae ago the sm nd systematics of alha77005 permit a time averaged super chondritic light ree depleted sm nd ratio if its crystallization age is less than tice 0 72 ae chronology rb sr internal isochrons for all three meteorites and a sm nd internal isochron for zagami are concordant at 180 myr 39ar 40ar plateau ages of shergotty and zagami maskelynite are 250 260 myr these ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite the concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity the time of this event is most precisely given by the rb sr age of 180 4 myr for zagami the crystallization ages of the meteorites were not precisely determined extreme upper limits are determined by sm nd model ages relative to an eucrite initial 143nd144nd 0 505835 at 4 6 ae ago these model ages for shergotty zagami and alha77005 are 3600 3500 and 2850 myr respectively the sm nd whole rock age of 1340 60 myr for the three meteorites gives the crystallization age if the sm nd ratios of the precursor materials were always the same we consider this 1340 myr age as a best estimate upper limit best estimate lower limits for shergotty and zagami are taken from the average 39ar 40ar ages of 1200 and 900 myr of pyroxene separates the average 39ar 40ar age of a whole rock sample of alha77005 was 1600 myr and can be partitioned between a low temperature feldspar phase and a high temperature olivine pyroxene inclusions phase the average apparent 39ar 40ar age of the low temperature phase is 1050 myr which is chosen as the best estimate lower limit to the age the crystallization ages of shergotty zagami and alha77005 probably lie within the ranges of 1200 1300 900 1300 and 1000 1300 myr respectively the rb sr whole rock age of 4400 400 myr and single stage babi model ages of 4800 5100 myr are interpreted as reflecting differentiation of the parent body at 4600 myr ago the complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history

correlations of gulf coast petroleums on the basis of branched acyclic alkanes abstract mass 211 chromatograms of c 15 n alkane free saturated hydrocarbon fractions of oils represent numerous homologous series of methyl branched acyclic alkanes including isoprenoids mass 211 patterns facilitated the subdivision fingerprinting of 88 of 282 gulf coast petroleums into merely seven classes evaluation of reservoir ages suggests that the patterns undergo progressive change with time an end member pattern occurs in jurassic reservoirs a second closely related occurs principally in oils found in lower cretaceous reservoirs minor changes result in an upper cretaceous pattern which is duplicated in numerous oils occuring in all succeeding tertiary and quaternary stages over most of coastal and offshore louisiana laterally extensive source formations and pervasive vertical migration are indicated although branched alkane concentrations can be brought toward thermodynamic equilibrium by mineral catalysis the consistency of the observations indicates that this is minor suggesting a biomarker status for the compounds involved both facies variability and metabolic evolution might be significant in the generation of the observed patterns

39ar 40ar age and thermal history of martian dunite nwa 2737 abstract we report an 39 ar 40 ar age determination of a whole rock sample of the olivine rich martian meteorite northwest africa nwa 2737 those extractions releasing 0 48 of the 39 ar define an 39 ar 40 ar isochron age of 160 190 ma when evaluated in various ways higher temperature extractions show increasing ages that eventually exceed the reported sm nd age of 1 42 ga at least part of this excess 40 ar may have been shock implanted from the martian atmosphere we considered two possible interpretations of the ar ar isochron age utilizing the measured ar diffusion characteristics of nwa 2737 and a thermal model which relates ar diffusion to the size of a cooling object after shock heating one interpretation that 40 ar was only partially degassed by an impact event 11 ma ago the cre age appears possible only if nwa 2737 was shock heated to temperatures 600 c and was ejected from mars as an object a few 10 s of cm in diameter the second interpretation which we prefer is that nwa experienced an earlier more intense shock event which left it residing in a warm ejecta layer and a less intense event 11 ma ago which ejected it into space our evaluation would require nwa 2737 to have been heated by this first event to a temperature of 300 500 c and buried in ejecta to a depth of 1 20 m these conclusions are compared to model constraints on meteorite ejection from mars reported in the literature the second mars ejection impact 11 ma ago probably heated nwa 2737 to no more than 400 c nwa 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space

