Compositional variation and mixing of impact melts on microscopic scales

Abstract— We investigated the compositional characteristics of schlieren-rich, holohyaline impact glasses from Ries, Wabar, and Meteor Crater using a Cameca SX 100 scanning electron microprobe. This instrument is capable of producing detailed maps of major elements at spatial resolutions of <10 μm. The objective was to characterize the composition of an unusually large number of individual schlieren and to evaluate details of the process that causes melts of lithologically diverse target rocks to mix on scales of micrometers. The Ries and Meteor Crater impacts involved lithologically heterogeneous targets; whereas, Wabar Crater formed in relatively uniform dune sand. Texturally heterogeneous, schlieren-rich glasses from the Ries Crater illustrate that schlieren of highly variable color can be surprisingly similar in composition, as first detailed by Stähle (1972). Consistent with these earlier findings, most schlieren represent mixtures of diverse rock melts; their compositions deviate only subtly from the average melt and do not resemble monomineralic melts nor binary mixtures of major rock-forming minerals. A specific population of schlieren is enriched in mafic elements (Mg, Fe, and Ca), which suggests incomplete homogenization of an amphibolite progenitor. In the case of Wabar Crater, a compositionally simple melt of dune sand mixed with projectile (IIIA iron meteorite) materials, and specific schlieren are variable mixtures of these two progenitors. The optically homogeneous glass from Meteor Crater is compositionally homogeneous as well, which suggests ideal mixing of such diverse lithologies as platform carbonates, sandstone, and a class IIIA iron meteorite. The mixing of projectile and target melts at Wabar and Meteor Crater unambiguously demonstrates that melts initially produced in distinctly different stratigraphic/structural locations will undergo wholesale mixing, if not homogenization. Also, the projectile melts unquestionably formed relatively early in the cratering process, and their dissemination throughout the prospective melt volume, albeit at variable concentration levels, suggests that the entire mixing process may be an early cratering feature. This also follows from the fact that we investigated ballistic melt ejecta, which thereby eliminates all of those mixing processes that may additionally operate during the pooling and generation of massive melt-ponds following gravitational collapse of large, structurally complex craters. Substantial turbulence ranging from field dimensions to microscopic scales seems inescapable to accomplish the observed degree of mixing, yet this is not readily inferred from current models of macroscopic material motions during hypervelocity impact.     

Meteoritic and other constraints on the internal structure and impact history of small asteroids

Studies of the internal structure of asteroids, which are crucial for understanding their impact history and for hazard mitigation, appear to be in conflict for the S-type asteroids, Eros, Gaspra, and Ida. Spacecraft images and geophysical data show that they are fractured, coherent bodies, whereas models of catastrophic asteroidal impacts, family and satellite formation, and studies of asteroid spin rates, and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble. These conflicting views may be reconciled if 10-50 km S-type asteroids formed as rubble piles, but were later consolidated into coherent bodies. Many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration, metamorphism, igneous and impact processes. Ordinary chondrites, which are the best analogs for S asteroids, are commonly breccias. Some may have formed in cratering events, but many appear to have formed during disruption and reaccretion of their parent asteroids. Some breccias were lithified during metamorphism, and a few were lithified by injected impact melt, but most are regolith and fragmental breccias that were lithified by mild or moderate shock, like their lunar analogs. Shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together, and by friction and pressure welding of silicate and metallic Fe,Ni grains. We suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble. Internal voids would be partly filled with regolith by impact-induced seismic shaking. Consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body. Fractures on Ida that were created by antipodal impacts and are concentrated in and near large craters, and small positive gravity anomalies associated with the Psyche and Himeros craters on Eros, are consistent with this concept. Spin data suggest that smaller asteroids 0.6-6 km in size are unconsolidated rubble piles. C-type asteroids, which are more porous than S-types, and their analogs, the volatile-rich carbonaceous chondrites, were probably not lithified by shock.     

