Browsing by Author "Walton, Erin L."
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- ItemA new Martian meteorite from Oman: mineralogy, petrology, and shock metamorphism of olivine-phyric basaltic shergottite Sayh al Uhaymir 150(2005) Walton, Erin L.; Spray, John G.; Bartoschewitz, RainerThe Sayh al Uhaymir (SaU) 150 meteorite was found on a gravel plateau, 43.3 km south of Ghaba, Oman, on October 8, 2002. Oxygen isotope (δ17O 2.78; δ18O 4.74), CRE age (∼1.3 Ma), and noble gas studies confirm its Martian origin. SaU 150 is classified as an olivine-phyric basalt, having a porphyritic texture with olivine macrocrysts set in a finer-grained matrix of pigeonite and interstitial maskelynite, with minor augite, spinel, ilmenite, merrillite, pyrrhotite, pentlandite, and secondary (terrestrial) calcite and iron oxides. The bulk rock composition, in particular mg (68) [molar Mg/(Mg + Fe) × 100], Fe/Mn (37.9), and Na/Al (0.22), are characteristic of Martian meteorites. Based on mineral compositions, cooling rates determined from crystal morphology, and crystal size distribution, it is deduced that the parent magma formed in a steady-state growth regime (magma chamber) that cooled at <2 °C/hr. Subsequent eruption as a thick lava flow or hypabyssal intrusion entrained a small fraction of xenocrystic olivine and gave rise to a magmatic foliation, with slow cooling allowing for near homogenization of igneous minerals. SaU 150 experienced an equilibration shock pressure of 33–45 GPa in a single impact event. Post-shock heat gave rise to localized melting (∼11 vol%). Larger volume melts remained fluid after pressure release and crystallized dendritic olivine and pyroxene with fractal dimensions of 1.80–1.89 and 1.89–1.95, respectively, at −ΔT >70–365 °C. SaU 150 is essentially identical to SaU 005/094, all representing samples of the same fall that are similar to, but distinct from, the DaG shergottites.
- ItemA previously unrecognized example of the shock-induced breakdown of biotite to garnet from the Steen River impact structure, Canada(2016) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Tschauner, OliverShock metamorphic effects from quartz and feldspar have been studied in detail, and to a lesser extent olivine and pyroxene. Less is known about how shock is manifest in other minerals such as double chain inosilicates, phyllosilicates, carbonates and sulfates. In this study, we investigate shock in crystalline basement rocks from the Steen River impact structure (SRIS), a 25-km diameter complex crater in NW Alberta, Canada.
- ItemA revised shock history for the youngest unbrecciated lunar basalt - northwest Africa 032(2020) Mijajlovic, Tatiana; Xi Xue; Walton, Erin L.Northwest Africa (NWA) 032 is the youngest radio metrically dated mare basalt, with concordant Rb-Sr and Sm-Ndages of 2.947 ± 0.016 Ga and 2.931 ± 0.092, respectively . Measurement of the cosmogenic nuclides present in NWA 032 suggest an Earth-Moon transfer age of 0.5Ma, typical of lunar meteorites . NWA 032 is an unbrecciate dolivine-pyroxene-phyric basalt, with olivine, pyroxene and plagioclase as major minerals (Fig.1). A previous description of shock effects in NWA 032 allowed for a shock pressure estimate of ~4060 GPa ; however, the shock state of plagioclase feldspar (shock-amorphized vs crystalline) was inconclusive, owing to the fine grain size of this mineral (≤1μm). The purpose of our study is to characterize the shock deformation and transformation effects in NWA 032 using field emission scanning electron microscopy (FESEM) and micro-Raman spectroscopy, focusing on the structural state of feldspar, shock deformation recorded in igneous olivine and pyroxene, as well as characterizing the crystallization products of shock melting. The latter have been demonstrated as useful criteria to evaluate shock conditions . Our results more tightly constrain the shock history experienced by NWA 032.
