Bosumtwi impact structure,Ghana: Geochemistry of impactites and target rocks,and search for a meteoritic component

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.     

Strontium and neodymium isotope systematics of target rocks and impactites from the El'gygytgyn impact structure: Linking impactites and target rocks

The 3.6 Ma El'gygytgyn structure, located in northeastern Russia on the Chukotka Peninsula, is an 18 km diameter complex impact structure. The bedrock is formed by mostly high‐silica volcanic rocks of the ~87 Ma old Okhotsk‐Chukotka Volcanic Belt (OCVB). Volcanic target rocks and impact glasses collected on the surface, as well as drill core samples of bedrock and impact breccias have been investigated by thermal ionization mass spectrometry (TIMS) to obtain new insights into the relationships between these lithologies in terms of Nd and Sr isotope systematics. Major and trace element data for impact glasses are added to compare with the composition of target rocks and drill core samples. Sr isotope data are useful tracers of alteration processes and Nd isotopes reveal characteristics of the magmatic sources of the target rocks, impact breccias, and impact glasses. There are three types of target rocks mapped on the surface: mafic volcanics, dacitic tuff and lava of the Koekvun’ Formation, and dacitic to rhyolitic ignimbrite of the Pykarvaam Formation. The latter represents the main contributor to the impact rocks. The drill core is divided into a suevite and a bedrock section by the Sr isotope data, for which different postimpact alteration regimes have been detected. Impact glasses from the present‐day surface did not suffer postimpact hydrothermal alteration and their data indicate a coherent alteration trend in terms of Sr isotopes with the target rocks from the surface. Surprisingly, the target rocks do not show isotopic coherence with the Central Chukotka segment of the OCVB or with the Berlozhya magmatic assemblage (BMA), a late Jurassic felsic volcanic suite that crops out in the eastern part of the central Chukotka segment of the OCVB. However, concordance for these rocks exists with the Okhotsk segment of the OCVB. This finding argues for variable source magmas having contributed to the build‐up of the OCVB.     

Documentation of shock features in impactites from the Dhala impact structure,India

The fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between <2 and >60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.     

The contribution of impulsive meteoritic impact vapourization to the Hermean exosphere

The exosphere of Mercury has been the object of many investigations and speculations regarding its composition, formation, depletion and dynamics. While vapourization of Mercurian surface materials by meteorite impacts has been often considered to be a less important contributor to the exosphere than other potential processes, larger objects coming from the Main Asteroid Belt could cause high local and transient enhancements in the density of the exosphere. Vapourization by such impacts is an almost stoichiometric process, and thus would contain valuable information about the surface composition. We investigate some exospheric effects of impact vapourization for meteorites with radii of 1, 10 cm, and 1 m, with particular reference to the missions that will explore Mercury during the next decade (MESSENGER and BepiColombo). Because of their higher probabilities, impacts of objects in the two smaller size ranges will surely occur during the lifetimes of the two missions. The enhancement of the exospheric density on the dayside of Mercury would be appreciable for the 10-cm and 1-m meteorites (some orders of magnitude, especially for Al, Mg, Si, and Ca). Such events could allow detection, for the first time, of refractory species like Al, Mg, and Si, which are expected to exist on the surface but have not yet been detected in the exosphere. Ca could be detectable in all cases, even if produced by impacting objects as small as 1 cm in radius. The lower exospheric background on the night side should allow easier identification of Na and K produced by impulsive events, even if their generally high background values make this eventuality less likely.     

Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure,India

The existence of mass‐independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra‐terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best‐preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM‐type chondrite.     

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.     

