Accretionary lapilli from the Sudbury impact event

Meteorite impact‐generated accretionary lapilli are not well studied. The recently discovered distal ejecta from the 1850 Ma Sudbury impact event contain abundant accretionary lapilli generated during the impact and deposited at great distances from the crater. We petrographically and geochemically examined lapilli from five sites (McClure, Connors Creek, Hwy 588, Pine River, and Grand Trunk Pacific, approximately 480–750 km from the center of the Sudbury structure). The compositions of quartz, K‐feldspar, calcite, biotite, and chlorite minerals are similar to each other in all of the samples, although the relative proportions of the minerals vary from site to site. The lapilli occur in a matrix of coarse‐grained quartz, carbonate, and feldspar grains. Zonation within lapilli appears to be due to grain size distribution rather than compositional variation. The inner zones are coarser grained than outer zones. The relative abundances of calcite, phyllosilicates, and feldspars are similar in each zone within individual lapilli. A meteoritic component is indicated by up to 1.8 ppb Ir in one lapillus from the Pine River site, and Ni and Cr ratios are on a chondritic trend line for many of the lapilli. Mechanisms previously proposed for accretionary lapilli formation seem inadequate to explain deposition of distal accretionary lapilli resulting from impact events. A new mechanism for upper atmospheric accretion is proposed, whereby ash ejected from impact events concentrates at altitudes of neutral buoyancy, where it then accretes and is deposited later than ballistically emplaced particles. Likely, multiple processes are taking place in the chaotic postimpact environment.     

The South Range Breccia Belt of the Sudbury Impact Structure: A possible terrace collapse feature

Abstract— The South Range Breccia Belt (SRBB) is an arcuate, 45 km long zone of Sudbury Breccia in the South Range of the 1.85 Ga Sudbury Impact Structure. The belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of Huronian Supergroup (2.49–2.20 Ga), Nipissing Diabase (2.2 Ga), and Proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters. The SRBB matrix is composed of a fine‐grained (～100 μm) assemblage of biotite, quartz, and ilmenite, with trace amounts of plagioclase, zircon, titanite, epidote, pyrite, chalcopyrite, pyrrhotite, and occasionally chlorite. The SRBB hosts the Frood‐Stobie, Vermilion, and Kirkwood quartz diorite offset dykes, the former being associated with one of the largest Ni‐Cu‐PGE sulphide deposits in the world. Optical petrography and whole‐rock geochemistry concur with previous studies that have suggested that the matrix of the SRBB is derived from comminution and at least partial frictional melting of the wall rock Huronian Supergroup lithologies. Rare earth element (REE) data from all sampled lithologies associated with the SRBB exhibit crustal signatures when normalized to C1 chondrite values. Additionally, REE data from the quartz diorites, disseminated sulphides in Sudbury Breccia, and a sample of an aphanitic biotite‐hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites, quartz gabbro, and granophyre units of the Sudbury Igneous Complex (SIC), which suggests that these lithologies are representative of bulk SIC melt. We suggest that the SRBB was formed by high strain‐rate (>1 m/s), gravity‐driven seismogenic slip of the inner ring of the Sudbury Impact Structure during postimpact crustal readjustment (crater modification stage). Failure of the hanging wall may have facilitated the injection of bulk SIC melt into the SRBB, along with the Ni‐Cu‐PGE sulphides of the Frood‐Stobie deposit. Postimpact Penokean (1.9–1.7 Ga) tectonism, particularly northwest‐directed shearing along the South Range Shear Zone and associated thrust faulting, could account for the present subvertical orientation of the SRBB, and the apparent lack of a connection at depth with the SIC.     

