Intra‐crater sedimentary deposits at the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— Detailed field mapping has revealed the presence of a series of intra‐crater sedimentary deposits within the interior of the Haughton impact structure, Devon Island, Canadian High Arctic. Coarse‐grained, well‐sorted, pale gray lithic sandstones (reworked impact melt breccias) unconformably overlie pristine impact melt breccias and attest to an episode of erosion, during which time significant quantities of impact melt breccias were removed. The reworked impact melt breccias are, in turn, unconformably overlain by paleolacustrine sediments of the Miocene Haughton Formation. Sediments of the Haughton Formation were clearly derived from pre‐impact lower Paleozoic target rocks of the Allen Bay Formation, which form the crater rim in the northern, western, and southern regions of the Haughton structure. Collectively, these field relationships indicate that the Haughton Formation was deposited up to several million years after the formation of the Haughton crater and that they do not, therefore, represent an immediate, post‐impact crater lake deposit. This is consistent with new isotopic dating of impactites from Haughton that indicate an Eocene age for the impact event (Sherlock et al. 2005). In addition, isolated deposits of post‐Miocene intra‐crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the Haughton Formation, impact melt breccias, and other pre‐impact target rock formations. These deposits provide clear evidence for glaciation at the Haughton crater. The wealth and complexity of geological and climatological information preserved as intra‐crater deposits at Haughton suggests that craters on Mars with intra‐crater sedimentary records might present us with similar opportunities, but also possibly significant challenges.     

The case for an Imbrium origin of the Apollo thorium-rich impact-melt breccias

Abstract— Mafic, Th-rich impact-melt breccias, most of which are identified with the composition known as low-K Fra Mauro (LKFM), are the most common rock type in the nonmare regoliths of the Apollo lunar landing sites. The origin of mafic impact-melt breccias bears on many lunar problems: the nature of the late meteoroid bombardment (cataclysm); the spatial distribution of KREEP, both near the surface and at depth; the ages of the major basins; and the composition of the early crust of the nearside lunar highlands. Thus, it is crucial that the origin of mafic impact-melt breccias be accurately understood. Because of both intra- and intersite differences in compositions of mafic impact-melt breccia samples, apparent differences in crystallization age, and differences in siderophile-element ratios, previous studies have argued that either (1) most mafic impact-melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that (2) they are all from the Imbrium (Apollos 14 and 15), Nectaris (Apollo 16), and Serenitatis (Apollo 17) basins. Here, we reconsider the hypothesis that virtually all of the Th-rich, mafic impact-melt breccias from the Apollo missions are products of the Imbrium impact. Ejecta deposit modeling based on modern crater scaling indicates that the Imbrium event produced ejecta deposits that average hundreds of meters thick or more at all Apollo highland sites, which is thicker than some previous estimates. Substantial amounts of Imbrium ejecta should have been sampled at every Apollo highland site. We suggest that the mafic impact-melt breccias may be the principal form of those ejecta. The Imbrium projectile impacted into Th-rich material that we regard as part of a unique, mafic, lunar geochemical province we call the High-Th Oval Region. Based on the surface distribution of Th, only basins within the High-Th Oval Region excavated Th-rich material; the Th concentrations of the highlands as observed by the Apollo orbiting γ-ray experiments are consistent with the estimates from ejecta modeling. Of the younger basin-forming impacts, only Imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact-melt breccias in the Apollo-sampled regolith. The High-Th Oval Region still may have been molten or hot at shallow depths ～4 Ga ago when the Imbrium projectile struck. We reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances. We argue that siderophile-element “fingerprints” of mafic impact-melt breccias are not inconsistent with production of all common types by a single projectile. We suggest that the narrow range of ages of 3.7–4.0 Ga for all successfully dated mafic impact-melt breccias may reflect a single event whose age is difficult to measure precisely, rather than a number of discrete impact events closely spaced in time, such that reported age variations among mafic impact-melt breccias reflect the ability to measure ⁴⁰Ar/³⁹Ar ages with greater precision than the accuracy with which measured portions of mafic impact-melt breccias have recorded the time of their formation.     

Shocked rocks and impact glasses from the El'gygytgyn impact structure,Russia

Abstract— The El'gygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid‐Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and ⁴⁰Ar/³⁹Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock‐metamorphosed volcanic rocks, impact melt rocks, and bomb‐shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event. Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35–45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45–55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass “bombs” composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the El'gygytgyn impact melt rocks.     

Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars

Abstract— Impact cratering is an important geological process on Mars and the nature of Martian impact craters may provide important information as to the volatile content of the Martian crust. Terrestrial impact structures currently provide the only ground‐truth data as to the role of volatiles and an atmosphere on the impact‐cratering process. Recent advancements, based on studies of several well‐preserved terrestrial craters, have been made regarding the role and effect of volatiles on the impact‐cratering process. Combined field and laboratory studies reveal that impact melting is much more common in volatile‐rich targets than previously thought, so impact‐melt rocks, melt‐bearing breccias, and glasses should be common on Mars. Consideration of the terrestrial impact‐cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on Mars. Furthermore, recent studies at the Haughton and Ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta. It is apparent that the relative abundance of volatiles in the near‐surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of Martian ejecta deposits. This study shows the value of using terrestrial analogues, in addition to observational data from robotic orbiters and landers, laboratory experiments, and numerical modeling to explore the Martian impact‐cratering record.     

A case study of impact‐induced hydrothermal activity: The Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— The well‐preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid‐size complex impact craters. A moderate‐ to low‐temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ?100–120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (?250 down to ?90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ?150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton.     

Shock metamorphic history of >4 Ga Apollo 14 and 15 zircons

During impact events, zircons develop a wide range of shock metamorphic features that depend on the pressure and temperature conditions experienced by the zircon. These conditions vary with original distance from impact center and whether the zircon grains are incorporated into ejecta or remain within the target crust. We have employed the range of shock metamorphic features preserved in >4 Ga lunar zircons separated from Apollo 14 and 15 breccias and soils in order to gain insights into the impact shock histories of these areas of the Moon. We report microstructural characteristics of 31 zircons analyzed using electron beam methods including electron backscatter pattern (EBSP) and diffraction (EBSD). The major results of this survey are as follows. (1) The abundance of curviplanar features hosting secondary impact melt inclusions suggests that most of the zircons have experienced shock pressures between 3 and 20 GPa; (2) the scarcity of recrystallization or decomposition textures and the absence of the high‐pressure polymorph, reidite, suggests that few grains have been shocked to over 40 GPa or heated above 1000 °C in ejecta settings; (3) one grain exhibits narrow, arc‐shaped bands of twinned zircon, which map out as spherical shells, and represent a novel shock microstructure. Overall, most of the Apollo 14 and 15 zircons exhibit shock features similar to those of terrestrial zircon grains originating from continental crust below large (~200 km) impact craters (e.g., Vredefort impact basin), suggesting derivation from central uplifts or uplifted rims of large basins or craters on the Moon and not high‐temperature and ‐pressure ejecta deposits.     

Impact melt dikes in the Sudbury multi-ring basin (Canada): Implications from uranium-lead geochronology on the Foy Offset Dike

Abstract To investigate the origin of Offset Dikes and their age relationships to major impact generated lithologies in the Sudbury multi-ring impact structure, such as the Main Mass of the Sudbury “Igneous” Complex, zircon and baddeleyite were dated by the U-Pb chronometer. The rocks analysed are one diorite and two quartz diorites from inside the Foy Offset, one quartz diorite from the contact zone, and two country rock samples collected at 10 and 30 m distances from the contact within the Levack Gneiss Complex. The 21 analysed zircon and baddeleyite fractions yield a crystallization age of 1852 +4/-3 (2σ) Ma for the accessory minerals in the Foy Offset Dike and an age of 2635 ± 5 Ma for the shocked Levack country rock, in which zircons show significant shock effects (multiple sets of planar fractures), in contrast to the totally unshocked zircons of the Offset Dike. Within given errors, the new age of 1852 Ma is identical to the pooled 1850 ± 1 Ma U-Pb age determined by Krogh et al. (1984) as the crystallization age of accessory phases in different lithologies of the Sudbury “Igneous” Complex, which has been interpreted to represent the coherent impact melt sheet of the Sudbury Structure. This excellent agreement of the ages substantiates that emplacement of the Offset Dikes occurred coevally with the formation of the impact melt sheet. Total absence of inherited zircons in the central part of the Foy Offset indicates melting of the precursor material at temperatures well above 1700 °C, which emphasizes the origin of the dike lithologies by impact melting.     

