Browsing by Author "Herd, Christopher D. K."

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    Anatomy of a young impact crater in Central Alberta: prospects for the ‘missing’ Holocene impact record
    (2008) Herd, Christopher D. K.; Froese, D. F.; Walton, Erin L.; Herd, E. P. K.; Duke, J.
    Small impact events recorded on the surface of Earth are significantly underrepresented based on expected magnitude-frequency relations. We report the discovery of a 36-m-diameter late Holocene impact crater located in a forested area near the town of Whitecourt, Alberta, Canada. Although undetectable using visible imagery, the presence of the crater is revealed using a bare-Earth digital elevation model obtained through airborne light detection and ranging (LiDAR). The target material comprises deglacial Quaternary sediments, with impact ejecta burying a late Holocene soil dated to ca. 1100 14C yr B.P. Most of the 74 iron meteorites (0.1–1196 g) recovered have an angular exterior morphology. These meteorites were buried at depths <25 cm and are interpreted to result from fragmentation of the original projectile mass, either at low altitude or during the impact event. Impact of the main mass formed the simple bowl-shaped impact structure associated with an ejecta blanket and crater fill. The increasing availability of LiDAR data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features.
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    Carboxylic acid abundances in the Tagish Lake meteorite: lithological differences and implications for formic acid abundances in carbonaceous chondrites
    (2009) Hilts, Robert; Herd, Christopher D. K.; Morgan, Don; Edwards, LeAnne; Huang, Yongsong
    The most abundant soluble organic compounds in carbonaceous chondrites are typically carboxylic acids (Pizzarello et al 2001). Strait-chain monocarboxylic acids up to C-12 have been the focus of considerable attention owing to the exciting possibility that they may have been incorporated into the molecular architecture of prebiotic protomembranes on the ancient Earth (e.g., Silva et al 2004.) The most abundant monocarboxylic acid in interstellar space is formic acid (e.g. Remigen et al. 2003; Snyder 2008). It is generally accepted that the organic material in carbonaceous chondrites such as the Tagish Lake meteorite, which includes carboxylic acids, is derived from interstellar or nebular sources (Cronin et al 1988 and Cronin et al 1993). It is somewhat surprising, therefore, that up until now only what have been described as small or moderate formic acid concentrations have been found in aqueous extracts of carbonaceous chondrites (Huang et al 2005, Naraoka et al 1999, Yuen et al 1973, Shimoyama et al 1986, Yuen et al 1984 and Krishnamurthy et al 1992). Previous reports have ascribed the unexpectedly low formic acid abundances to either compound loss during extraction and subsequent work up, or to depletion caused by evaporation and/or aqueous leaching of the compound from the meteorite upon its exposure to the Earth's hydrosphere (Huang et al 2005 and Naraoka et al 1999). Here we present our analysis of the water-soluble monocarboxylic acids in two different lithologies within the Tagish Lake meteorite using the SPME-GCMS procedures recently developed by Huang et al 2005 to compare the two lithologies in this respect. Our results conclusively show that formic acid is, by a wide margin, the most abundant monocarboxylic acid in both of the Tagish Lake lithologies investigated thus far. This is in stark contrast to all previous studies of other meteorites in which it was concluded that the formic acid concentration was the lowest or one of the lowest of those monocarboxylic acids present in the extract (Huang et al 2005, Naraoka et al 1999, Yuen et al 1973, Shimoyama et al 1986, Yuen et al 1984 and Krishnamurthy et al 1992). Moreover, our serendipitous discovery that formic acid has a very low response factor when run on either GCMS(quadrupole) or GC-FID (Allen et al 1987) instruments suggests that previous studies on carbonaceous chondrites may have dramatically underestimated the quantities of formic acid present. Also, a close inspection of the relative abundances for the straight-chain monocarboxylic acids in each Tagish Lake lithology has led us to conclude that the overall oxidation levels for the water soluble organics from the two lithologies are different. Lastly, we have found that the monocarboxylic acids within the Tagish Lake meteorite are enriched in deuterium compared to terrestrial organics, with delta D values ranging from + 247 to + 581%o. These results confirm that the acids originate from interstellar space and that terrestrial contamination has been largely avoided.