Origin and emplacement of Offset Dykes in the Sudbury impact structure: Constraints from Hess

Abstract— The Hess Offset is a steeply dipping dyke located 12–15 km north of the 1.85 Ga Sudbury igneous complex (SIC) within the 200–250 km diameter Sudbury impact structure. It is up to 60 m wide and strikes subconcentrically to the SIC for at least 23 km. The main phase of the dyke is granodioritic, but it conforms with what is locally referred to as Quartz Diorite: a term used for all the Offset Dykes of the Sudbury impact structure. Rare earth element data shows that the Hess Offset is genetically related to the SIC. Hess is most closely affiliated with an evolved Felsic Norite component of SIC and not bulk impact melt. This indicates that Hess was emplaced during fractionation of the impact melt sheet, rather than immediately following impact. The main Quartz Diorite phase of the dyke comprises a quartz + plagioclase + hornblende + biotite ± clinopyroxene ± orthopyroxene assemblage. Critically, the Hess Offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte-rich, shatter cone-bearing annulus about the SIC. This fault system was active during the modification stage of the impact process.     

Petrography and geochemistry of the impact to postimpact transition layer at the El'gygytgyn impact structure in Chukotka,Arctic Russia

The 3.6 Ma El'gygytgyn impact structure, located in northeast Chukotka in Arctic Russia, was largely formed in acidic volcanic rocks. The 18 km diameter circular depression is today filled with Lake El'gygytgyn (diameter of 12 km) that contains a continuous record of lacustrine sediments of the Arctic from the past 3.6 Myr. In 2009, El'gygytgyn became the focus of the International Continental Scientific Drilling Program (ICDP) in which a total of 642.4 m of drill core was recovered. Lithostratigraphically, the drill cores comprise lacustrine sediment sequences, impact breccias, and deformed target rocks. The impactite core was recovered from 316.08 to 517.30 meters below lake floor (mblf). Because of the rare, outstanding recovery, the transition zone, ranging from 311.47 to 317.38 m, between the postimpact lacustrine sediments and the impactite sequences, was studied petrographically and geochemically. The transition layer comprises a mixture of about 6 m of loose sedimentary and volcanic material containing isolated clasts of minerals and melt. Shock metamorphic effects, such as planar fractures (PFs) and planar deformation features (PDFs), were observed in a few quartz grains. The discoveries of silica diaplectic glass hosting coesite, kinked micas and amphibole, lechatelierite, numerous impact melt shards and clasts, and spherules are associated with the impact event. The occurrence of spherules, impact melt clasts, silica diaplectic glass, and lechatelierite, about 1 m below the onset of the transition, marks the beginning of the more coherent impact ejecta layer. The results of siderophile interelement ratios of the transition layer spherules give indications of the relative contribution of the meteoritical component.     

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock,Sudbury impact structure,Canada

The 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.     

New insights on petrography and geochemistry of impactites from the Lonar crater,India

The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter‐ to decimeter‐sized impact‐melt bombs, and aerodynamically shaped millimeter‐ and submillimeter‐sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top‐most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter‐sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH‐type chondrite with the submillimeter‐sized spherules containing ~6 wt% impactor components.     

The Basal Onaping Intrusion in the North Range: Roof rocks of the Sudbury Igneous Complex

The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so‐called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex (SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation. We suggest henceforth that this unit be referred to as the “Upper Contact Unit” of the SIC.     

Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475

The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.     

The nature and emplacement of distal aqueous-rich ejecta deposits from Hale crater,Mars

Hale crater formed in the Early to Middle Amazonian and is one of the best preserved large craters on Mars. We focus on the emplacement of previously mapped distal continuous ejecta and newly recognized discontinuous distal ejecta deposits reaching up to 450 km northeast of Hale. The distal continuous ejecta deposits are typically tens of meters thick, likely water-rich, and subsequent dewatering of some resulted in flow along gradients of 10 m km^-1 for distances of tens of kilometers. The discontinuous distal ejecta are typically <10 m thick with volumes generally <0.5 km³ and embay Hale secondaries, which occur up to ~600 km from Hale. Both continuous and discontinuous distal ejecta deposits are typically smooth at scales of tens to hundreds of meters, relatively dark-toned, devoid of eolian bedforms, inferred to be mostly fine-grained, and were likely emplaced within hours to 1–2 days after impact. The occurrence of well-preserved discontinuous distal ejecta at Hale is unusual compared to other large Martian craters and could be due to impact into an ice-rich substrate that enabled their formation and (or) their survival after minimal postimpact degradation relative to older craters. The pristine nature of distal continuous and discontinuous distal deposits at Hale and the preservation of associated secondaries imply (1) low erosion rates after the Hale impact, comparable to those estimated elsewhere during the Amazonian; (2) the impact did not significantly influence long-term global or regional scale geomorphic activity or climate; and (3) the Hale impact occurred after late alluvial fan activity in Margaritifer Terra.     