Araguainha impact crater,Brazil. I. The interior part of the uplift

Abstract— The central uplift of the 40-km wide Araguainha impact structure, Brazil, consists of a ring, about 8 km in diameter, of up to 150-m high blocks of Devonian Furnas sandstone, which surround a central depression of elliptical shape (4.5 × 3.0 km). The depression is occupied by a pre-Devonian alkali-feldspar granite, shocked by pressures of 20–25 GPa and permeated by cataclastic shear zones and dikes of shocked granitic material. The granite is flanked and partly covered by several impact breccias: (1) Impact breccia with melt matrix overlies the granite in places and forms hills, bordering the granitic center in the S and SW. It is chemically identical with the granite and consists of thermally altered granitic clasts in a matrix of sanidine, quartz, biotite, muscovite, chlorite and riebeckite. (2) Polymict breccias form hills which border the central depression in the N and NW. Components are unshocked and shocked sediments, shock-melted sandstone, shocked granite and shock melt rocks in irregular masses and individual bodies, embedded in a fine-grained matrix. ⁴⁰Ar/³⁹Ar analyses show that the melt rocks solidified 246 Ma ago, indicating that the impact occurred at near the Permian-Triassic boundary, possibly when the area was covered by a shallow sea. The present chemistry and petrography of the melt rocks suggest that by reacting with seawater granitic impact melt was depleted of K and Rb and enriched in Na, and that later diagenetic processes produced replacement of feldspar by quartz and deposition of hematite. (3) Monomict breccias, consisting of unshocked, shocked and shock-fused quartz sandstones, form hills which surround the central depression in the SE and S. The Araguainha structure is an eroded complex crater, produced by an impact, 246 Ma ago. The depth of excavation was about 2.4 km, comprising Permian, Permo-Carboniferous and Devonian sediments and the granitic basement. The diameter of the transient crater was about 24 km. Erosion and weathering have removed most of the original crater fill and ejecta deposits, with the exception of remnants, preserved in the central uplift.     

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project,Yaxcopoil, Mexico

Abstract— The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil‐1 (Yax‐1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact‐generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax‐1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán‐6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán‐6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax‐1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km‐ thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.     

Secondary and sesquinary craters on Europa

We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa as secondary to the primary impacts of comets on Io, Europa, and Ganymede. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa's surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. The mass of basaltic impact ejecta from Io to reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa's ice crust. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady-Kipp fragments originally developed by Melosh. Our model successfully reproduces the observed size-number distributions of small craters on both Mars and Europa. However, the model predicts that a significant fraction of the 200-500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from Io that had gone into orbit about Jupiter. This prediction is not supported by observation, which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters. Iogenic basalts are also interesting because they provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa's surface.     

Chicxulub impactites: Geochemical clues to the precursor rocks

Abstract— The 65 Ma Chicxulub impact structure, Mexico, with a diameter of ～180 km is the focus of geoscientific research because of its link to the mass extinction event at the Cretaceous‐Tertiary (K/T) boundary. Chicxulub, now buried beneath thick post‐impact sediments, is probably one of the best‐preserved terrestrial impact structures known. Because of its inaccessibility, only limited samples on the impact lithologies from a few drill cores are available. We report major element and Sr‐, Nd‐, O‐, and C‐isotopic data for Chicxulub impact‐melt lithologies and basement clasts in impact breccias of drill cores C‐1 and Y‐6, and for melt particles in the Chicxulub ejecta horizon at the K/T boundary in Beloc, Haiti. The melt lithologies with SiO₂ ranging from 58 to ～63 wt% show significant variations in the content of Al, Ca, and the alkalies. In the melt matrix samples, δ¹³C of the calcite is about ?3%o. The δ¹⁸O values for the siliceous melt matrices of Y‐6 samples range from 9.9 to 12.4%o. Melt lithologies and the black Haitian glass have rather uniform ⁸⁷Sr/⁸⁶Sr ratios (0.7079 to 0.7094); only one lithic fragment displays ⁸⁷Sr/⁸⁶Sr of 0.7141. The Sr model ages T^SrUR for most lithologies range from 830 to 1833 Ma; unrealistic negative model ages point to an open Rb‐Sr system with loss of Rb in a hydrothermal process. The ¹⁴³Nd/¹⁴⁴Nd ratios for all samples, except one basement clast with ¹⁴³Nd/¹⁴⁴Nd of 0.5121, cluster at 0.5123 to 0.5124. In an ?_Nd‐?_Sr diagram, impactites plot in a field delimited by ?_Nd of ?2 to ?6, and ?_Sr of 55 to 69. This field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and Cretaceous sediments. At least one additional intermediate to mafic precursor component is required to explain the data.     

