| 1. |
- Alwmark, Carl, et al.
(författare)
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A 3-D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X-ray tomographic microscopy-Method and applications
- 2011
-
Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 46:8, s. 1071-1081
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Tidskriftsartikel (refereegranskat)abstract
- A method is described for imaging in 3-D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X-ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long-term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment-dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non-destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4-6, L4-6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two-thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.
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| 2. |
- Alwmark, Carl, et al.
(författare)
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Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body
- 2015
-
Ingår i: Geology. - 0091-7613. ; 43:4, s. 279-282
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Tidskriftsartikel (refereegranskat)abstract
- Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar system's asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years.
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| 3. |
- Alwmark, Carl, et al.
(författare)
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Shocked quartz grains from the Malingen structure, Sweden-Evidence for a twin crater of the Lockne impact structure
- 2014
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 49:6, s. 1076-1082
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Tidskriftsartikel (refereegranskat)abstract
- The Malingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Malingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Malingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne-Malingen pair).
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| 4. |
- Alwmark, Sanna, et al.
(författare)
-
Shock barometry of the Siljan impact structure, Sweden
- 2011
-
Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 46:12, s. 1888-1909
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Tidskriftsartikel (refereegranskat)abstract
- The Siljan impact structure in Sweden is the largest confirmed impact structure in Western Europe. Despite this, the structure has been poorly studied in the past, and detailed studies of shock metamorphic features in the target lithologies are missing. Here, we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at Siljan. At 21 localities from an area approximately 20 km in diameter located centrally in the structure, the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter. The most strongly shocked samples, recording pressures of up to 20 GPa, occur at the very central part of the structure, and locally in these samples, higher pressures causing melting conditions in the affected rocks were reached. Pressures recorded in the studied samples decrease outwards from the center of the structure, forming roughly circular envelopes around the proposed shock center. Based on the distribution pattern of shocked quartz at Siljan, the original transient cavity can be estimated at approximately 3238 km in diameter. After correcting for erosion, we conclude that the original rim to rim diameter of the Siljan crater was somewhere in the size range 5090 km.
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| 5. |
- Gnos, E., et al.
(författare)
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The Wabar impact craters, Saudi Arabia, revisited
- 2013
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 48:10, s. 2000-2014
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Tidskriftsartikel (refereegranskat)abstract
- The very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0-2.0myr(-1) southward. Shock lithification took place even at the smallest, 11m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20-30m.
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| 6. |
- Herrmann, Maria, et al.
(författare)
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The effect of low-temperature annealing on discordance of U–Pb zircon ages
- 2021
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Discordant U–Pb data of zircon are commonly attributed to Pb loss from domains with variable degree of radiation damage that resulted from α-decay of U and Th, which often complicates the correct age interpretation of the sample. Here we present U–Pb zircon data from 23 samples of ca. 1.7–1.9 Ga granitoid rocks in and around the Siljan impact structure in central Sweden. Our results show that zircon from rocks within the structure that form an uplifted central plateau lost significantly less radiogenic Pb compared to zircon grains in rocks outside the plateau. We hypothesize that zircon in rocks within the central plateau remained crystalline through continuous annealing of crystal structure damages induced from decay of U and Th until uplifted to the surface by the impact event ca. 380 Ma ago. In contrast, zircon grains distal to the impact have accumulated radiation damage at shallow and cool conditions since at least 1.26 Ga, making them vulnerable to fluid-induced Pb-loss. Our data are consistent with studies on alpha recoil and fission tracks, showing that annealing in zircon occurs at temperatures as low as 200–250 °C. Zircon grains from these samples are texturally simple, i.e., neither xenocrysts nor metamorphic overgrowths have been observed. Therefore, the lower intercepts obtained from regression of variably discordant zircon data are more likely recording the age of fluid-assisted Pb-loss from radiation-damaged zircon at shallow levels rather than linked to regional magmatic or tectonic events.
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| 7. |
- Holm-Alwmark, Sanna, et al.
(författare)
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Combining shock barometry with numerical modeling : Insights into complex crater formation—The example of the Siljan impact structure (Sweden)
- 2017
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 52:12, s. 2521-2549
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Tidskriftsartikel (refereegranskat)abstract
- Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz-bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best-fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best-fit model results in a final crater (rim-to-rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak-ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure.
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| 8. |
- Holm-Alwmark, Sanna, et al.