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

the origins of petroleum in the northern gulf of mexico abstract the distribution and chemistry of oils in the northern gulf of mexico and the adjacent onshore can be explained by multiple sources fades variations maturation and post generation alteration genetic families include jurassic smackover flexure trend upper cretaceous and paleogene oils smackover oils have high sulfur contents pristane phytane ratios c 35 c 34 hopane ratios 1 0 and c 30 c 29 hopane ratios usuall 1 0 flexure trend oils are similar and contain abundant extended hopanes high sulfur contents and v ni v 0 5 these oils are inferred to be sourced in upper jurassic lower cretaceous strata upper cretaceous oils contain a distinctive suite of tricyclic and nonhopanoid triterpanes oleanane is low or absent paleogene oils can be recognized by the presence of 18 h oleanane and a lack of extended hopanes these oils occur from south texas to southern and offshore louisiana two subfamilies can be recognized based on the relative abundance of triterpanes and steranes these oils have a presumed paleogene source mixing of oil types is quite prevalent at the geographic boundaries of oil types the initial control on the distribution of oil and gas in the northern gulf of mexico is provided by the areal extent of source rocks secondary control is due to cenozoic deposition which provides the thermal stress to generate as well as destroy oil salt tectonics provide conduits for migrating fluids to escape the zone of thermal destruction mesozoic source strata again become important in the deep gulf of mexico where cenozoic sediments thin

shergottites dhofar 019 sau 005 shergotty and zagami 40ar 39ar chronology and trapped martian atmospheric and interior argon we report a high resolution 40ar 39ar study of mineral separates and whole rock samples of olivine phyric dhofar 019 sayh al uhaymir sau 005 and basaltic shergotty zagami shergottites excess argon is present in all samples the highest 40ar 36ar trapped ratios are found for argon in pyroxene melt inclusions 1500 maskelynite 1200 impact glass 1800 of shergotty and impact glass of sau 005 1200 a high 40ar 36ar trapped componentusually uniquely ascribed to martian atmosphere can also originate from the martian interior indicating a heterogeneous martian mantle composition as additional explanation of variable high 40ar 36ar trapped ratios in shocked shergottites we suggest argon implantation from a transient atmosphere during impact induced degassing the best 40ar 39ar age estimate for dhofar 019 is 642 72 ma maskelynite sau 005 samples are between 700900 ma old relatively high 40ar 39ar ages of melt inclusions within dhofar 019 1086 252 ma and sau 005 olivine 885 66 ma could date entrapment of a magmatic liquid during early olivine crystallization or reflect unrecognized excess 40ar components the youngest 40ar 39ar age of shergotty separates maskelynite is 370 ma that of zagami is 200 ma the 40ar 39ar chronology of dhofar 019 and sau 005 indicate 1 ga ages apparent ages uncorrected for trapped e g martian atmosphere mantle argon components approach 4 5 ga but are not caused by inherited 40ar because excess 40ar is supported by 36artrapped young ages obtained by 40ar 39ar and other chronometers argue for primary rather than secondary events the cosmic ray exposure ages calculated from cosmogenic argon are 15 7 0 7 ma dhofar 019 1 0 1 6 ma sau 005 2 1 2 5 ma shergotty and 2 2 3 0 ma zagami

40ar 39ar studies of whole rock nakhlites evidence for the timing of formation and aqueous alteration on mars 20 25 mg whole rock samples of the nakhlites lafayette and nakhla have been analyzed via the 40ar 39ar technique in part to verify their formation ages but primarily in an attempt to determine the timing of aqueous alteration in these martian meteorites as in previous studies plateaus in apparent age are observed at about 1300 ma 1322 10 for lafayette 1332 10 and 1323 11 for nakhla presumably corresponding to crystallization ages the plateaus are not entirely flat perhaps reflecting the effects of recoil during creation of 39ar in the nuclear irradiation the first 5 20 of the k derived ar released from all three samples give apparent ages 1300 ma coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance we believe the low temperature pattern represents more recent than 1300 ma formation of martian aqueous alteration products such as iddingsite no low temperature plateaus are observed this is consistent with petrographic evidence for multiple formation events although the lack of low temperature plateaus is far from conclusive on the other hand if there was a single time of alteration we believe that it will be difficult if not impossible to determine it using the k ar system