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    Carboxylic acid abundances in the Tagish Lake meteorite: lithological differences and implications for formic acid abundances in carbonaceous chondrites
    (2009) Hilts, Robert; Herd, Christopher D. K.; Morgan, Don; Edwards, LeAnne; Huang, Yongsong
    Analysis of two different Tagish Lake rocks found: 1) a very low GCMS response for formic acid, 2) formic acid concns above 100 ppm and 3) that formic acid to higher homologue ratios indicate the average level of oxidation for the soluble organics.
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    Cold curation of pristine astromaterials: insights from the Tagish Lake meteorite
    (2016) Herd, Christopher D. K.; Hilts, Robert; Skelhorne, Aaron W.; Simkus, Danielle N.
    The curation and handling of volatile-bearing astromaterials is of prime importance in current and future plans for sample return missions to targets containing organic compounds, ices, or other volatile components. We report on the specific curation constraints required for the preservation of the Tagish Lake meteorite, a C2 ungrouped chondrite that contains significant concentrations of organic matter, including compounds of prebiotic interest or volatile in character, and which was recovered from a frozen lake surface a few days after its fall. Here we review the circumstances of the meteorite’s handling, its complement of intrinsic and contaminant organic compounds, and an unusual reaction between some of the specimens and the Al foil in which they were enclosed. From our results we derive the requirements for curation of the meteorite, and describe a specialized facility that enables its curation and handling. The Subzero Facility for Curation of Astromaterials consists of a purified Ar glove box enclosed within a freezer chamber, and enables investigations relevant to curation of samples at or below -10 °C. We provide several recommendations based on insights obtained from the commissioning and initial use of the facility that are relevant to collection of freshly fallen meteorites, curation of volatile-bearing meteorites and other astromaterials, and planning and implementation of curation plans for future sample return missions to volatile-bearing targets.
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    Compound-specific carbon isotope compositions of aldehydes and ketones in the Tagish Lake meteorite
    (2016) Simkus, Danielle N.; Aponte, José C.; Hilts, Robert; Elsila, Jamie E.; Herd, Christopher D. K.
    Investigating the aldehyde and ketone content of astromaterials provides insight into the abiotic formation of prebiotic compounds on asteroid parent bodies, as aldehydes and ketones are the chemical precursors to α-amino acids in Strecker synthesis reactions. Elucidating amino acid synthesis reactions is important for understanding the origin of prebiotic compounds on Earth and the potential for life beyond our planet. Aldehydes and ketones have been previously detected via gas chromatography mass spectrometry (GC-MS) in a few carbonaceous chondrite meteorites, including Murchison, Bells and Ivuna. However, carbon isotope compositions of aldehydes and ketones in meteorites have not yet been measured. As such, their relationship to amino acids and other compound classes in meteorites have not been fully investigated. Here, we present the first report of compound specific carbon isotope measurements of aldehydes and ketones in meteorite samples.
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    Compound‐specific carbon isotope compositions of aldehydes and ketones in the Murchison meteorite
    (2019) Simkus, Danielle N.; Aponte, José C.; Hilts, Robert; Elsila, Jamie E.; Herd, Christopher D. K.
    Compound‐specific carbon isotope analysis (δ13C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker‐cyanohydrin synthesis in the early solar system. Previous δ13C investigations have targeted α‐amino acid and α‐hydroxy acid Strecker products and reactant HCN; however, δ13C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O‐(2,3,4,5,6‐pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and δ13C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. δ13C values, ranging from −10.0‰ to +66.4‰, were more 13C‐depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar 12C/13C ratios. These relatively 13C‐depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ13C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration‐driven chemical pathways.
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    Creation of a cryogenic, inert atmosphere sample curation facility: establishing baselines for sample return missions
    (2012) Herd, Christopher D. K.; Hilts, Robert; Skelhorne, Aaron W.
    A new facility for handling and curiation of meteorites within a sub-zero environment under an Ar atmosphere has been established. Baseline contaminants within the glovebox containing the astromaterials will be described.
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    Creation of a cryogenic, inert atmosphere sample curation facility: update
    (2013) Hilts, Robert; Skelhorne, Aaron W.; Herd, Christopher D. K.