The melt‐bearing impactites of the Ritland structure,Norway–Implications for melt formation in small impact craters

A melt‐bearing impactite unit is preserved in the 2.7 km diameter shallow marine Ritland impact structure. The main exposure of the melt‐bearing unit is in an approximately 100 m long cliff about 700 m southwest of the center of the structure. The melt and clast content vary through this maximum 2 m thick unit, so that lithology ranges from impact melt rock to suevite. Stratigraphic variations with respect to the melt content, texture, mineralogy, and geochemistry have been studied in the field, and by laboratory analysis, including thin section microscopy. The base of the melt‐bearing unit marks the transition from the underlying lithic basement breccia, and the unit may have been emplaced by an outward flow during the excavation stage. There is an upward development from a melt matrix‐dominated lower part, that commonly shows flow structures, to an upper part characterized by more particulate matrix with patchy melt matrix domains, commonly as deformed melt slivers intermingled with small lithic clasts. Melt and lithic fragments in the upper part display a variety of shapes and compositions, some of which possibly represent fallback material from the ejecta cloud. The upper boundary of the melt‐bearing impactite unit has been placed where the deposits are mainly clastic, probably representing slump and avalanche deposits from the modification stage. These deposits are therefore considered sedimentary and not impactites, despite the component of small melt fragments and shocked minerals within the lowermost part, which was probably incorporated as the debris moved down the steep crater walls.     

Chemical variations and genetic relationships between the Hess and Foy offset dikes at the Sudbury impact structure

Offset dikes are found concentrically around—and extending radially outward from—the Sudbury Igneous Complex (SIC), which represents an ~3 km thick differentiated impact melt sheet. The dikes are typically composed of an inclusion‐rich, so‐called quartz diorite (IQD) in the center of the dike, and an inclusion‐poor quartz diorite (QD) along the margins of the dike. New exposures of the intersection between the concentric Hess and radial Foy offset dikes provide an excellent opportunity to understand the relationship between the radial and concentric offset dikes and their internal phases. The goal was to constrain the timing of the dike emplacements relative to the impact and formation of the SIC. Results herein suggest that (1) the timing between the emplacement of the QD and IQD melts was geologically short, (2) the Hess and Foy dikes coexisted as melts at the same time and the intersection between them represents a mixture of the two, (3) the Foy dike has a slightly more evolved chemical composition than the Hess dike, and (4) the IQD melt from the Foy dike underwent some degree of chemical fractionation after its initial emplacement.     

Impactites of the Yaxcopoil‐1 drilling site,Chicxulub impact structure: Petrography,geochemistry, and depositional environment

Abstract— The impact breccias encountered in drill hole Yaxcopoil‐1 (Yax‐1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine‐grained top (unit 3; 23 m thick; nuée ardente) and a coarse breccia (unit 4; ～15 m thick) below. As such, they consist of a mélange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (～24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic‐matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super‐heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax‐1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.     