Oxygen isotope composition of meteoritic ablation debris from the Transantarctic Mountains: Constraining the parent body and implications for the impact scenario

Abstract– Microscopic meteoritic ablation spheres recently found on top of the Victoria Land in Transantarctic Mountains, and in the L2 Dome C and DF2691 Dome Fuji ice core layers document a major impact of a 10⁸ kg (or larger) cosmic body in the Antarctic region about 480 kyr ago. Although of broadly chondritic composition, the exact nature of the impactor is unknown, and whether the impactor struck the Antarctic ice sheet or exploded in the atmosphere is a matter of debate. Based on oxygen isotope analyses of ablation spheres from the Transantarctic Mountains by means of IR‐laser fluorination coupled with mass spectrometry, we suggest that they represent the debris of an atmospheric airburst of a primitive asteroid of CV, CO, or CK composition, or a comet with composition similar to the short‐period comet 81P/Wild 2.     

Portales Valley: A meteoritic sample of the brecciated and metal-veined floor of an impact crater on an H-chondrite asteroid

Abstract— A shower of meteorite fragments fell at ～0730 h local time on 1998 June 13 near the town of Portales, New Mexico. Thus far, 51 pieces of the Portales Valley (H6) meteorite have been recovered. This meteorite has an unusually large number of metallic veins. Some of these veins are also unusually thick, having widths on the order of centimeters. These wide veins have fine Widmanstätten structure, which is the first time it has been seen in an ordinary chondrite. This structure indicates the metallic veins and the host chondrite cooled slowly. These veins appear to have been produced by shock-metamorphic processes, which we infer produced a >20 km diameter impact crater on an H-chondrite planetesimal.     

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Chesapeake Bay impact structure: Morphology,crater fill,and relevance for impact structures on Mars

Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.     

A survey of possible impact structures on 25143 Itokawa

We determined the morphologies and dimensions of possible impact craters on the surface of Asteroid 25143 Itokawa from images taken by the Hayabusa spacecraft. Circular depressions, circular features with flat floors or convex floors, and circular features with smooth surfaces were identified as possible craters. The survey identified 38 candidates with widely varying morphologies including rough, smooth and saddle-shaped floors, a lack of raised rims and fresh material exposures. The average depth/diameter ratio was 0.08±0.03: these craters are very shallow relative to craters observed on other asteroids. These shallow craters are a result of (1) target curvature influencing the cratering process, (2) raised rim not being generated by this process, and (3) fines infilling the craters. As many of the crater candidates have an unusual appearance, we used a classification scheme that reflects the likelihood of an observed candidate's formation by a hypervelocity impact. We considered a variety of alternative interpretations while developing this scheme, including inherited features from a proto-Itokawa, spall scars created by the disruption of the proto-Itokawa, spall scars following the formation of a large crater on Itokawa itself, and apparent depressions due to random arrangements of boulders. The size-frequency distribution of the crater candidates was close to the empirical saturation line at the largest diameter, and then decline with decreasing diameter.     

Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

We present petrologic and isotopic data on Northwest Africa (NWA) 4799, NWA 7809, NWA 7214, and NWA 11071 meteorites, which were previously classified as aubrites. These four meteorites contain between 31 and 56 vol% of equigranular, nearly endmember enstatite, Fe,Ni metal, plagioclase, terrestrial alteration products, and sulfides, such as troilite, niningerite, daubréelite, oldhamite, and caswellsilverite. The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that these rocks were not formed through igneous processes like the aubrites, but rather by impact processes. In addition, the presence of pre‐terrestrially weathered metal (7.1–14 vol%), undifferentiated modal abundances compared to enstatite chondrites, presence of graphite, absence of diopside and forsterite, low Ti in troilite, and high Si in Fe,Ni metals suggest that these rocks formed through impact melting on chondritic and not aubritic parent bodies. Formation of these meteorites on a parent body with similar properties to the EHa enstatite chondrite parent body is suggested by their mineralogy. These parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to ³⁹Ar‐⁴⁰Ar ages, indicating that this region of the solar system was heavily bombarded early in its history. By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high‐temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe‐metal‐rich and FeO‐poor silicate abundances, and low oxygen fugacity.     