constraints on the u pb isotopic systematics of mars inferred from a combined u pb rb sr and sm nd isotopic study of the martian meteorite zagami uranium lead rb sr and sm nd isotopic analyses have been performed on the same whole rock mineral and leachate fractions of the basaltic martian meteorite zagami to better constrain the u pb isotopic systematics of martian materials although the rb sr and sm nd systems define concordant crystallization ages of 166 6 ma and 166 12 ma respectively the u pb isotopic system is disturbed nevertheless an age of 156 6 ma is derived from the 238u 206pb isotopic system from the purest mineral fractions maskelynite and pyroxene the concordance of these three ages suggest that the 238u 206pb systematics of the purest zagami mineral fractions have been minimally disturbed by alteration and impact processes and can therefore be used to constrain the behavior of u and pb in the zagami source region the value of the zagami source region can be estimated with some confidence from the 238u 206pb isochron to be 3 96 0 02 disturbance of the u pb isotopic systems means that this represents a minimum value the value of the zagami source is significantly lower than the values estimated for most basaltic magma sources from earth and the moon this is surprising given the high initial 87sr 86sr ratio 0 721566 82 and low initial end value 7 23 0 17 determined for zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body this suggests that mars is characterized by a low bulk planet u pb ratio a feature that is consistent with its relatively volatile rich nature the leachates contain terrestrial common pb that was probably added to the meteorite during handling curation or sawing the mineral fractions particularly those with significant amounts of impact melt glass contain a second contaminant the presence of this contaminant results in pb pb ages that are older than the crystallization age of zagami indicating that the contaminant is characterized by a high 207pb 206pb ratio such a contaminant could be produced by removal of single stage pb from a relatively high martian reservoir before 1 8 ga and therefore could be an ancient manifestation of hydrous alteration of martian surface material

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

paleoreconstruction by biological markers abstract during diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons the following four basic events needed for paleoreconstruction can be monitored by biological markets 1 sourcing 2 maturation 3 migration and 4 biodegradation actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated cretaceous and phosphoria sourced oils in the wyoming thrust belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry identification of recently discovered biological markers head to head isoprenoids allows subtle source differentiation between oils from sumatra the degree of crude oil maturation in basins from california alaska russia wyoming and louisiana can be assessed by specific novel biomarker ratios 20s 20r sterane epimers field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock an example of immature sumatra rock is given extensive migration is documented by biomarkers in several oils the effect of biodegradation on biomarker steranes and terpanes is exemlified in a california basin all biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo reconstruction

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

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

classification of offshore gulf of mexico oils and gas condensates five groups of petroleums can be recognized in the offshore gulf of mexico on the basis of carbon isotope and sulfur isotope ratios sulfur content and relative abundances of nickel and vanadium of 355 oils analyzed 341 96 1 are evidently closely related and cannot be subdivided on the basis of carbon or sulfur isotopes we suggest that these oils represent a single broad superfamily however two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium group 1a sigma 13 c 26 5 0 4 parts per mil sigma 34 s 5 6 2 4 parts per mil s 0 25 0 15 v ni v 0 27 0 10 and group 1b sigma 13 c 26 5 0 3 parts per mil sigma 34 s 5 9 1 2 parts per mil s 0 73 0 43 v ni v 0 67 0 07 group 1a oils are broadly distributed across the louisiana and adjacent texas continental shelf whereas group 1b oils are found along the louisiana shelf edge and continental slope group 1b oils have substantial genetic affinities to those from the campeche shelf offshore mexico of mesozoic origin the remaining 14 analyzed petroleums include supermature condensates from offshore texas and oils from offshore southern florida believed to be cogenetic with those from the jurassic smackover formation a three dimensional appraisal of geohistory suggests that petroleum generation began in the eocene in the region of the rio grande delta progressed coastwise to the mississippi delta area by the miocene and then occurred in regions of progres ively deeper water during the pliocene pleistocene

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

the 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

the rocks of mars from far and near award of the meteoritical society 2001 h y mcsween jr the leonard medal address the rocks of mars from far and near

an interpretation of cretaceous and tertiary biota in polar regions abstract rich fossil floras and vertebrate faunas lived within the arctic and atarctic circles during the cretaceous and paleogene these occurrences seem explicable by the nature of the thermal environments in polar regions and by the physiological responses of plants and animals allied to those that lived there there seems no need to invoke a lower tilt to the earth s axis to provide greater warmth and light in polar regions to account for these fossil records