(författare)
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Resolving the age of the Puchezh-Katunki impact structure (Russia) against alteration and inherited 40Ar* – No link with extinctions
- 2021
-
Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 301, s. 116-140
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of a “death from above” cause for biotic crises and extinction events is intriguing, to say the least, but such claims must be supported by reliable and reproducible data, not only impact diagnostic criteria, but also accurate and precise radioisotopic ages of the impact structures/events. To date, only one example of such an impact-related global extinction event is confirmed, at the end of the Cretaceous period. Here we present and discuss results of newly obtained 40Ar/39Ar data from step heating analysis of impact melt rock samples from the 40 km-in-diameter Puchezh-Katunki impact structure, Russia, which allow us to precisely and accurately date its formation at 195.9 ± 1.0 Ma (2σ; P = 0.10). Based on these new data, we challenge the proposed temporal correlation with as many as five different extinction events (including the end-Triassic mass extinction) that were based on previous age estimations ranging from ∼164 to 203 Ma. Our new age for the formation of the Puchezh-Katunki impact structure allows us to exclude a relationship between this impact event and a known extinction event. We also show that careful sample preparation and methodology can overcome problems with inherited and trapped 40Ar, issues that are common when dating impact melt rocks. This is supported by 40Ar* diffusion and mixing numerical models showing that the most prominent negative effects in the case of the Puchezh-Katunki impact melt rock samples are caused by hydrothermal alteration and undegassed melt rock domains present in an otherwise homogenized melt rock. Numerical modeling also shows that the 40Ar* from high-Ca inherited crystals or clasts is decoupled from the melt rock during step heating experiments allowing to safely recover a plateau age. Finally, our results highlight the importance of improving the database of ages of impact structures and show that caution should be practiced when suggesting connections between specific impact events and extinction events, especially in the case of poorly dated impact structures.
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| 9. |
- Holm-Alwmark, Sanna, et al.
(författare)
-
Shocked quartz in distal ejecta from the Ries impact event (Germany) found at ~ 180 km distance, near Bernhardzell, eastern Switzerland
- 2021
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.
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| 10. |
- Martell, Josefin, et al.
(författare)
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Shock deformation in zircon grains from the Mien impact structure, Sweden
- 2021
-
Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 56:2, s. 362-378
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Tidskriftsartikel (refereegranskat)abstract
- Recognition of impact-induced deformation of minerals is crucial for the identification and confirmation of impact structures as well as for the understanding of shock wave behavior and crater formation. Shock deformed mineral grains from impact structures can also serve as important geochronometers, precisely dating the impact event. We investigated zircon grains from the Mien impact structure in southern Sweden with the aim of characterizing shock deformation. The grains were found in two samples of impact melt rock with varying clast content, and in one sample of suevitic breccia. We report the first documentation of so-called “FRIGN zircon” (former reidite in granular neoblastic zircon) from Mien (pre-erosion diameter 9 km), which confirms that this is an important impact signature also in relatively small impact structures. Furthermore, the majority of investigated zircon grains contain other shock-related microtextures, most notably granular and microporous textures, that occur more frequently in grains found in the impact melt than in the suevitic breccia. Our findings show that zircon grains that are prime candidates for establishing a new and improved age refinement of the Mien impact structure are present in the impact melt.
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| 11. |
- Martell, Josefin, et al.
(författare)
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The scale of a martian hydrothermal system explored using combined neutron and x-ray tomography
- 2022
-
Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:19
-
Tidskriftsartikel (refereegranskat)abstract
- Nakhlite meteorites are igneous rocks from Mars that were aqueously altered ~630 million years ago. Hydrothermal systems on Earth are known to provide microhabitats; knowledge of the extent and duration of these systems is crucial to establish whether they could sustain life elsewhere in the Solar System. Here, we explore the three-dimensional distribution of hydrous phases within the Miller Range 03346 nakhlite meteorite using nondestructive neutron and x-ray tomography to determine whether alteration is interconnected and pervasive. The results reveal discrete clusters of hydrous phases within and surrounding olivine grains, with limited interconnectivity between clusters. This implies that the fluid was localized and originated from the melting of local subsurface ice following an impact event. Consequently, the duration of the hydrous alteration was likely short, meaning that the martian crust sampled by the nakhlites could not have provided habitable environments that could harbor any life on Mars during the Amazonian.