Evidence for a meteoritic origin of the September 15, 2007, Carancas crater

Abstract— On September 15th, 2007, around 11:45 local time in Peru, near the Bolivian border, the atmospheric entry of a meteoroid produced bright lights in the sky and intense detonations. Soon after, a crater was discovered south of Lake Titicaca. These events have been detected by the Bolivian seismic network and two infrasound arrays operating for the Comprehensive Nuclear‐Test‐Ban Treaty Organization, situated at about 80 and 1620 km from the crater. The localization and origin time computed with the seismic records are consistent with the reported impact. The entry elevation and azimuthal angles of the trajectory are estimated from the observed signal time sequences and back‐azimuths. From the crater diameter and the airwave amplitudes, the kinetic energy, mass and explosive energy are calculated. Using the estimated velocity of the meteoroid and similarity criteria between orbital elements, an association with possible parent asteroids is attempted. The favorable setting of this event provides a unique opportunity to evaluate physical and kinematic parameters of the object that generated the first actual terrestrial meteorite impact seismically recorded.     

Some general properties of helmeted coronal structures

A simplified analysis of helmeted coronal structures is carried out and some of the gross properties of such structures discussed. It is found that the magnetically closed region can have but a limited extension into the corona. For temperatures in excess of 1.5 × 10⁶ °K, the maximum height above the limb is about 1.6 R . The maximum possible extension of the helmet from the solar center is exactly one-half the distance to the critical point (where the flow velocity passes through the speed of sound). For this reason, a helmet streamer, at least out to a few solar radii, is essentially a magnetostatic structure - the flow adjacent to the helmet having little effect upon its properties. For given base dimensions, there is a maximum temperature for which a helmet streamer can exist - giving an indication of why such streamers do not appear over young active regions. If the temperature in the helmet and in the streaming region are approximately the same, the helmet height, helmet shape, external flow velocity, and rate of outward decline in the magnetic field are shown to be much more dependent upon the photospheric field distribution than upon the field strength. The density enhancement, however, is a strong function of the field strength. This enhancement is preserved out to the top of the helmet with both the density inside and outside decreasing approximately as predicted by hydrostatic equilibrium. The possible existence of both domed helmets and cusped helmets is demonstrated with the former existing at lower temperatures and the latter at higher temperatures. Cusped helmets occur, however, over a relatively narrow temperature range and are, hence, expected to be less common. The expansion velocity outside the helmet is higher than that predicted by radial flow but increases outward much more slowly. The magnetic field decreases outward proportionally to the square root of the density and inversely proportionally to the velocity - bearing, in general, no relation to a potential field since the rate of decline in field strength is determined by the temperature.On leave from AC Electronics Research Laboratories Santa Barvara, Calif., U.S.A.     

METEOMOD: A numerical model for the calculation of melting-crystallization relationships in meteoritic igneous systems

Abstract— The METEOMOD model is a computer program designed to calculate melting-crystallization relationships in igneous systems compositionally similar to ordinary chondrites and basaltic achondrites. The core of METEOMOD is a set of empirically calibrated equations, called geothermometers, which describe equilibria between silicate melt and minerals such as olivine, orthopyroxene, pigeonite, augite, plagioclase, and metallic Fe in terms of pressure, temperature, and liquid compositions. The silicate mineral geothermometers are calibrated from a database containing the compositions of melts and minerals produced in melting experiments on 113 meteoritic and 141 synthetic systems. The metallic iron-silicate melt geothermometer is calibrated from a database of 396 melting experiments. The Meteorite Melting Model or METEOMOD calculates crystallization temperatures and contents of major end members in mineral solid solutions with accuracies of ±10–15 °C and ±1–2 mol%, respectively. Input parameters for the program are (1) increment in crystallization degree; (2) one of 12f_O2 buffers routinely used in petrology; (3) shift from the buffer in log units, if any; (4) a choice of equilibrium or fractional crystallization trajectory; (5) terminal crystallization degree; (6) contents of ten major elements in wt%; (7) a set of minor and trace elements in parts per million; (8) the number of initial compositions to be modeled in a single computation run. The output consists of a series of tables that list equilibrium temperatures, O fugacities, and proportions of melt and minerals and their compositions, as a function of the degree of crystallization. The results of application of METEOMOD to modeling of melting-crystallization of the St. Severin LL chondrite are compared with the experimental data of Jurewicz et al. (1995).     