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

the earth as a planet paradigms and paradoxes the independent growth of the various branches of the earth sciences in the past two decades has led to a divergence of geophysical geochemical geological and planetological models for the composition and evolution of a terrestrial planet evidence for differentiation and volcanism on small planets and a magma ocean on the moon contrasts with hypotheses for a mostly primitive still undifferentiated and homogeneous terrestrial mantle in comparison with the moon the earth has an extraordinarily thin crust the geoid which should reflect convection in the mantle is apparently unrelated to the current distribution of continents and oceanic ridges if the earth is deformable the whole mantle should wander relative to the axis of rotation but the implications of this are seldom discussed the proposal of a mantle rich in olivine violates expectations based on evidence from extraterrestrial sources these and other paradoxes force a reexamination of some long held assumptions

terrestrial 81kr kr ages of antarctic meteorites the production rate of 38ar in meteorites p 38 has been determined as a function of the sample s chemical composition from 81kr kr exposure ages of four eucrite falls the cosmogenic 78kr 83kr ratio is used to estimate the shielding dependence of p 38 from the true 38ar exposure ages and the apparent 81kr kr exposure ages of nine antarctic eucrite finds terrestrial ages are calculated they range from about 3 105a pecora escarpment 82502 to very recent falls thiel mountains 82502 polymict eucrites from the allan hills a78132 a79017 and a81009 have within the limits of error the same exposure age 15 2 106a and the same terrestrial age 1 1 105a this is taken as strong evidence that these meteorites are fragments of the same fall a similar case are the elephant moraine polymict eucrites a79005 a79006 and 82600 with an exposure age of 26 106a and a terrestrial age of 1 8 105a eeta79004 may be different from this group because its exposure age and terrestrial age are 21 106a and 2 5 105a respectively the distribution of terrestrial ages of allan hills meteorites is discussed meteorites from this blue ice field have two sources directly deposited falls and meteorites transported to the allan hills inside the moving antarctic ice sheet during the surface residence time meteorites decompose due to weathering processes the weathering half life is about 1 6 105a from the different age distributions of allan hills and yamato meteorites it is concluded that meteorite concentrations of different antarctic ice fields need different explanations

40ar 39ar and cosmic ray exposure ages of nakhlites nakhla lafayette governador valadares and chassigny we present 40ar 39ar dating results of handpicked mineral separates and wholerock samples of nakhla lafayette and chassigny our data on nakhla and lafayette and recently reported ages for some nakhlites and chassigny misawa et al 2006 park et al 2009 point to formation ages of approximately 1 4 ga rather than 1 3 ga that is consistent with previous suggestions of close in time formation of nakhlites and chassigny in lafayette mesostasis we detected a secondary degassing event at approximately 1 1 ga which is not related to iddingsite formation it may have been caused by a medium grade thermal event resetting the mesostasis age but not influencing the k ar system of magmatic inclusions and the original igneous texture of this rock cosmic ray exposure ages for these meteorites and for governador valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3he 21ne and 38ar individual results are similar to literature data the considerable scatter of t3 t21 and t38 ages is due to systematic uncertainties related to bulk rock and target element chemistry production rates and shielding effects this hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and chassigny from a confined martian surface terrain eugster 2003 garrison and bogard 2005 cosmic ray exposure ages from stepwise release age spectra using 38ar and neutron induced 37ar from ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates although not from shielding conditions