Geology of the Morasko craters,Poznań, Poland—Small impact craters in unconsolidated sediments

Confirmed small impact craters in unconsolidated deposits are rare on Earth, and only a few have been the subjects of detailed investigations. Consequently, our knowledge of indicators permitting unambiguous identification of such structures is limited. In this work, detailed geological mapping was performed in the area of the Morasko craters, of which the largest crater is of about 96 m diameter. These craters were formed in the mid‐Holocene (~5000 yr ago) in unconsolidated sediments of a glacial terminal moraine. Fragments of the impactor—an iron meteorite—have been found in the craters’ vicinity for many decades. Despite numerous studies of the meteorite, no detailed research concerning the geological structure around the craters and of the ejecta deposits has been undertaken. The new data, including evaluation of over 52 sediment cores and 260 shallow drillings, permit the identification of four main sediment types: Neogene clays, diamicton with Neogene clay clasts containing charcoal pieces, diamicton without clasts, and sand with locally preserved paleosoil and charcoal pieces. Based on sedimentological properties, the ejecta deposits are mainly identified as diamicton with Neogene clay clasts, described as lithic impact breccia, covering locally preserved pre‐impact soil. Moreover, crater sections characterized by inverse stratigraphy of sediments are identified as belonging to overturned flaps.     

Formation of a small impact structure discovered within the Agoudal meteorite strewn field,Morocco

A relic impact structure was recognized within the strewn field of the Agoudal iron meteorite. The heavily eroded structure has preserved shatter cones in a limestone basement, and remnants of autochthonous and allochthonous breccias. Fragments of iron incorporated into the allochthonous breccia have a chemical composition (Ni = 5.16 wt%, Ir = 0.019 ppm) similar to that of the Agoudal meteorite, supporting a syngenetic origin of the strewn field and the impact structure. The total recovered mass of Agoudal meteorite fragments is estimated at approximately 500 kg. The estimated size of the SE–NW‐oriented strewn field is 6 × 2 km. Model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1.4 m in diameter with an impact angle of approximately 60° from the horizontal. However, the most probable is an impact of a larger, 3–4 m diameter meteoroid, resulting a strewn field with approximately 10 craters, 10–30 m in diameter each, plus numerous meteorite fragments. The calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the Agoudal meteorite.     

Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil crater,Egypt

Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz‐rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high‐pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s^?1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures.     

The Whitecourt meteorite impact crater,Alberta, Canada

Abstract– The <1,100 yr old Whitecourt meteorite impact crater, located south of Whitecourt, Alberta, Canada, is a well‐preserved bowl‐shaped structure having a depth and diameter of approximately 6 and 36 m, respectively. There are fewer than a dozen known terrestrial sites of similar size and age. Unlike most of these sites, however, the Whitecourt crater contains nearly all of the features associated with small impact craters including meteorites, ejecta blanket, observable transient crater boundary, raised rim, and associated shock indicators. This study indicates that the crater formed from the impact of an approximately 1 m diameter type IIIAB iron meteoroid traveling east‐northeast at less than approximately 10 km s^?1, striking the surface at an angle between 40° and 55° to horizontal. It appears that the main mass survived atmospheric transit relatively intact, with fragmentation and partial melting during impact. Most meteoritic material has a jagged, shrapnel‐like morphology and is distributed downrange of the crater.     

Laser‐induced melting experiments: Simulation of short‐term high‐temperature impact processes

This study introduces an experimental approach using direct laser irradiation to simulate the virtually instantaneous melting of target rocks during meteorite impacts. We aim at investigating the melting and mixing processes of projectile (iron meteorite; steel) and target material (sandstone) under idealized conditions. The laser experiments (LE) were able to produce features very similar to those of impactites from meteorite craters and cratering experiments, i.e., formation of lechatelierite, partial to complete melting of sandstone, and injection of projectile droplets into target melts. The target and projectile melts have experienced significant chemical modifications during interaction of these coexisting melts. Emulsion textures, observed within projectile‐contaminated target melts, indicate phase separation of silicate melts with different chemical compositions during quenching. Reaction times of 0.6 to 1.4 s could be derived for element partitioning and phase‐separation processes by measuring time‐depended temperature profiles with a bolometric detector. Our LE allow (i) separate melting at high temperatures to constrain primary melt heterogeneities before mixing of projectile and target, (ii) quantification of element partitioning processes between coexisting projectile and target melts, (iii) determination of cooling rates, and (iv) estimation of reaction times. Moreover, we used a thermodynamic approach to calculate the entropy gain during laser melting. The entropy changes for laser‐melting of sandstone and iron meteorite correspond to shock pressures and particle velocities produced during the impact of an iron projectile striking a quartz target at a minimum impact velocity of ~6 km s^?1, inducing peak shock pressures of ~100 GPa in the target.     