Establishing a 14.6 ± 0.2 Ma age for the Nördlinger Ries impact (Germany)—A prime example for concordant isotopic ages from various dating materials

Abstract– ⁴⁰Ar/³⁹Ar dating of recrystallized K‐feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km Nördlinger Ries crater (southern Germany) yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (2σ). This new age for the Nördlinger Ries is the first age obtained from (1) monomineralic melt (2) separated from an impact‐metamorphosed target rock clast within (3) Ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure. The new age goes very well with the ⁴⁰Ar/³⁹Ar step‐heating and laser probe dating results achieved from mixed‐glass samples (suevite glass and tektites) and is slightly younger than the previously obtained fission track and K/Ar and ages of about 15 Ma, as well as the K/Ar and ⁴⁰Ar/³⁹Ar age data obtained in the early 1990s. Taking all the ⁴⁰Ar/³⁹Ar age data obtained from Ries impact melt lithologies into account (data from the literature and this study), we suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event.     

Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.     

Martian rampart crater ejecta: Experiments and analysis of melt-water interaction

Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction.     

Impacts into non-polar ice-rich paleodeposits on Mars: Excess ejecta craters, perched craters and pedestal craters as clues to Amazonian climate history

We compare three previously independently studied crater morphologies - excess ejecta craters, perched craters, and pedestal craters - each of which has been proposed to form from impacts into an ice-rich surface layer. Our analysis identifies the specific similarities and differences between the crater types; the commonalities provide significant evidence for a genetic relationship among the morphologies. We use new surveys of excess ejecta and perched craters in the southern hemisphere in conjunction with prior studies of all of the morphologies to create a comprehensive overview of their geographic distributions and physical characteristics. From these analyses, we conclude that excess ejecta craters and perched craters are likely to have formed from the same mechanism, with excess ejecta craters appearing fresh while perched craters have experienced post-impact modification and infilling. Impacts that led to these two morphologies overwhelmed the ice-rich layer, penetrating into the underlying martian regolith, resulting in the excavation of rock that formed the blocky ejecta necessary to armor the surface and preserve the ice-rich deposits. Pedestal craters, which tend to be smaller in diameter, have the same average deposit thickness as excess ejecta and perched craters, and form in the same geographic regions. They rarely have ejecta around their crater rims, instead exhibiting a smooth pedestal surface. We interpret this to mean that they form from impacts into the same type of ice-rich paleodeposit, but that they do not penetrate through the icy surface layer, and thus do not generate a blocky ejecta covering. Instead, a process related to the impact event appears to produce a thin, indurated surface lag deposit that serves to preserve the ice-rich material. These results provide a new basis to identify the presence of Amazonian non-polar ice-rich deposits, to map their distribution in space and time, and to assess Amazonian climate history. Specifically, the ages, distribution and physical attributes of the crater types suggest that tens to hundreds of meters of ice-rich material has been episodically emplaced at mid latitudes in both hemispheres throughout the Amazonian due to obliquity-driven climate variations. These deposits likely accumulated more frequently in the northern lowlands, resulting in a larger population of all three crater morphologies in the northern hemisphere.     

SIMS U-Pb dating of micro-zircons in the lunar meteorites Dhofar 1528 and Dhofar 1627

About half of the lunar meteorites in our collections are feldspathic breccias. Acquiring geochronologic information from these breccias is challenging due to their low radioactive-element contents and their often polymict nature. We used high-spatial-resolution (5 μm) NanoSIMS (nanoscale secondary ion mass spectrometry) U-Pb dating technique to date micro-zircons in the lunar feldspathic meteorites Dhofar 1528 and Dhofar 1627. Three NanoSIMS dating spots of two zircon grains from Dhofar 1528 show a discordia with an upper intercept at 4354 ± 76 Ma and a lower intercept at 332 ± 1407 Ma (2σ, MSWD = 0.01, p = 0.91). Three spots of two zircon grains in Dhofar 1627 define a discordia with an upper intercept at 3948 ± 30 Ma and a lower intercept at 691 ± 831 Ma (2σ, MSWD = 0.40, p = 0.53). Both samples likely experienced shock metamorphism caused by impacts. Based on the clastic nature, lack of recrystallization and the consistent U-Pb and Pb-Pb dates of the zircons in Dhofar 1528, the U-Pb date of 4354 Ma is interpreted as the crystallization age of its Mg-suite igneous precursor. Some of the Dhofar 1627 zircons show poikilitic texture, a crystallization from the matrix impact melt, so the U-Pb date of 3948 Ma corresponds to an impact event, likely the Imbrium basin-forming event. These data are the first radiometric ages for these two meteorites and demonstrate that in situ (high spatial resolution) U-Pb dating has potential for extracting geochronological information about igneous activities and impact events from lunar feldspathic and polymict breccias.     