- ItemA revised shock history for the youngest unbrecciated lunar basalt – Northwest Africa 032(2020) Mijajlovic, Tatiana; Xue, Xie; Walton, Erin L.Northwest Africa (NWA) 032 is considered to be the youngest radiometrically-dated mare basalt, with concordant Rb-Sr and Sm-Nd ages of 2.947 ± 0.016 Ga and 2.931 ± 0.092, respectively . These ages are ~175 Ma older than those from 40Ar39Ar (2.779 ± 0.014 Ga) . NWA 032 contains a high modal abundance of pyroxene (50.7 vol%), plagioclase (29.4 vol%), and olivine (11.3 vol%) . The texture is that of an unbrecciated olivine-pyroxene phyric basalt. Olivine phenocrysts are zoned with Mg rich cores (Fo34-50) and thin, discontinuous Fe-rich rims (Fo30). Fine grained (≤ 1μm) elongated, tapered plagioclase crystals (An80-90) are present within the groundmass, interspersed with pyroxene (En1-25Wo15-25) of similar shape and size. These two minerals occur in a plumose texture, radiating from a common nucleation point. Pyroxene may be categorized based on grain size as either groundmass (<1 μm), intermediate crystals (~50 μm) or larger phenocrysts (~100 μm) . The mineralogy of NWA 032 makes it ideal for the study and classification of shock features based on the updated shock classification scheme , which relies on petrographic observations of deformation and transformation in olivine, pyroxene and plagioclase – the three most abundant minerals in NWA 032. A previous description of shock effects in NWA 032 allowed for a shock pressure estimate of ~40-60 GPa ; however, the shock state of plagioclase feldspar (shock-amorphized vs crystalline) was inconclusive, owing to the fine grain size of this mineral (≤1μm). The purpose of our study is to characterize the shock deformation and transformation effects in NWA 032 using a combination of field emission scanning electron microscopy (FESEM) and Raman spectroscopy, focusing on the shock state of feldspar, as well as characterizing the crystallization products of shock melting. The latter have been demonstrated as useful criteria to evaluate shock conditions . Our results more tightly constrain the shock history experienced by NWA 032.
- ItemAnatomy 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.
- ItemConstraining the shock conditions experienced by Haughton crystalline basement rocks: a combined Raman spectroscopy and electron backscatter diffraction study of Anomaly Hill zircons(2020) Walton, Erin L.; Jurak, Haley A. M.; Timms, Nick E.; Osinski, Gordon R.Haughton is a 23-km diameter impact structure on Devon Island, Canada [1, 2]. The target stratigraphy comprised ~1880 m of Lower Paleozoic sedimentary rocks, unconformably overlying the Precambrian Canadian Shield . Near the centre of the structure, is a location characterized by negative gravimetric and positive magnetic anomalies, known as “Anomaly Hill” . Highlyshocked, pumice-like lithic clasts are abundant at this locale, and include gneissic and carbonaterich clasts [4, 5]. In this study, we examined 20 zircon grains from a single crystalline clast collected at Anomaly Hill, to reveal microstructures at the micrometer to nanometer scale. Earlier work on Haughton zircons  did not incorporate EBSD, and so, is missing a wealth of information to facilitate the identification of key microstructures including FRIGN (former reidite in granular neoblasts) zircon, non-FRIGN granular textures, neoblasts versus sub-grain rotation formation of subdomains, and various dissociation textures, as described in [7, 8]. The goal of our study is to constrain the shock conditions experienced by crystalline basement rocks at Haughton using zircon, a mineral that is increasingly recognized as a sensitive shock indicator.
- ItemCrystallization, 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.
- ItemDwell time at high pressure of meteorites during impact ejection from Mars(2020) Bowling, Timothy J.; Johnson, Brandon C.; Wiggins, Sean E.; Walton, Erin L.; Melosh, Henry Jay; Sharp, Thomas G.Martian meteorites are currently the only rock samples from Mars available for direct study in terrestrial laboratories. Linking individual specimens back to their source terrains is a major scientific priority, and constraining the size of the impact craters from which each sample was ejected is a critical step in achieving this goal. During ejection from the surface of Mars by hypervelocity impacts, these meteorites were briefly compressed to high temperatures and pressures. The period of time that these meteorites spent at high pressure during ejection, or the ‘dwell time’, has been used to infer the size of the crater from which they were ejected. This inference requires assumptions that relate shock duration to impactor size, and the relation used by many authors is neither physically motivated nor accurate. Using the iSALE2D shock physics code we simulate vertical impacts at high resolution to investigate the dwell time that basaltic rocks from Mars (shergottites) spend at high pressure and temperature during ejection. Future simulation of oblique impacts will lead to more accurate dwell time estimates. Ultimately, we find that dwell time is insensitive to changes in impact velocity but for a given impact, dwell times are longer for material originating from greater depth and material that experiences higher shock pressures. Using our results, we provide scaling laws for estimating impactor size. During the formation of craters 1.9, 14, and 104 km in diameter, material capable of escaping Mars will have mean dwell times of 1, 10, and 100 ms, respectively.
- ItemDynamic 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.