Dissemination and fractionation of projectile materials in the impact melts from Wabar Crater,Saudi Arabia

Abstract— We have analyzed small, ballistically dispersed melt samples in the form of aerodynamically shaped spheres, dumbbells, teardrops, etc., from Wabar Crater, Saudi Arabia, and have compared these to our previous study of the more massive, black and white melt specimens. The smaller melt samples differ from the more massive melts in that they are petrographically and chemically more homogeneous, possess fewer, more diffuse schlieren and contain much less clastic detritus. These observations suggest higher peak temperatures for the smaller melt samples than for the massive black and white melts which represent Wabar's major melt-zone. Analyses of the Wabar and Nejed (paired with Wabar) meteorites permit detailed comparison of the unaltered projectile with impactor residues in the melts. Siderophile element concentrations indicate that the small glass beads commonly contain > 10% meteoritic component, compared to < 5% for the massive black and white melts. One glass bead was found to contain ～ 17% meteoritic component. Based on models for melt production during cratering, we deduce that more meteoritic material was mixed with the upper stratigraphic horizons of Wabar's melt zone than with the lower parts. Siderophile elements in all Wabar melt specimens are fractionated relative to the Wabar-Nejed meteorite and have Fe/Ni ratios up to ～ 1.8 times that of Wabar-Nejed for the most siderophile element-rich glasses. The abundance sequence of siderophiles in the melts relative to the projectile is Fe ? Co > Ni ? Ir ? As » Au. Although this sequence seems incompatible with simple vapor fractionation of either elements or oxides, we believe that a complex vapor fractionation process most likely produced the observed siderophile element abundances. Our sample suite should be representative of all materials found in and around the Wabar structure, and we conclude that substantial quantities of the projectile were lost to the atmosphere, most likely as vapor. No fractionation of lithophile elements is observed in the glasses relative to the target rocks. Although fractionation of the impactor must have occurred prior to intimate mixing of projectile and target, details of the actual fractionation mechanism(s) remain poorly understood. The results of this study indicate that caution is necessary when attempting to define impactor types and masses from compositional data for impact melts from other craters.     

The Sirente crater field,Italy

Abstract— We propose the Sirente crater field to be the first discovered impact craters in Italy. They are located in the Sirente plain within the mountains of the Abruzzo region, central Italy. The craters are distributed in a field 450 m long and 400 m wide. This field consists of ?17 smaller craters close to a larger main crater. The main crater is located in the southern end of the crater field and is 140 m long and 115 m wide, measured rim‐to‐rim. It has a well‐developed, saddle‐shaped rim that rises at a maximum 2.2 m above the surrounding plain. Radiocarbon dating of the target surface preserved below the rim gave a calibrated age of formation at about a.d. 412 (1650 ± 40 radiocarbon years b.p.). This young age is consistent with the apparent little modification of the rim. The morphology of the main crater and its relation to a crater field strongly points to its origin by impact from a projectile that broke up during its passage through the atmosphere. Quartz is very rare in the target and no planar deformation features have been found so far. The rim material and the upper 4 m of the main crater infill are impregnated with ferric oxides, which gives a more reddish colour compared to the other sediments of the plain. Rusty crusts with high Fe and Mn content occur in the rim material, but have not been found in the plain's sediments. Some of these crusts can be separated by magnet, and have sporadic micron‐sized Ni‐rich granules. The main crater is in the size range of the craters with explosive dispersion of the projectile and has many features comparable to both large experimental and meteoritic impact craters formed in loose sediments. We suggest that this crater represents a rare example of well‐preserved, small impact crater formed in unconsolidated target materials.     

NEW LIGHT ON THE ORIGIN OF THE HOLY BLACK STONE OF THE KA'BA

At various times the Black Stone of the Ka'ba has been considered to be lava, basalt, a meteorite and lately an agate. This paper discusses the possibility that it may consist of impactite glass, derived from the meteoritic craters of Al-Hadidah, called Wabar.     

Mineralogy,petrology, and distribution of meteorites at the Whitecourt crater,Alberta, Canada

The Whitecourt meteorite impact crater, Alberta, Canada is a rare example of a well‐preserved small impact structure, with which thousands of meteorite fragments are associated. As such, this crater represents a unique opportunity to investigate the effect of a low‐energy impact event on an impacting iron bolide. Excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types. The meteorites' distribution, and internal and external characteristics support a low‐altitude breakup of the impactor which caused atmospherically ablated (regmaglypted) meteorites to fall close to the crater and avoid impact‐related deformation. In contrast, shrapnel fragments sustained deformation at macro‐ and microscales resulting from the catastrophic disruption of the impactor. The impactor was significantly fragmented along pre‐existing planes of weakness, including kamacite lamellae and inclusions, resulting in a bias toward low‐mass (<100 g) fragments. Meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite, consistent with other low‐Ni IIIAB iron meteorites. Considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity, lower mass impactor than previously inferred.     