The crystalline lunar spherules: Their formation and implications for the origin of meteoritic chondrules

Abstract— Crystalline lunar spherules (CLS) from three thin sections of Apollo 14 regolith breccias (14318,6; 14318,48 and 14315,20) have been examined. The objects have been classified and their abundances, size distributions, bulk compositions, and (where possible) plagioclase compositions determined. By number, 64% consist predominantly of very fine-grained equant plagioclase grains but can also contain larger (～50 μm) feldspar crystals (type X), while 22% contain plagioclase lathes in a fine-grained mafic mesostasis (type Y). Plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue CL of the normal feldspar of the breccias. Type Z spherules (5%) contain feldspar with blue CL and minor amounts of olivine and pyroxene. Type Q spherules (4%) contain feldspar with yellow CL but in a luminescent mesostasis (of quartz or feldspar?). A few spherules are mixtures of type Y and type X textures. Most type X spherules, and a few type Y spherules, have fine-grained opaque rims. Compound objects were also found and consist of two or more CLS that appear to have collided while still plastic or molten. The CLS are thought to be impact spherules that crystallized in free flight, their coarse textures suggesting fairly slow cooling rates (～ <1 °C/s). The abundance of the CLS resembles that of chondrules in the CM chondrite Murchison, and their cumulative size-frequency distributions are very similar to those of the chondrules in several meteorite classes. The bulk compositions of the CLS do not resemble regoliths at any of the Apollo sites, including Apollo 14, or any of the common impact glasses, but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain. The Apollo 14 site is located on a region containing Imbrium ejecta, and we suggest that the CLS derive from the Imbrium impact. Ballistic calculations indicate that only impact events of this size on the Moon are capable of producing melt spherules with the required free flight times and slow cooling rates. Smaller impacts produce glassy spherules and agglutinates. As has been pointed out many times, the CLS have many properties in common with meteoritic chondrules. While much remains unclear, difficulties with a nebular origin and new developments in chondrule chronology, studies of asteroid surfaces and impact ejecta behavior, and the present observations indicate that meteoritic chondrules could have formed by impact.     

Depositional processes of impactites from the YAX-1 drill core in the Chicxulub impact structure inferred from vertical profiles of PDF orientations and grain size distributions of shocked quartz

Core samples from the Chicxulub impact structure provide insights into the formation processes of a shallow-marine-target, complex crater. Although previous studies investigated the impactites (generally suevitic and polymict breccias) of the Yaxcopoil-1 (YAX-1) drill core in the Chicxulub impact structure, the interpretation of its deposition remains controversial. Here, we analyze planar deformation features (PDFs), grain size, and abundance of shocked quartz throughout the YAX-1 impactite sequence (794–895 m in depth). PDF orientations of most quartz grains in YAX-1 impactites show a distribution of both low angles ({104}, {103}, {102}) and high angles (orientations higher than 55° to c-axis), while the lower part of the impactite sequence contains quartz showing only PDF orientations of low angles. High-abundance, coarse-grained shocked quartz is found from the lower to middle parts of the impactites, whereas it abruptly changes to low-abundance, fine-grained shocked quartz within the upper part. In the uppermost part of the impactites, repeated oscillations in contents of these two components are observed. PDF orientation pattern suggests most of the shocked quartz grains experienced a range of shock pressure, except two samples in the lower part of impactites, which experienced only a high level of shock. We suggest that the base and lower part of the impactite sequence were formed by ejecta curtain and melt surge deposits, respectively. Our results are also consistent with the interpretation that the middle part of the impactite sequence is fallback ejecta from the impact plume. Additionally, we support the contention that massive seawater resurges into the crater occurred during the deposition of the upper and uppermost part of the impactites.     

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.