- ItemEvaluating baddeleyite oxygen isotope analysis by secondary ion mass spectrometry (SIMS)(2018) Davies, J. H. F. L.; Stern, R. A.; Heaman, L. M.; Mose, D. E.; Walton, Erin L.; Vennemann, T.Two baddeleyite megacrysts were evaluated as potential reference materials (RMs) for SIMS oxygen isotope analysis, and utilized to understand and calibrate instrumental mass fractionation (IMF).
- ItemEvidence 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.
- ItemFrictional melting processes and the generation of shock veins in terrestrial impact structures: evidence from the Steen River impact structure, Alberta, Canada(2016) Walton, Erin L.Shock‐produced melt within crystalline basement rocks of the Steen River impact structure (SRIS) are observed as thin (1 – 510 μm wide), interlocking networks of dark veins which cut across and displace host rock minerals. Solid‐state phase transformations, such as ferro‐pargasite to an almandine‐andraditemajorite garnet and amorphization of quartz and feldspar, are observed in zones adjacent to comparatively wider (50─500 μm) sections of the shock veins. Shock pressure estimates based on the coupled substitution of Na+, Ti4+ and Si4+ for divalent cations, Al3+ and Cr3+ in garnet (14─19 GPa) and the pressure required for plagioclase (Ab62‐83) amorphization at elevated temperature (14−20 GPa) are not appreciably different from those recorded by deformation effects observed in non‐veined regions of the bulk rock (14─20 GPa). This spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact‐generated melt and large deviatoric stresses experienced by minerals along vein margins.
- ItemGeologic 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.
- ItemHeterogeneous mineral assemblages in Martian meteorite Tissint as a result of a recent small impact event on Mars(2014) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Filiberto, Justin R.The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoodite + or - stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite + magnesiowuestite or clinopyroxene + magnesiowuestite. The pressure stabilities of these mineral assemblages are approximately 15 GPa and >19 GPa, respectively. The approximately 200-mu m-wide margin of a thicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprising glass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint > or = 29-30 GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicate perovskite plus magnesiowuestite. Small volumes of shock melt (<100 mu m) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 s in larger volumes of melt (1 mm (super 2) ). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10-20 ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at approximately 1 Ma was not exceptionally large.
- ItemHydrous 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, SylviaHydrous 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.
- ItemInsight into the post-impact hydrothermal system at the Chicxulub impact structure, Mexico(2020) Walton, Erin L.; Groeneveld, Tianna M.The 66-million-year-old Chicxulub impact structure was formed by the hypervelocity impact of an asteroid into the Yucatán peninsula. Hypervelocity impacts generate hydrothermal circulation systems in the resulting craters and create long-term thermal anomalies in near-surface areas of the crust. Drill cores have sampled preserved impactites from the Chicxulub crater with the most recent core, M0077A, intersecting the peak ring. Analysis of the mineralogy, composition, and occurrence of secondary minerals in the core may be used to understand conditions in the postimpact hydrothermal environment. In this study, the secondary minerals within ten thin sections and five polished offcut tiles from drill core M0077A were investigated. The thin sections were sampled from ~750-1300 metres below sea floor (mbsf), while the tiles were constrained to a depth of ~1250-1330 mbsf. Identified secondary minerals include andradite-grossular garnet assemblages, anhydrite, Fe-oxides, calcite, Mg-Fe-Al-rich clay group minerals, sulfides, Casulphates, titanite, albite, fluorite, epidote, quartz, and halite. These minerals occur as vein- and vug-filling assemblages in the shocked granite and suevite breccia. The andradite-grossular garnet assemblages are of particular interest as they suggest that high-temperatures (300-400 C⁰) dominated the early stages of the post-impact hydrothermal system. Secondary minerals anhydrite, titanite, epidote, and Fe-oxides support the temperature range indicated by the garnets, but do not constrain the temperature as tightly. The temperature range indicated by the garnets is sufficiently hot for the hydrothermal system to persist for at least two million years, based on previous numerical modeling1,3. Calcite, Mg-Fe-Al-rich clay group minerals, sulfides, and halite formed as the hydrothermal system cooled. The secondary minerals identified attest to the depth and range of thermal and chemical modification that occurred in the Earth’s crust due to impact events, both immediately after and in the ensuing millions of years.
- ItemInvestigating the response of biotite to impact metamorphism: examples from the Steen River impact structure, Canada(2018) Walton, Erin L.; Sharp, Thomas G.; Hu, Jinping; Tschauner, OliverImpact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm−3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.
- ItemInvestigation 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.
- ItemInvestigation 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, MichaelThe 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%).
- ItemIsotopic 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.