    Introduction: As reported previously [1] we have established a cryogenic facility for curation and handling of planetary materials at the University of Alberta. Established in order to enable curation and handling of the organic-rich Tagish Lake meteorite [e.g., 2], the facility consists of a state-of-the art Ar gas glove box, housed within a -20 °C controlled environment chamber. The Ar gas is purified with an MB 20 G gas purifier (MBraun, Inc.) and continually recirculated. A Class 1000 clean room serves as a room temperature anteroom and source of clean air for the -20 °C chamber. Results of ongoing studies of baseline contaminants and operating parameters are reported here. Effect of temperature on volatile organics: The glove box atmosphere was previously sampled using a solid phase microextraction fibre (SPME), exposed for 28 days at room temperature (Sept. 29 to Oct. 27, 2011). GC-MS analysis of the SPME fibre revealed common organic chemicals such as styrene and aniline [1]. To test our hypothesis that low temperatures should lead to lower levels of organic vapours in the glove box atmosphere, an SPME fibre was stored in the glove box at the standard operating temperature of -20 °C for a period of 54 days (Sept. 26 to Nov. 19, 2012). GC-MS analysis on the fibre revealed that styrene and aniline were absent in the low temperature sample. Apparently, the combination of a lower temperature and decreasing rate of outgassing from glove box components over time combined with repeated flushing of the box with Ar gas effectively eliminated the vast majority of the vapor phase organic contaminants from the box. The evolution of the composition of the atmosphere over the next 10-12 months will be monitored by periodic sampling with SPME fibres. Glove composition: The original HypalonTM gloves provided with the glove box were found to be too stiff for at the nominal -20 °C operating temperature. They were subsequently replaced with gloves made from polyurethane, which retains serviceable elasticity down to -20 °C. In order to determine the nonvolatile organic residue on the surface of the new polyurethane gloves, the drippings from the passage of approximately 1 mL of ultrapure dichloromethane over a 2 cm x 2 cm piece of polyurethane glove material was analyzed by GC-MS. Not surprisingly, the GC trace of the glove dichloromethane rinse was found to contain the two monomeric species, viz. methylene diphenyl diisocyanate and 2,2'-(1,4-butanediyl)bis-oxirane, from which the polyurethane polymer material that forms the basis of the gloves was derived. Clearly, a dichloromethane rinse of the gloves prior to their installation is essential to stop the transfer of these nonvolatile organics to the interior of the glove box and ultimately the surfaces of any planetary materials being processed therein. References: [1] Hilts R.W. et al. 2012. Meteoritics & Planetary Science 47:A186. [2] Herd C.D.K. et al. 2011. Science 332:1304-1307.
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    Crystallization, melt inclusion, and redox history of a Martian meteorite; olivine-phyric shergottite Larkman Nunatak 06319
    (2010) Peslier, A. H.; Hnatyshin, D.; Herd, Christopher D. K.; Walton, Erin L.; Brandon, Alan D.; Lapen, T. J.; Shafer, J. T.
    The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo (sub 76-55) from core to rim), pyroxene (from core to rim En (sub 70) Fs (sub 25) Wo (sub 5) , En (sub 50) Fs (sub 25) Wo (sub 25) , and En (sub 45) Fs (sub 45) Wo (sub 10) ), and Cr-rich spinel in a matrix of maskelynite (An (sub 52) Ab (sub 45) ), pyroxene (En (sub 30-40) Fs (sub 40-55) Wo (sub 10-25) ,), olivine (Fo (sub 50) ), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo (sub >72) ) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ-2), followed by crystallization of the groundmass over a range of FMQ-1 to FMQ+0.3. Variation is nearly continuous throughout the differentiation sequence. LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ-4 to -2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.
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    Development of a GC-MS-SPME method for the determination of amines in meteorites
    (2016) Hilts, Robert; Skelhorne, Aaron W.; Simkus, Danielle N.; Herd, Christopher D. K.
    A GC-MS-SPME analytical method for the direct determination of amines in aqueous solution has been developed. The key step in the procedure is the conversion of the amines into their non-volatile ammonium salts by protonation with HCl.
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    Development of a new SPME-GCMS method for the determination of amines in meteorites
    (2016) Hilts, Robert; Skelhorne, Aaron W.; Herd, Christopher D. K.; Simkus, Danielle N.