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

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

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

isotopic composition of trapped and cosmogenic noble gases in several martian meteorites isotopic abundances of the noble gases were measured in the following martian meteorites two shock glass inclusions from elephant moraine eet 79001 shock vein glass from shergotty and yamato y 793605 and whole rock samples of allan hills alh 84001 and queen alexandra range que 94201 these glass samples when combined with literature data on a separate single glass inclusion from eet 79001 and a glass vein from zagami permit examination in greater detail of the isotopic composition of ne ar kr and xe trapped from the martian atmosphere the isotopic composition of martian ne if actually present in these glasses remains poorly defined the 40ar 36ar ratio of trapped martian atmospheric ar is probably considerably lower than the nominal ratio of 3000 measured by viking and data on impact glasses suggest a value of 1900 the atmospheric 36ar 38ar ratio is 4 0 martian atmospheric kr may be enriched in lighter isotopes by 0 5 amu compared to both solar wind kr and to the martian composition previously reported the isotopic composition of xe in these glasses agrees with that previously reported in the literature the martian atmospheric 36ar 132xe and 84kr 132xe elemental ratios are higher than those reported by viking by factors of 2 5 1 6 depending on the 40ar 36ar ratio adopted and 1 8 respectively and are discussed in a separate paper cosmogenic gases indicate space exposure ages of 2 7 0 6 ma for que 94201 and shergotty and 14 1 ma for alh 84001 small amounts of 21ne produced by energetic solar protons may be present in que 94201 but are not present in alh 84001 or y 793605 the space exposure age for y 793605 is 4 9 0 6 ma and appears to be distinctly older than the ages for basaltic shergottites however uncertainties in cosmogenic production rates still makes somewhat uncertain the number of martian impact events required to produce the exposure ages of martian meteorites

a review of meteorite evidence for the timing of magmatism and of surface or near surface liquid water on mars 1 there is widespread photogeological evidence for ubiquitous water flowing on the surface of mars however the age of surface and near surface water cannot be deduced with high precision from photogeology while there is clear evidence for old and young fluvial features in the photogeologic record the uncertainty in the absolute calibration of the martian crater flux results in uncertainties of 1 5 gyr in the middle period of martian geologic history aqueous alteration of primary igneous minerals produces secondary minerals in martian meteorites here we use the ages of secondary alteration minerals in martian meteorites to obtain absolute ages when liquid water was at or near the surface of mars aqueous alteration events in martian meteorites occurred at 3929 37 ma carbonates in alh84001 633 23 ma iddingsite in nakhlites and 0 170 ma salts in shergottites furthermore these events appear to be of short duration suggesting episodic rather than continuous aqueous alteration of the meteorites the martian meteorites appear to be contaminated by martian surface pb characterized by a 207pb 206pb ratio near 1 lead of this composition could be produced by water based alteration on the martian surface the high 129xe 132xe ratio in the martian atmosphere compared to martian meteorites indicates fractionation of i from xe within 100 myr after nucleosynthesis of 129i such fractionation is difficult to achieve through magmatic processes however water very efficiently fractionates i from xe raising the intriguing possibility that mars had a liquid water ocean within its first 100 myr 1

noble gases in mineral separates from three shergottites shergotty zagami and eeta79001 this study provides a complete data set of all five noble gases for bulk samples and mineral separates from three martian shergottites shergotty bulk pyroxene maskelynite zagami bulk pyroxene maskelynite and elephant moraine eet a79001 lithology a bulk pyroxene we also give a compilation of all noble gas and nitrogen studies performed on these meteorites our mean values for cosmic ray exposure ages from 3he 21ne and 38ar are 2 48 myr for shergotty 2 73 myr for zagami and 0 65 myr for eeta79001 lith a serious loss of radiogenic 4he due to shock is observed cosmogenic neon results for bulk samples from 13 martian meteorites new data and literature data are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic ray effects while a contribution of this low energy irradiation is strongly indicated for all of the shergottites spallation ne in chassigny allan hills alh 84001 and the nakhlites is fully explained by galactic cosmic ray spallation implanted martian atmospheric gases are present in all mineral separates and the thermal release indicates a near surface siting we derive an estimate for the 40ar 36ar ratio of the martian interior component by subtracting from measured ar in the k poor pyroxenes the small radiogenic component as well as the implanted atmospheric component as indicated from 129xe excesses unless compromised by the presence of additional components a high ratio of 2000 is indicated for martian interior argon similar to that in the martian atmosphere since much lower ratios have been inferred for chassigny and alh 84001 the result may indicate spatial and or temporal variations of 40ar 36ar in the martian mantle