Modification of premare impact craters by volcanism and tectonism

Many lunar craters greater than 10 km in diam exhibit a variety of morphological characteristics which are not produced by meteorite impact or meteorite erosion. Most such craters are located in or near the margins of the maria. Although some could have resulted from processes such as cauldron resurgence, caldera formation, or ring dike emplacement, most have formed by modification of impact craters by endogenic processes including erosion by flowing lava, fissure volcanism, plutonism and uplift of crater floors along ring fractures of impact origin.     

Discovering research value in the Campo del Cielo,Argentina, meteorite craters

Abstract The Campo del Cielo meteorite crater field in Argentina contains at least 20 small meteorite craters, but a recent review of the field data and a remote sensing study suggest that there may be more. The fall occurred ～4000 years ago into a uniform loessy soil, and the craters are well enough preserved so that some of their parameters of impact can be determined after excavation. The craters were formed by multi-ton fragments of a type IA meteoroid with abundant silicate inclusions. Relative to the horizontal, the angle of infall was ～9°. Reflecting the low angle of infall, the crater field is elongated with apparent dimensions of 3 × 18.5 km. The largest craters are near the center of this ellipse. This suggests that when the parent meteoroid broke apart, the resulting fragments diverged from the original trajectory in inverse relation to their masses and did not undergo size sorting due to atmospheric deceleration. The major axis of the crater field as we know it extends along N63°E, but the azimuths of infall determined by excavation of Craters 9 and 10 are N83.5°E and N75.5°E, respectively. This suggests that the major axis of the crater field is not yet well determined. The three or four largest craters appear to have been formed by impacts that disrupted the projectiles, scattering fragments around the outsides of the craters and leaving no large masses within them; these are relatively symmetrical in shape. Other craters are elongated features with multi-ton masses preserved within them and no fragmentation products outside. There are two ways in which field research on the Campo del Cielo crater field is found to be useful. (1) Studies exist that have been used to interpret impact craters on planetary surfaces other than the Earth. This occurrence of a swarm of projectiles impacting at known angles and similar velocities into a uniform target material provides an excellent field site at which to test the applicability of those studies. (2) Individual craters at Campo del Cielo can yield the masses of the projectiles that formed them and their velocities, angles and azimuths of impact. From these data, there is a possibility to estimate parameters for the parent meteoroid at entry and, thus, learn enough about its orbit to judge whether or not it was compatible with an asteroidal origin. Preliminary indications are that it was. Campo del Cielo is a IA iron meteorite and Sikhote-Alin, an observed fall, is a IIB iron meteorite in Wasson's classification. The Sterlitamak iron, also an observed fall, is a medium octahedrite in the Prior-Hey classification. It would be interesting to compare their orbital parameters.     

Rumanová H5 chondrite,Slovaki

Abstract— An H5 chondrite was found near the village of Rumanová, Slovakia. dominant minerals of the meteorite are enstatite, olivine, kamacite, taenite and troilite. The minor minerals are oligoclase, augite, pigeonite, accessory chromite, whitlockite and chlorapatite. The composition of olivine (Fa_19.0) and low-Ca orthopyroxene (Fs_17.0), and the density and chemical composition of the meteorite correspond to those of an H chondrite. Normal zoning of Ni in metal grains and parallel planar fractures in olivine suggest weak shock metamorphism of stage S3. Due to moderate oxidation of metal, iron hydroxides were formed corresponding to weathering stage W2.     

A meteorite impact crater field in eastern Bavaria? A preliminary report

Abstract— Numerous circular depressions north of Burghausen in eastern Bavaria, with diameters ranging from meters to tens of meters in size and dispersed over an area of at least 11 times 7 km, are suspected to have an extraterrestrial origin since they resemble other small meteorite impact craters. The depressions are bowl‐shaped, have high circularity and a characteristic rim. Most of them were formed in unconsolidated glacial gravels and pebbles intermixed with fine‐grained sand and clay. Magnetic investigations reveal weak anomalies with amplitudes of less than ±10 nano Tesla (nT). In some cases, the origins of the anomalies are suspected to be due to human activity within the structures. So far, no traces of meteoritic material have been detected. An evident archaeological or local geological explanation for the origin of the craters does not exist. A World War I and II explosive origin can be excluded since trees with ages exceeding 100 years can be found in some craters. One crater was described in 1909. Carbon‐14 dating of charcoal found in one crater yielded an age of 1790 ± 60 years. Hence, a formation by meteorite impacts that occurred in Celtic or early medieval times should be considered. A systematic archaeological excavation of some structures and an intensified search for traces of meteoritic material are planned.     