    Introduction: Amines are polar, water-soluble organics found in all meteorites that contain amino acids. It has been proposed that meteoritic amines are produced by the thermal decarboxylation of amino acids[1]. The analysis of amines using GC-MS techniques has proven to be problematical owing to the high water solubility and high volatility of these compounds. In addition, the strong interactions of polar amine molecules with the stationary phase of a typical polysiloxane column lead to extensive tailing, poor resolution of peaks and low detector response [2]. To circumvent these disadvantages the highly polar amines have been converted to less polar derivatives that are more amenable to GC analysis by substitution of amine N-H hydrogens with weakly polar moieties such as acyl, silyl, dinitrophenyl and methyl groups [2]. The derivatives of amines generally have much weaker interactions with stationary phases and thus are more volatile in terms of their chromatographic behavior, which leads to better separation on the column. In the last decade base-deactivated columns that give good separations of underivatized aliphatic and aromatic amines have been successfully developed [3]. Consequently, volatile amines in aqueous media can now be routinely and reproducibly analyzed by the combination of SPME(solid- phase microextraction) techniques with base-deactivated GC columns that have been specifically engineered to directly separate amines(see for example [4]). The great advantage of this analytical method is that it sidesteps derivatization,which invariably entails the loss of amines, especially those that are more volatile, through side reactions and incomplete transfer during isolation and workup. Results and Discussion: An aqueous test solution containing an in-house collection of amines standards, viz. N-ethylaniline (100 ppm), dipropylamine (100 ppm), methylamine (100 ppm) and piperidine (100 ppm) was prepared in 80 mL of Millipore water. The majority of this solution (70 mL) was set aside for SPME-GC(CP Volamine)MS analysis while the remaining 10 mL was adjusted to ca. pH = 2 using concentrated HCl to convert all of the amines into their corresponding, non-volatile ammonium salts. The water from this acidified mixture was removed on a rotary evaporator , affording a colourless residue. The solid was dissolved in ca. 3 mL of Millipore water and the pH was brought up to 8 by adding ca. 1 mL of 8 M NaOH(aq). The SPME –GC(CP-Volamine) trace of the reconstituted amine mixture was found to contain the same four amine standards as were seen in solution prior to the protonation step. Thus, this result proves that our amine to ammonium to amine methodologycan be used for the direct determination of volatile alkyl amines and aromatic amines. Application of our new SPME-GC(CP-Volamine)MS method to an aqueous extract of a 2-gram sample of the Tagish Lake stone10a afforded a GC trace that contained three different polar aromatic compounds, namely acetophenone , 4-phenylpyridine and 2-phenyl-1,2-propane diol. In the absence of any isotope ratio data, we cannot conclude whether these three aromatic species are indigenous or terrestrial contaminants. It should be pointed out, however, that acetophenone, has been liberated from the Tagish Lake meteorite by the application of heat [5], and thus it does not seem unreasonable to conclude that the acetophenone seen in the trace for 10a likely has an extraterrestrial origin. References: [1] M.A. Sephton, Astronomy and Geophysics, Vol. 45(2), p. 2.8-2.14 (2004). [2] H. Kataoka, Journal of Chromatography A, 733 (1996) 19-34. [3] "An Advanced Base Deactivated Capillary Column for analysis of Volatile amines, Ammonia and Alcohols.", Jaap de Zeeuw, Ron Stricek and Gary Stidsen, Restek Corp. Bellefonte, USA. [4] L. Muller, E. Fattore and E. Benfenati, Journal of Chromatography A, 791 (1997), 221-230. [5] H. Yabuta, G.D. Cody and C.M.O.D. Alexander (2007). Abstract # 2304, 38th Lunar and Planetary Science Conference.
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    Dynamic crystallization of shock melts in Allan Hills 77005: implications for melt pocket formation in Martian meteorites
    (2007) Walton, Erin L.; Herd, Christopher D. K.