sterane and triterpane patterns as diagnostic tools in the mapping of oils condensates and source rocks of the gulf of mexico region abstract four hundred and eighty one gulf of mexico oils and condensates can be subdivided into nine major families based on their sterane m z 217 218 and triterpane m z 191 177 biomarker mass chromatogram patterns these families are 1 carbonate sourced oils reservoired in jurassic cretaceous reservoirs onshore and plio pleistocene reservoirs offshore 2 austin chalk reservoired oils 3 tuscaloosa oils 4 wilcox reservoired oils la 5 south texas palaeogene reservoired oils 6 mississippi delta oils 7 post mature oils of uncertain affinities 8 migration contaminated oils showing attributes of a mixing of immature and post mature sources and 9 oils and condensates lacking biomarkers each oil family is concentrated in specific age defined producing trends ranging in age from jurassic to pleistocene for tertiary reservoirs these producing trends shift geographically with time in a manner analogous to the clockwise migrating cenozoic depocenter model for the northern gulf of mexico at least six ages of marine source rock deposition may be implicated in the genesis of these oils 1 the kimmeridgian oxfordian including smackover and norphlet formations 2 the aptian including pine island bexar and sunniland formations 3 the albian paluxy formation 4 the cenomanian turonian including eagle ford and tuscaloosa formations 5 the coniacian santonian eagle ford and austin chalk and 6 the palaeocene wilcox midway sparta formations the jurassic and cretaceous source rocks correspond to described oceanic anoxic events of probable global significance this paper will concentrate on describing the biomarker patterns of the oils and postulated source rocks but due to reasons of space isotopic elemental and other data will only be briefly discussed

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

origin and evolution of the nakhla meteorite inferred from the sm nd and u pb systematics and ree ba sr rb and k abundances analyses of sm nd and u th pb systematics ree ba sr rb and k concentrations were carried out for whole rock and mineral separates from the nakhla meteorite the 1 26 07 b y sm nd age obtained in this work is in good agreement with those previously obtained by the rb sr and ar ar methods the high initial nd value of 16 suggests that nakhla was derived from a light ree depleted old planetary mantle source u th pb data after correction for pre analytical terrestrial pb contamination assuming an age of 1 26 b y suggest that the age of the nakhla source is 4 33 b y the agreement in the age determined by three independent radiometric methods and the high initial nd value strongly suggest that the 1 3 b y age dates one thorough igneous event in the parent body which not only reset these isotopic systems but also established the chemical and petrologic characteristics observed for the nakhla meteorite using a three stage sm nd evolution model in combination with lil element data and estimated partition coefficients we have tested partial melting and fractional crystallization models to estimate lil element abundances in a possible nakhla source our model calculations suggest that partial melting of the light ree depleted source followed by extensive fractional crystallization 50 of the partial melt could account for the ree abundances in the nakhla constituent minerals the estimated source is depleted in the light ree ba rb and k and therefore may resemble the morb source in the earth s upper mantle or the upper 60 300 km of the moon the significantly younger age of nakhla than the youngest lunar rock the young differentiation age inferred from the u th pb data and the estimated lil element abundances including those of k u and th in the source suggest that the nakhla meteorite may have been derived from a relatively large well differentiated planetary body such as mars

petroleum generation in the southeast texas basin implications for hydrocarbon occurrence at the south liberty salt dome geochemical characteristics of hydrocarbons from the south liberty field liberty county texas were integrated with local stratigraphy pressure temperature seismic data and formation water chemistry to determine the source maturity and migration pathways of hydrocarbons associated with the salt dome in the southeast texas gulf coast fourteen crude oil samples from the soft geopressured cook mountain and yegua both eocene and frio oligocene reservoirs 989 2886 m 3245 9469 ft were analyzed by whole oil high resolution gas chromatography gc and gc mass spectrometry pressure and temperature data from 36 wells were used to model the thermal maturation history in the vicinity of the dome south liberty oils were found to belong to a single genetic family sourced from rocks with a similar level of maturity biomarkers indicate they were formed in a marginal marine environment with notable terrestrial input from a likely lower tertiary source rock probably the downdip lower claiborne and or wilcox group oils were generated within the peak oil window vitrinite reflectance 0 6 0 9 at expulsion temperatures of 125 to 130c thermal modeling indicates the lower claiborne and or upper wilcox beds attained optimum maturity in and around the dome between 8 to 20 ma and 18 to 31 ma respectively oil migration toward the structure probably occurred through faults breaching the deep seated lower claiborne upper wilcox source two severely biodegraded samples were found and these were associated with cooler formation water 50 to 55c with a greater meteoric water component 48 60 comparison with brazoria county oils 100 km 62 mi to the southwest indicates that the south liberty oils belong to the same genetic family with minor differences resulting from a greater input from terrestrial kerogen