Contrasting Aerodynamic Morphology and Geochemistry of Impact Spherules from Lonar Crater,India: Some Insights into Their Cooling History

The ~50 or 570 ka old Lonar crater, India, was excavated in the Deccan Trap flood basalt of Cretaceous age by the impact of a chondritic asteroid. The impact-spherules known from within the ejecta around this crater are of three types namely aerodynamically shaped sub-mm and mm size spherules, and a sub-mm sized variety of spherule, described as mantled lapilli, having a core consisting of ash-sized grains, shocked basalt and solidified melts surrounded by a rim of ash-sized materials. Although, information is now available on the bulk composition of the sub-mm sized spherules (Misra et al. in Meteorit Planet Sci 7:1001–1018, 2009), almost no idea exists on the latter two varieties. Here, we presented the microprobe data on major oxides and a few trace elements (e.g. Cr, Ni, Cu, Zn) of mm-sized impact spherules in unravelling their petrogenetic evolution. The mm-sized spherules are characterised by homogeneous glassy interior with vesicular margin in contrast to an overall smooth and glassy-texture of the sub-mm sized spherules. Undigested micro-xenocrysts of mainly plagioclase, magnetite and rare clinopyroxene of the target basalt are present only at the marginal parts of the mm-sized spherules. The minor relative enrichment of SiO₂ (~3.5 wt% in average) and absence of schlieren structure in these spherules suggest relatively high viscosity of the parent melt droplets of these spherules in comparison to their sub-mm sized counterpart. Chemically homogeneous mm-sized spherule and impact-melt bomb share similar bulk chemical and trace element compositions and show no enrichment in impactor components. The general depletion of Na₂O within all the Lonar impactites was resulted due to impact-induced volatilisation effect, and it indicates the solidification temperature of the Lonar impactites close to 1,100 °C. The systematic geochemical variation within the mm-sized spherules (Mg# ~0.38–0.43) could be attributed to various level of mixing between plagioclase-dominated impact melts and ultrafine pyroxene and/or titanomagnetite produced from the target basalt due to impact. Predominance of schlieren and impactor components (mainly Cr, Ni), and nearly absence of vesicles in the sub-mm sized spherules plausibly suggest that these quenched liquid droplets could have produced from the impactor-rich, hotter (~1,100 °C or more) central part of the plume, whereas the morpho-chemistry of the mm-sized spherules induces their formation from the relatively cool outer part of the same impact plume.     

The evolution of the Onaping Formation at the Sudbury impact structure

Abstract– The 1.4–1.6 km thick Onaping Formation consists of a complex series of breccias and “melt bodies” lying above the Sudbury Igneous Complex (SIC) at the Sudbury impact structure. Based on the presence of shocked lithic clasts and various “glassy” phases, the Onaping has been described as a “suevitic” breccia, with an origin, at least in part, as fallback material. Recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases, both as clast types and discrete intrusive bodies. The nature of the Onaping and that of other “suevitic” breccias overlying impact melt sheets is reviewed. The relative thickness, internal stratigraphic and lithological character, and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the Onaping. The Sudbury structure formed in a foreland basin and water played an essential role in the evolution of the Onaping, as indicated by a major hydrothermal system generated during its formation. Taken together, observations and interpretations of the Onaping suggest a working hypothesis for the origin of the Onaping that includes not only impact but also the interaction of sea water with the impact melt, resulting in repeated explosive interactions involving proto‐SIC materials and mixing with pre‐existing lithologies. This is complicated by additional brecciation events due to the intrusion of proto‐SIC materials into the evolving and thickening Onaping. Fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two‐thirds of the Onaping.     

Sedimentological and petrographic analysis of drill core FC77-1 from the flank of the central uplift,Flynn Creek impact structure,Tennessee