From regolith to rock by shock

A model for shock-lithification of terrestrial and lunar regolith is proposed that accounts for: (1) observed petrographic properties and densities of shock-lithified material from missile impact craters at White Sands, New Mexico and from Meteor Crater, Arizona; (2) observed petrographic textures of lunar soil and lunar soil analogues experimentally shocked to known pressures in laboratory experiments; (3) theoretical calculations of the behavior of air and water under shock compression; and (4) measured Hugoniot and release adiabat data on dry and wet terrestrial soils and lunar regolith. In this model it is proposed that air or an air-water mixture initially in the pores of terrestrial soil affects the behavior of the soil-air-water system under shock-loading. Shock-lithified rocks found at Meteor Crater are classified as ‘strongly lithified’ and ‘weakly lithified’ on the basis of their strength in hand specimen; only weakly lithified rocks are found at the missile impact craters. These qualitative strength properties are related to the mechanisms of bonding in the rocks. The densities of weakly lithified samples are directly related to the pressures to which they were shock-loaded. A comparison of the petrographic textures and densities of weakly lithified samples with textures and densities of ‘regolith’ shock-loaded to known pressures suggests that weakly lithified terrestrial samples formed at pressures well under 100 kb, probably under 50 kb. If terrestrial soils are shock-loaded to pressures between 100 and 200 kb by impact events of short duration, the pore pressure due to hot air or air-water mixtures exceeds the strength of the weak lithification mechanisms and fragmentation, rather than lithification, occurs. At pressures above 200 kb, lithification can occur because the formation of glass provides a lithification mechanism which has sufficient strength to withstand the pore pressure. During shock-lithification of lunar regolith at pressures below 50 kb, the material is compressed to intrinsic crystal density and remains at approximately that density upon release from the shocked state. It is proposed, however, that at pressures in excess of 50 kb, the release of trapped volatiles from lunar soil grains into fractures causes an expansion of the regolith during unloading from the shocked state.     

Kgagodi Basin: The first impact structure recognized in Botswana

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ～250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component. Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.     

Fracture‐related intracrystalline transformation of olivine to ringwoodite in the shocked Sixiangkou meteorite

Abstract— Magnesium‐iron olivine in the Sixiangkou L6 chondrite contains abundant fractures induced by plastic deformation during shock metamorphism. This study reports the discovery of lamellar ringwoodite that incoherently nucleated and grew along planar and irregular fractures in olivine. Magnesium‐iron interdiffusion took place between olivine matrix and crystallizing ringwoodite at high pressures and high temperatures, which resulted in higher FeO content in ringwoodite lamellae than in olivine. This suggests that a quasi‐hydrostatic high pressure lasting for several minutes should have been produced in the shock veins of the meteorite. The intracrystalline transformation of olivine to ringwoodite also has implications for phase transitions in subducting lithospheric slabs because planar and irregular fractures are commonly produced in olivine that suffered plastic deformation.     

Magnetometer survey of the proposed Sirente meteorite crater field,central Italy: Evidence for uplifted crater rims and buried meteorites

Abstract— The Sirente crater field consists of a 120 m wide, rimmed main depression flanked to the northwest by about 30 smaller depressions. It has been dated to the first centuries A.D. An impact origin is suggested, but not confirmed. The small size combined with the properties of the target material (carbonate mud) would neither allow shock features diagnostic of impact, nor projectile vaporization. Consequently, a meteoritic component in the sediments would be very localized. At impacts of this size the projectile most likely is an iron meteorite. Any iron meteorites on the ground surface would, in Iron Age Europe, have been removed shortly after the event. However, if the depressions are of impact origin they should contain meteorites at great depth in analogy with known craters. The magnetic properties of iron meteorites differ distinctly from the very low magnetic sediments and sedimentary rocks of the Sirente area. We have used a proton precession magnetometer/gradiometer to produce magnetic anomaly maps over four of the smaller depressions (～8 m diameter), as well as two crossing profiles over a fifth depression (～22 m diameter). All show distinct magnetic anomalies of about 20 nT, the larger depression up to 100 nT. Magnetic modeling shows a best fit for structures with upturned strata below their rims, excluding a karstic origin but supporting an explosive formation. The 100 nT anomaly can only be explained by highly‐magnetic objects at a few meters depth. All together, the magnetic data provides a strong indication for an impact origin of the crater field.