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| 12. |
- Plan, Anders, et al.
(författare)
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Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon
- 2021
-
Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 56:10, s. 1795-1828
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Tidskriftsartikel (refereegranskat)abstract
- Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stageIII) from the ~200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4 microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO4 during shock.
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| 13. |
- Sánchez Gómez, Sebastián Tomás, et al.
(författare)
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A possible 5 km wide impact structure with associated 22 km wide exterior collapse terrain in the Alhabia–Tabernas Basin, southeastern Spain
- 2023
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Ingår i: Meteoritics and Planetary Science. - 1086-9379. ; 58:10, s. 1512-1539
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Tidskriftsartikel (refereegranskat)abstract
- The Tabernas–Alhabia Basin is a structural depression situated in the province of Almería, southeastern Spain. The basin is filled with Neogene, Pliocene, and Pleistocene sediments resting discordantly on a Paleozoic metamorphic basement. During the marine Tortonian sedimentation, a bed of breccia (Gordo megabed) was formed. It consists of rotated sedimentary megablocks commonly capped and/or surrounded by a polymict breccia composed mainly of up to dm-sized clasts of the crystalline (schist) basement. Previous work has suggested the bed to be a seismite corresponding to events induced by earthquakes. Here, we link the formation of the Gordo megabed with an ∼5 km wide, rimmed depression with exposed breccias on the northern flank of the Sierra de Gádor mountain. This semicircular structure, developed in mainly schists and dolostone of the basement, is delimited to the W, S, and E by an up to 350 m high escarpment with overturned stratigraphy. Toward the north, this crater-like structure opens toward the Gordo megabed of the Tabernas Basin. In the southern sector, the overturned strata transform outward for into a blocky allochthonous breccia with decreasing thickness and clast size. In the interior of the structure, there are occurrences of graded breccia and arenite superposed on a blocky, autochthonous breccia. Based on the presence of mineralogical shock metamorphic evidence, potential shatter cones, and a high Ir anomaly (∼500 ppb) as well as the position of the structure near the town of Alhama de Almería, we propose to call it the Alhama de Almería impact structure.
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| 14. |
- Zachén, Gabriel, et al.
(författare)
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Shocked Mesosiderites: Hidden in Plain Sight?
- 2022
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Summary: Four mesosiderites were classified, and petrographic analyses were performed to look for indicators of shock. Of the four samples, Lamont and QUE 86900 were found to contain shock-related features, such as planar microstructures and quenched melts, respectively.
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| 15. |
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| 16. |
- Alwmark, Sanna
(författare)
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Terrestrial consequences of hypervelocity impact – shock metamorphism, shock barometry, and newly discovered impact structures
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Impact cratering was once considered a rare geological process of no, or little, importance to the evolution of the Solar System and planet Earth. After more than 50 years of space exploration and the discovery of numerous (~190 as of October 2016) impact structures on Earth, this view has changed, and it is now clear that impact craters are in fact one of the most common morphological features on solid bodies in the Solar System. The formation of a (hypervelocity) impact crater involves extreme conditions that cannot be compared with any other natural geological process, with extreme pressures and temperatures causing melting and/or vaporization of both projectile and portions of the target rocks. Upon impact, shock waves are generated at the projectile-target interface, which pass through the target rocks at supersonic velocity. The passage of the shock waves induce irreversible changes, so called shock metamorphic effects in the target rocks, including the formation of high pressure mineral polymorphs, diaplectic glasses, and microdeformation features in minerals. The most investigated of these microstructures are planar deformation features (PDFs) in quartz. These are straight, parallel, closely spaced (2-10 µm apart), sets of (when fresh) glass lamellae only naturally formed by impact cratering. PDFs are oriented parallel to specific crystallographic planes, with the most frequently reported orientations being parallel to low Miller-Bravais index planes (e.g., {10‾13}, {10‾12}). The orientation pattern of a PDF population differ depending on the pressure that the host quartz grain was subjected to, meaning that the orientations of PDFs can be used as a shock barometer, allowing e.g., production of shock barometry profiles that illustrate shock attenuation at impact structures. The research presented in this thesis focuses on impact craters, and the process by which they form, impact cratering, with special emphasis on shock metamorphic features in target rocks at the Siljan impact structure (Sweden). The results and discussion highlight the importance of the way datasets of PDF statistics are obtained and processed, using manual and/or automated methods of indexing. The interpretation of the dataset can influence the shock barometry models, and the need for a unified method is discussed. With regards to the Siljan impact structure, the pre-erosional rim-to-rim diameter of the crater was estimated to be on the order of 60 km, based on a combination of shock barometry and numerical simulation, produced by a collision between a ~5 km diameter projectile and Earth. Results of the numerical modeling are consistent with a sedimentary thickness overlying the crystalline basement at the time of impact of ~2.5 km, and post-impact erosion of the crater on the order of 3 to 3.5 km.The thesis also encompasses studies of two other, newly confirmed, Swedish impact structures, Målingen and Hummeln. The possible means of formation for both Målingen and Hummeln had been discussed for many years before the first bona fide evidence for the impact origin of the two structures was presented in papers included in this thesis.Furthermore, terrestrial impact structures with reliable ages (i.e., errors on age of less than 2 %) are discussed in the context of possible variations in the impactor flux to Earth over time. According to the results, there is presently no evidence for the existence of a periodic contribution to the terrestrial impact population.