On the origin of the lunar smooth-plains

Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.     

Cover

Cover: This oblique view of the lunar crater Pierazzo (3.3°N, 100.2°W, D≈9km) was taken by NASA’s Lunar Reconnaissance Orbiter Camera’s Narrow Angle Camera in late 2017. The camera was pointed off-nadir to provide this oblique view which, coupled with the moon’s curvature, provides an observation angle of 74°. This young crater features many large deposits of impact melt, typically dark material that is seen strewn throughout the image not only outside the crater (and is found over 40 km from the impact site), but in numerous deposits inside the crater. An extensive analysis of the impact melt was recently published by Veronica Bray et al. (2018, Icarus 201, p. 26–36). Small, bright splotches litter the ejecta and are mostly new craters that post-date the larger Pierazzo impact, though some might be caused by ejected blocks from the crater hitting its own ejecta. The crater is named in honor of Elisabetta (“Betty“) Pierazzo (1963–2011), who studied impact craters, including the production of impact melt material. We selected this image for the cover of this special issue because we think that it presents a good overview of this issue: rather than emphasizing any one study or type of paper in this special issue, it, at a simple glance, shows the force of an impact, the intriguing complexity inherent to their structure, and that even relatively young features are prone to modifi cation by the ongoing process of impact cratering. Credit: NASA/GSFC/ASU     

Multiple impact events on the L-chondritic parent body: Insights from SIMS U-Pb dating of Ca-phosphates in the NWA 7251 L-melt breccia

Here we report in situ secondary ionization mass spectrometry Ca-phosphate U-Pb ages for an L-impact melt breccia (NWA 7251), which are integrated with petrological and mineral chemical studies of this meteorite. NWA 7251 is a heavily shocked rock that is composed mainly of the chondrite host, impact melt portion, and melt veins (crosscutting and pervasive type). The host is an L4 chondrite that has been shocked to S4. The impact melt portion has a fine-grained igneous texture, and is composed mainly of olivine, low-Ca pyroxene, high-Ca pyroxene, and albitic glass. The impact melt was generated at pressure of >30–35 GPa and temperature of >1300–1500 °C during an impact event. The Ca-phosphate grains in the host were affected by a shock heating event. Most of the Ca-phosphate grains in the melt were neocrystallized, but relatively large grains enclosed by or adjacent to metal veins or melt globules are likely inherited. The U-Pb isotopic systematics of Ca-phosphates in NWA 7251 yield an upper intercept age of 4457 ± 56 Ma and a lower intercept age of 574 ± 82 Ma on the normal U-Pb concordia diagram. The age of 4457 ± 56 Ma is interpreted to be related to an early shocking event rather than the thermal metamorphism of the parent body. The impact melt and veins in NWA 7251 were generated at 574 ± 82 Ma, resulting from disruption of the L chondrite parent body.     

Petrography and geochemistry of ejecta from the Sudbury impact event

Ejecta from the Connors Creek site in Michigan (500 km from the Sudbury Igneous Complex [SIC]), the Pine River site in western Ontario (650 km from the SIC), and the Coleraine site in Minnesota (980 km from the SIC) were petrographically and geochemically analyzed. Connors Creek was found to have approximately 2 m of ejecta, including shocked quartz, melt droplets, and accretionary lapilli; Pine River has similar deposits about 1 m in thickness, although with smaller lapilli; Coleraine contains only impact spherules in a 20 cm‐thick layer (impact spherules being similar to microkrystites or microtektites). The ejecta transition from chaotic deposits of massively bedded impactoclastic material with locally derived detritus at Connors Creek to a deposit with apparently very little detrital material that is primarily composed of melt droplets at Pine River to a deposit that is almost entirely composed of melt spherules at Coleraine. The major and trace element compositions of the ejecta confirm the previously observed similarity of the ejecta deposits to the Onaping Formation in the SIC. Platinum‐group element (PGE) concentrations from each of the sites were also measured, revealing significantly elevated PGE contents in the spherule samples compared with background values. PGE abundances in samples from the Pine River site can be reproduced by addition of approximately 0.2 wt% CI chondrite to the background composition of the underlying sediments in the core. PGE interelement ratios indicate that the Sudbury impact event was probably caused by a chondritic impactor.