Drill core FC77-1 on the flank of the central uplift, Flynn Creek impact structure, Tennessee, contains 175 m of impact breccia lying upon uplifted Lower Paleozoic carbonate target stratigraphy. Sedimentological analysis of this 175-m interval carbonate breccia shows that there are three distinct sedimentological units. In stratigraphic order, unit 1 (175–109 m) is an overall coarsening-upward section, whereas the overlying unit 2 (109–32 m) is overall fining-upward. Unit 3 (32–0 m) is a coarsening-upward sequence that is truncated at the top by postimpact erosion. Units 1 and 3 are interpreted as debris or rock avalanches into finer sedimentary deposits within intracrater marine waters, thus producing progressively coarser, coarsening-upward sequences. Unit 2 is interpreted to have formed by debris or rock avalanches into standing marine waters, thus forming sequential fining-upward deposits. Line-logging of clasts ranging from 5 mm to 1.6 m, and thin-section analysis of selected drill core samples (including clasts < 5 mm), both show that the Flynn Creek impact breccia consists almost entirely of dolostone clasts (90%), with minor components of cryptocrystalline melt clasts, chert and shale fragments, and clastic grains. Cryptocrystalline melt clasts, which appear isotropic in thin section, are in fact made of exceedingly fine quartz crystals that exhibit micro-Fourier transform infrared (FTIR) and micro-Raman spectra consistent with crystalline quartz. These cryptocrystalline melt clasts are the first melt clasts of any kind to be reported from Flynn Creek impact structure.     

Empirical and theoretical comparisons of the Chicxulub and Sudbury impact structures

Abstract— Chicxulub and Sudbury are 2 of the largest impact structures on Earth. Research at the buried but well‐preserved Chicxulub crater in Mexico has identified 6 concentric structural rings. In an analysis of the preserved structural elements in the eroded and tectonically deformed Sudbury structure in Canada, we identified ring‐like structures corresponding in both radius and nature to 5 out of the 6 rings at Chicxulub. At Sudbury, the inner topographic peak ring is missing, which if it existed, has been eroded. Reconstructions of the transient cavities for each crater produce the same range of possible diameters: 80–110 km. The close correspondence of structural elements between Chicxulub and Sudbury suggests that these 2 impact structures are approximately the same size, both having a main structural basin diameter of ?150 km and outer ring diameters of ?200 km and ?260 km. This similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt (melt produced directly upon decompression from high pressure impact) and impact melt (shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall) in large impacts. Our empirical comparisons suggest that Sudbury has ?70% more impact melt than does Chicxulub (?31,000 versus ?18,000 km³) and 85% more shock melt (27,000 km³ versus 14,500 km³). To examine possible causes for this difference, we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts. We use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models. The results demonstrate that the differences in melt volumes can be readily explained if Chicxulub was an asteroid impact and Sudbury was a comet impact. The estimated 70% difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked. Preheating of the target rocks at Sudbury by the Penokean Orogeny cannot explain the excess melt at Sudbury, the majority of which resides in the suevite. The greater amount of suevite at Sudbury compared to Chicxulub may be due to the dispersal of shock melt by cometary volatiles at Sudbury.     

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     

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.     

Evidence for shock‐induced anhydrite recrystallization and decomposition at the UNAM‐7 drill core from the Chicxulub impact structure

Drill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SO_x gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation.     

Microscopic impactor debris in the soil around Kamil crater (Egypt): Inventory,distribution, total mass,and implications for the impact scenario

We report on the microscopic impactor debris around Kamil crater (45 m in diameter, Egypt) collected during our 2010 geophysical expedition. The hypervelocity impact of Gebel Kamil (Ni‐rich ataxite) on a sandstone target produced a downrange ejecta curtain of microscopic impactor debris due SE–SW of the crater (extending ~300,000 m², up to ~400 m from the crater), in agreement with previous determination of the impactor trajectory. The microscopic impactor debris include vesicular masses, spherules, and coatings of dark impact melt glass which is a mixture of impactor and target materials (Si‐, Fe‐, and Al‐rich glass), plus Fe‐Ni oxide spherules and mini shrapnel, documenting that these products can be found in craters as small as few tens of meters in diameter. The estimated mass of the microscopic impactor debris (<290 kg) derived from Ni concentrations in the soil is a small fraction of the total impactor mass (~10 t) in the form of macroscopic shrapnel. That Kamil crater was generated by a relatively small impactor is consistent with literature estimates of its pre‐atmospheric mass (>20 t, likely 50–60 t).