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| 17. |
- Holm-Alwmark, Sanna, et al.
(författare)
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Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden
- 2022
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Ingår i: Geology. - 0091-7613. ; 50:3, s. 377-382
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Tidskriftsartikel (refereegranskat)abstract
- Little is known about the microstructural behavior of magnetite during hypervelocity impact events, even though it is a widespread accessory mineral and an important magnetic carrier in terrestrial and extraterrestrial rocks. We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden. Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. Deformation twins along {111} with shear direction of <112> are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features (PDFs) in quartz and shock twins in zircon, range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is observed at >5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures >5 GPa, which may be useful in quartz-poor samples. Despite significant hydrothermal alteration and the variable transformation of host magnetite to hematite, shock effects are preserved, which demonstrates that magnetite is a reliable mineral for preserving shock deformation over geologic time.
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| 18. |
- Horgan, Briony, et al.
(författare)
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Mineralogy, Morphology, and Emplacement History of the Maaz Formation on the Jezero Crater Floor From Orbital and Rover Observations
- 2023
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Ingår i: Journal of Geophysical Research: Planets. - 2169-9097. ; 128:8
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Tidskriftsartikel (refereegranskat)abstract
- The first samples collected by the Perseverance rover on the Mars 2020 mission were from the Maaz formation, a lava plain that covers most of the floor of Jezero crater. Laboratory analysis of these samples back on Earth would provide important constraints on the petrologic history, aqueous processes, and timing of key events in Jezero crater. However, interpreting these samples requires a detailed understanding of the emplacement and modification history of the Maaz formation. Here we synthesize rover and orbital remote sensing data to link outcrop-scale interpretations to the broader history of the crater, including Mastcam-Z mosaics and multispectral images, SuperCam chemistry and reflectance point spectra, Radar Imager for Mars' subsurface eXperiment ground penetrating radar, and orbital hyperspectral reflectance and high-resolution images. We show that the Maaz formation is composed of a series of distinct members corresponding to basaltic to basaltic-andesite lava flows. The members exhibit variable spectral signatures dominated by high-Ca pyroxene, Fe-bearing feldspar, and hematite, which can be tied directly to igneous grains and altered matrix in abrasion patches. Spectral variations correlate with morphological variations, from recessive layers that produce a regolith lag in lower Maaz, to weathered polygonally fractured paleosurfaces and crater-retaining massive blocky hummocks in upper Maaz. The Maaz members were likely separated by one or more extended periods of time, and were subjected to variable erosion, burial, exhumation, weathering, and tectonic modification. The two unique samples from the Maaz formation are representative of this diversity, and together will provide an important geochronological framework for the history of Jezero crater.
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| 19. |
- Meier, Matthias M.M., et al.
(författare)
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A tale of clusters : No resolvable periodicity in the terrestrial impact cratering record
- 2017
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 467:3, s. 2545-2551
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Tidskriftsartikel (refereegranskat)abstract
- Rampino & Caldeira carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ∼26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ('robust'; 2SE < 2 per cent) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ∼80 per cent of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira originates in a subset of 'clustered' impacts (i.e. with overlapping ages). The ∼26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly spaced) clusters. The presence of a few impact age clusters (e.g. from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record.
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| 20. |
- Paige, David A., et al.