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

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

geochemical study on a well in the western canada basin relation of the aromatic distribution pattern to maturity of organic matter abstract thirty one rock samples from a western canada basin well have been analysed for aromatics using glass capillary gas chromatography with simultaneous flame ionization and sulfur selective flame photometric detection a uniform aromatic distribution pattern with a marked predominance of di and tricyclic aromatic hydrocarbons was observed throughout a depth interval of 3000 meters comprising cretaceous and jurassic sediments the very high relative abundance of sulfur aromatics at two narrow intervals in the triassic and mississippian is attributed to a major change in facies gradual changes with increasing depth have been observed for a series of compound ratios which had been calculated from the isomers of methyl and dimethylphenanthrene and their parent compounds these changes reflect the increasing thermal maturation of the sedimentary organic matter a methylphenanthrene index mpi has been introduced which exhibited a very good correlation with vitrinite reflectance data the mpi is the first maturity parameter which is based on a series of aromatic hydrocarbons this hydrocarbon internal maturity parameter permits the recognition and definition of maturity levels in extracts which can be compared to the maturity e g vitrinite reflectance of source rocks

the age of snc meteorites and the antiquity of the martian surface we report new sm nd lu hf and pb pb mineral and whole rock isotope data for the basaltic shergottite zagami as well as pb pb whole rock isotope data for the basaltic shergottite los angeles the lherzolitic shergottite dar al gani 476 dag 476 and the clinopyroxenite nakhla in agreement with previous findings our new sm nd and lu hf mineral ages on the martian meteorite zagami are young 155 and 185 ma respectively the 207pb 206pb 204pb 206pb compositions of the insoluble fractions of shergottites zagami los angeles and literature data for shergotty and eeta79001 form an excellent alignment indicative of a 4 0 ga crystallization age the range of pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial pb and argues against mixing relationships the age of 4 048 0 017 ga mswd 1 5 provided by the pb isotope compositions of the zagami whole rock and residues is therefore taken to date the crystallization of this rock which so far was believed to be only not vert similar 180 ma old based on this result we argue that the lithosphere of mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the martian surface this interpretation is consistent with photographic interpretations of erosional features on mars it also relieves the constraint imposed by the presence of anomalies of 142nd and 182w both products of extinct radioactive nuclides that the martian mantle should have preserved primordial isotopic heterogeneities thus allowing for the planet interior to be actively convecting

paleoclimate and polar wander abstract fossil faunas floras and 18 o temperatures indicate that weak or no meridional temperature gradients existed over the continents from triassic to eocene for much of this time tropical pacific 18 o temperatures were some ten degrees colder than at present while arctic and antarctic regions apparently enjoyed warm temperate to semitropical climates polar wander involving displacement of the poles to what are now tropical marine latitudes is offered to explain this uniformly warm paleoclimate this hypothesis is supported by a theoretical model which is discussed

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

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

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

c 27 c 29 monoaromatic anthrasteroid hydrocarbons in cretaceous black shales the structural elucidation of aromatic steroid hydrocarbons occurring in sediments and petroleums has been the subject of increasing interest during recent years1 7 aromatization of the precursor sterols of marine origin has been shown to start at early stages of diagenesis in immature sediments8 9 in elucidating these first aromatization reactions we have recently identified two major series of ring a monoaromatic steroids 1 and 2 fig 1 in cretaceous black shales from the southern and eastern north atlantic collected during the deep sea drilling project dsdp 8 we report here the identification of two novel series of geochemical markers the ring b mono aromatic anthrasteroid hydrocarbons 3 and 4 as the two other major series occurring in the same immature dsdp samples fig 1