    A series of crystallization experiments have been performed on synthetic glasses matching the composition of a melt pocket found in Allan Hills (ALH) 77005 in order to evaluate the heterogeneous nucleation potential of the melt and the effect of oxygen fugacity on crystallization. The starting temperature of the experiments varied from superliquidus, liquidus and subliquidus temperatures. Each run was then cooled at rates of 10, 500 and 1000 degrees C/h at FMQ. The results of this study constrain the heating and cooling regime for a microporphyritic melt pocket. Within the melt pocket, strong thermal gradients existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite. Skeletal olivine in the melt pocket center crystallized from a melt containing few nuclei cooled at a fast rate. Nearer to the melt pocket margin elongate skeletal shapes progress to hopper and equant euhedral, reflecting an increase in nuclei in the melt at the initiation of crystallization and growth at low degrees of supercooling. Cooling from post-shock temperatures took place on the order of minutes. An additional series of experiments were conducted for a melt temperature of 1510 degrees C and a cooling rate of 500 degrees C/h at the FMQ buffer, as well as 1 and 2 log units above and below FMQ. Variation in chromite stability in these experiments is reflected in crystal shapes and composition, and place constraints on the oxygen fugacity of crystallization of the melt pocket. We conclude that the oxygen fugacity of the melt pocket was set by the Fe (super 3+) /Fe (super 2+) ratio imparted by melting of the host rock, rather than external factors such as incorporation of CO (sub 2) -rich Martian atmosphere, or melting and injection of oxidized surface (e.g., regolith) material. Comparison with previous crystallization experiments on melt pockets in Martian basalts indicate that the predominance of dendritic crystal shapes reflects the likelihood that those melt pockets with lower liquidus temperatures will be more completely melted, destroying most or all nuclei in the melt. In this case, crystal growth takes place at high degrees of supercooling, yielding dendritic shapes. It appears as though the melting process is just as important as cooling rate in determining the final texture of the melt pocket, as this process controls elimination or preservation of nuclei at the onset of cooling and crystallization.
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    Evidence of impact melting and post-impact decomposition of sedimentary target rocks from the Steen River impact structure, Alberta, Canada
    (2019) Walton, Erin L.; Timms, Nick E.; Hauck, Tyler E.; MacLagan, Ebberly A.; Herd, Christopher D. K.
    Hypervelocity bolide impacts deliver vast amounts of energy to the Earth's near surface. This crustal process almost universally includes sedimentary target rocks; however, their response to impact is poorly understood, in part because of complexities due to layering, pore space and the presence of volatiles that are difficult to model. The response of carbonates to bolide impact remains contentious, yet whether they melt or decompose and liberate gases by the reaction CaCO3(s) → CaO(s) + CO2(g)↑, has significant implications for post-impact climatic effects. We report on previously unknown carbonate impact melts at the Steen River impact structure, Canada, and the first description of naturally shocked barite, BaSO4. Carbonate melts are preserved as groundmass-supported calcite-rich clasts, sampled from an up to 164 m thick, continuous sequence of crater-fill polymict breccias. Electron microscopy reveals fluidal- and ocellar-textured calcite and barite, intimately associated with silicate melt, consistent with these phases being in the liquid state at the same time. Raman spectroscopy and electron backscatter diffraction (EBSD) mapping confirm the presence of high-pressure phases – reidite and coesite – within some Steen River carbonate melt-bearing breccias. These minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history. Preservation of reidite lamellae in zircon indicates a shock pressure >30 GPa <60 GPa and temperatures <1473 K. In addition to melting, we present compelling evidence for widespread (70–100%) decomposition of carbonate target rocks, mixed as lithic clasts into hot impact breccias. In this context, decomposition occurs strictly post-impact due to thermal equilibration-related heating. We demonstrate that this mechanism for CO2 outgassing is likely more widespread than previously recognized. The presence of andradite-grossular garnet serve as mineralogical markers of decomposition, analogous to limestone-replacing skarn deposits. Ca-rich garnet may therefore prove an important indicator mineral for post-shock decomposition of carbonate-bearing target rocks at other craters. These findings significantly advance our understanding of how sedimentary rocks respond to hypervelocity impact, and have wide-reaching implications for estimating the amount and timing of climatically-active volatile release due to impact events.
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    Geologic mapping of candidate source craters for martian meteorites
    (2019) Hamilton, Jarret S.; Herd, Christopher D. K.; Walton, Erin L.; Tornabene, Livio L.
    Martian meteorites are the only rock samples from Mars that are currently accessible for research in Earth-based laboratories. The meteorites are derived from the near-surface units adjacent to their source craters. These source craters eject material beyond the martian escape velocity during formation from random, hypervelocity impact on the planet’s surface. Specific source craters for any of the martian meteorites are unknown. This study uses results from a queried database to constrain potential source craters based on parameters such as ejection age, petrology, preservation, and crater diameter. Preliminary results indicate a number of candidate source craters that require detailed mapping to better understand their morphology, relative age, and volcanic context.