(författare)
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Ground penetrating radar observations of the contact between the western delta and the crater floor of Jezero crater, Mars
- 2024
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Ingår i: Science Advances. - 2375-2548. ; 10:4
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Tidskriftsartikel (refereegranskat)abstract
- The delta deposits in Jezero crater contain sedimentary records of potentially habitable conditions on Mars. NASA's Perseverance rover is exploring the Jezero western delta with a suite of instruments that include the RIMFAX ground penetrating radar, which provides continuous subsurface images that probe up to 20 meters below the rover. As Perseverance traversed across the contact between the Jezero crater floor and the delta, RIMFAX detected a distinct discontinuity in the subsurface layer structure. Below the contact boundary are older crater floor units exhibiting discontinuous inclined layering. Above the contact boundary are younger basal delta units exhibiting regular horizontal layering. At one location, there is a clear unconformity between the crater floor and delta layers, which implies that the crater floor experienced a period of erosion before the deposition of the overlying delta strata. The regularity and horizontality of the basal delta sediments observed in the radar cross sections indicate that they were deposited in a low-energy lake environment.
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| 21. |
- Quintero, Raiza R., et al.
(författare)
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Shocked quartz in sandstone from the buried Ilkurlka impact structure, Officer Basin, Western Australia
- 2024
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Ingår i: Meteoritics and Planetary Science. - 1086-9379. ; 59:1, s. 68-87
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Tidskriftsartikel (refereegranskat)abstract
- The Ilkurlka structure is an ~12 km diameter buried circular aeromagnetic anomaly within the Officer Basin in Western Australia. Prior studies postulated a range of origins, including meteorite impact. We report the presence of pervasive deformation in the first drill cores from the structure. Brecciated sandstone and siltstone contain arrays of quartz grains with concussion fractures and rare shocked quartz grains with planar deformation features (PDF). Universal stage measurements of two quartz grains reveal one grain with PDF parallel to (0001) orientation and three PDF sets parallel to (Formula presented.). A second grain contains three PDF sets parallel to (Formula presented.) and one set parallel to (Formula presented.). The shocked grains are interpreted to have formed in situ, rather than representing transported detrital shocked grains. These results suggest local shock compression of at least 10 GPa; however, preservation of primary porosity and overall paucity of shocked grains may indicate lower mean shock pressures. (U-Th)/He dating of 58 apatite grains from four samples across both cores shows a dominant age population at ~265 Ma and a minor age population at ~135 Ma. These dates overlap with regional events and thus do not provide an unambiguous impact age. An upper Carboniferous to lower Permian maximum impact age is provisionally proposed based on inferred missing target rock stratigraphy.
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| 22. |
- Sun, Vivian Z., et al.
(författare)
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Overview and Results From the Mars 2020 Perseverance Rover's First Science Campaign on the Jezero Crater Floor
- 2023
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Ingår i: Journal of Geophysical Research: Planets. - : John Wiley and Sons Inc. - 2169-9097 .- 2169-9100. ; 128:6
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Tidskriftsartikel (refereegranskat)abstract
- The Mars 2020 Perseverance rover landed in Jezero crater on 18 February 2021. After a 100-sol period of commissioning and the Ingenuity Helicopter technology demonstration, Perseverance began its first science campaign to explore the enigmatic Jezero crater floor, whose igneous or sedimentary origins have been much debated in the scientific community. This paper describes the campaign plan developed to explore the crater floor's Máaz and Séítah formations and summarizes the results of the campaign between sols 100–379. By the end of the campaign, Perseverance had traversed more than 5 km, created seven abrasion patches, and sealed nine samples and a witness tube. Analysis of remote and proximity science observations show that the Máaz and Séítah formations are igneous in origin and composed of five and two geologic members, respectively. The Séítah formation represents the olivine-rich cumulate formed from differentiation of a slowly cooling melt or magma body, and the Máaz formation likely represents a separate series of lava flows emplaced after Séítah. The Máaz and Séítah rocks also preserve evidence of multiple episodes of aqueous alteration in secondary minerals like carbonate, Fe/Mg phyllosilicates, sulfates, and perchlorate, and surficial coatings. Post-emplacement processes tilted the rocks near the Máaz-Séítah contact and substantial erosion modified the crater floor rocks to their present-day expressions. Results from this crater floor campaign, including those obtained upon return of the collected samples, will help to build the geologic history of events that occurred in Jezero crater and provide time constraints on the formation of the Jezero delta.
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