dhofar 378 martian shergottite evidence of early shock melting 1 shock heating of the dhofar 378 dho 378 martian shergottite produced melting vesiculation and flow of the plagioclase which upon cooling recrystallized into complex textures heating experiments on the similar zagami shergottite indicate that dho 378 was shock heated to 1000 1100 c and was cooled at 2 5 c h an 39ar 40ar analysis of dho 378 plagioclase indicates different ar diffusion domains and k ca ratios the lower temperature phase defines an ar ar isochron age of 141 32 ma the higher temperature phase released more 40ar but does not define an age the meteorite s thermal history was examined by constructing a generic model to compare cooling rates for objects of different sizes against fractional diffusion loss of ar for different cooling times using gas diffusion parameter values measured for dho 378 this model indicates that it is improbable that the major shock heating event occurred at the time that dho 378 was ejected from mars 3 ma ago rather we suggest that the time of shock heating is probably given by its ar ar age for dho 378 to cool sufficiently fast not to lose most of its 40ar 3 ma ago would require it to have been ejected into space as an impossibly small object larger and more reasonable mars ejection sizes indicate that dho 378 should have lost most of its 40ar on the basis of plagioclase texture and ar data we suggest that a major impact event 141 ma ago melted dho 378 plagioclase degassed most of its 40ar and deposited it in crater ejecta to cool a smaller and later impact ejected it into space 3 ma ago

origin of crude oil in the wilcox trend of louisiana and mississippi evidence of long range migration abstract geochemical characterization of crude oils from wilcox reservoirs in central louisiana and southwest mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper tuscaloosa and smackover reservoirs this observation is consistent with geologic constraints that suggest an origin of crude oil from within the wilcox group itself although shales of the shallow wilcox group in central louisiana and southwest mississippi contain gas prone kerogen and are thermally immature a more oil prone source facies is present in marine shales of the deep wilcox group in southcentral louisiana thermal maturity measurements based on pyrolysis suggest a broad area of effective wilcox source rock in southcentral louisiana migration distances from source to reservoir rocks of the downdip wilcox trend of southcentral louisiana appear to be relatively short however long range updip migration sometimes greater than 100 km from deeply buried wilcox source facies provides the best explanation for emplacement of crude oil in the shallow wilcox trend of central louisiana and southwest mississippi

rb sr sm nd and ar ar isotopic systematics of martian dunite chassigny abstract isotopic analysis of the martian meteorite chassigny yields a rb sr age of 1406 14 ma with an initial 87 sr 86 sr ratio of 0 702251 0 000034 a sm nd age of 1386 28 ma with an initial e 143nd value of 16 9 0 3 and an 39 ar 40 ar age of 1360 40 20 ma the concordance of these ages and the rb sr and sm nd initial isotopic signatures suggest that chassigny crystallized from low rb sr light rare earth element depleted source materials 1390 ma ago the ages and e 143nd values of chassigny and the nakhlites governador valadares and lafayette overlap suggesting that they could have come from very similar mantle sources nakhla northwest africa 998 and yamato 000593 appear to be from similar but distinct sources chassigny and all nakhlites so far studied have undergone similar evolution histories that is chassignites nakhlites were derived from a region where volcanism lasted at least 50 ma and crystallized from different lava flows or subsurface sills they probably were launched from mars by a single impact event the trapped martian atmospheric 40 ar 36 ar ratios in chassigny nakhlites and shergottite impact glass are similar and possibly indicate minimal change in this ratio over the past 600 ma

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

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

petroleum geology and geochemistry of the middle proterozoic mcarthur basin northern australia iii composition of extractable hydrocarbons abstract middle proterozoic organic rich sediments from the mcarthur basin in northern australia contain abundant hydrocarbons which are derived from syngenetic kerogen and hence are representative of the organic remains of microorganisms living at that time the major classes of hydrocarbons identified were n alkanes monomethyl branched alkanes cyclohexyl alkanes and acyclic isoprenoids there were also low abundances of pentacyclic triterpanes comprising hopanes and methyl hopanes low concentrations of steranes were present in most samples but like the triterpanes they were only easily detected in the least thermally altered sediments and hence were of limited use in detailed assessments of thermal maturity the presence of steranes in sediments of this age is strong evidence for the existence of eukaryotic organisms as far back as 1690 ma although the relatively high abundances of branched alkanes indicates that most of this primitive organic matter was probably derived from prokaryotes