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    Hydrous olivine alteration on Mars and Earth
    (2020) Vaci, Zoltan; Agee, Carl B.; Herd, Christopher D. K.; Walton, Erin L.; Tschauner, Oliver; Ziegler, Karen; Prakapenka, Vitali B.; Greenberg, Eran; Monique-Thomas, Sylvia
    Hydrous alteration of olivine macrocrysts in a Martian olivine phyric basalt, NWA 10416, and a terrestrial basalt from southern Colorado are examined using SEM, EPMA, TEM, and µXRD techniques. The olivines in the meteorite contain linear nanotubes of hydrous material, amorphous areas, and fluid dissolution textures quite distinct from alteration identified in other Martian meteorites. Instead, they bear resemblance to terrestrial deuteric alteration features. The presence of the hydrous alteration phase Mg-laihunite within the olivines has been confirmed by µXRD analysis. The cores of the olivines in both Martian and terrestrial samples are overgrown by unaltered rims whose compositions match those of a separate population of groundmass olivines, suggesting that the core olivines are xenocrysts whose alteration preceded crystallization of the groundmass. The terrestrial sample is linked to deep crustal metasomatism and the “ignimbrite flare-up” of the Oligocene epoch. The comparison of the two samples suggests the existence of an analogous relatively water-rich magmatic reservoir on Mars.
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    Investigation of impact melt clasts in allochthonous crater-fill deposits of the Steen River impact structure
    (2016) MacLagan, Ebberly A.; Herd, Christopher D. K.; Walton, Erin L.
    The Steen River impact structure (SRIS) is a buried complex crater located in northwestern Alberta that is thought to have formed around 91 ± 7 Ma. The goal of this study was to investigate the impact melt clasts in the SRIS breccia to determine their mechanisms of formation and emplacement.
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    Investigation of impact melt in allochthonous crater-fill deposits of the Steen River impact structure, Alberta, Canada
    (2018) MacLagan, Ebberly A.; Walton, Erin L.; Herd, Christopher D. K.; Dence, Michael
    The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix-supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K2O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter-sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale-derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile-rich nature of the shale protolith. The wide compositional range of impact-melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile-rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed-target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).
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    Isotopic and petrographic evidence for young Martian basalts
    (2008) Walton, Erin L.; Kelley, Simon P.; Herd, Christopher D. K.
    Radiometric age data for shergottites yield ages of 4.0 Ga and 180-575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new (super 39) Ar- (super 40) Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of (super 40) Ar/ (super 36) Ar versus (super 39) Ar/ (super 36) Ar, the more strongly shocked ALH 77005 (45-55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding (super 40) Ar/ (super 36) Ar ratios of 1600-2026. Apparent ages calculated from these extractions range from 374-8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30-44 GPa) yield a fitted age of 382 + or - 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest (super 40) Ar/ (super 36) Ar ratios (1260-1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess (super 40) Ar in the shock melts. The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martian surface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be > or =4.0 Ga; we contend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (Late Amazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We show that although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasively altered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.
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    Localized shock melting in lherzolitic shergottite Northwest Africa 1950: comparison with Allan Hills 77005
    (2007) Herd, Christopher D. K.; Walton, Erin L.
    The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low-Ca pyroxene poikilically enclosing cumulate olivine (Fo70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low- and high-Ca pyroxene, cumulate olivine (Fo68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large-scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter-size) melt pockets have crystallized magnesian olivine (Fo78–87) and chromite, embedded in an Fe-rich, Al-poor basaltic to picro-basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 103 crystals/mm2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 105 crystals/mm2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.
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    Martian regolith in Elephant Moraine 79001 shock melts? evidence from major element composition and sulfur speciation
    (2010) Walton, Erin L.; Jugo, Pedro J.; Herd, Christopher D. K.; Wilke, Max
    Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S (super 2-) versus S (super 6+) ). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt - pyroxene (En (sub 72-75) Fs (sub 20-21) Wo (sub 5-7) ) and olivine (Fo (sub 75-80) ). Reconstruction of glass+crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite. Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.