Fallout Suevite Research Articles

Origin of small pits in martian impact craters

We propose a numerical model for the formation of the closely-spaced pits found in the thin, ejecta-related deposits superposed on the floors, interior terrace blocks, and near-rim ejecta blankets of well-preserved martian impact craters. Our model predicts the explosive degassing of water from this pitted material, which is assumed to originally be water-bearing, impact melt-rich breccia at the time of deposition. This process is analogous to what occurred in the fall-out suevite deposits at the Ries impact structure in Germany. At Ries, impact heating of water-bearing target material resulted in the rapid degassing of its water and other volatiles. The martian environment plays an important role in enhancing the effects of this degassing by increasing the flow-speed of the escaping gas. The high flow-rate of gas through particulate materials, such as suevite, tends to quickly form segregation channels or vent pipes, similar to those found in the Ries deposits. These pipes act as conduits for the efficient high-speed escape of the gas and small clasts that it entrains. Escaping gas and entrained clasts abraded and eroded the conduit walls, flaring them to form pits above a network of pipes.

Petrographic investigation of melt and matrix relationships in Chicxulub crater Yaxcopoil-1 brecciated melt rock and melt rock-bearing suevite (846–885 m, units 4 and 5)

The drill core from the International Continental Drill Program’s Chicxulub Scientific Drilling Project Yaxcopoil-1 (Yax-1) borehole, in the annular trough of the Chicxulub crater, exhibits from 794 to 895m a continuous sequence of impactites consisting of reworked fallout, fallout suevite, and brecciated impact melt rock. These impact breccias exhibit a complex history of deposition, fracturing, matrix emplacement and hydrothermal alteration. Detailed investigation of the mineralogy and chemistry of these breccias has led to a better understanding of the complex events involved in their formation. We find that the paragenesis of the brecciated impact melt rock (unit 5, 861–885m) involved fracturing of melt rock and early K-metasomatism during a hydrothermal alteration episode as suggested by earlier work. However, the present work has identified the role of multiple episodes of precipitation of Mg-rich phyllosilicates and formation and dissolution of accessory minerals in a relatively high temperature (>300°C) hydrothermal event. The earliest matrix formation event involved precipitation of Mg-rich phyllosilicate, accessory quartz, calcite, apatite, and andradite garnet from a hydrothermal fluid with a brine or seawater component. The fluid could have partly incorporated elements and shock-metamorphosed mineral phases derived from sedimentary lithologies, including calcite and dolomite that underwent complex phase transformations such as melting, decomposition and possible back-reactions. The discovery of andradite garnet in the matrix confirms the presence of an early high temperature hydrothermal event previously identified by mineralogical, stable isotope and fluid inclusion studies. The mineral assemblage, including Mg-rich saponite, suggests the involvement of seawater by comparison with similar alteration assemblages in hydrothermal systems involving seawater around the world. The presence of a later, low-temperature phase of the hydrothermal system, with different fluid chemistry is indicated by the partial dissolution of andradite garnet, and continued precipitation of matrix phyllosilicate minerals, but without the accessory quartz, abundant calcite or andradite, and without an accompanying K-metasomatism event.

Boron isotope composition of secondary smectite in suevites at the Ries crater, Germany: Boron fractionation in weathering and hydrothermal processes

The 24-km diameter Ries crater, in Germany, is one of the best-preserved terrestrial complex impact structure and clay minerals in the groundmass of the Ries suevites have been recognized for several decades. It is generally accepted that these clays were formed by post-impact aqueous alteration of impact-generated glasses and/or finely comminuted crystalline basement material at temperatures above ambient conditions. The Ries impact structure provides a good opportunity to study the evolution of post-impact development of impact craters. Here we present a study of boron isotopic compositions (δ 11B) and B concentrations of the secondary smectite clay fraction in fall-out and crater suevites from the Ries crater (Germany). These data were used to study the boron fractionation in clay mineral precipitation processes and to model, using B-isotope fractionation, the characteristics (pH, temperature, and salinity) of fluids during fluid–rock interaction at different positions within the crater. The results of this study show that the boron isotopic composition of smectite in fall-out suevites sampled from 4 different locations (average − 24.5 ± 1.8‰) was significantly different from that of the smectite in crater suevites sampled from the Nördlingen 1973 drill core at different depth intervals, from 370 m to 525 m (average − 4.07‰). Similarly, the δ 11B composition of the fluids responsible for the alteration calculated from smectite isotopic composition in the fall-out suevites (+ 7.5 ± 1.6‰), differed from the alteration fluid composition in crater suevites (+ 17.6 ± 10.8‰); indicating a different origin of the fluids responsible for the alteration of the fall-out and crater suevites in the Ries crater. Our results suggest that the alteration in fall-out suevites occurred at lower temperatures than the crater suevites and at a lower pH, which is consistent with the smectite precipitation in equilibrium with meteoritic fluids. The boron isotopic composition of smectite in the crater suevites, suggests secondary clay formation at higher temperatures and/or at elevated pH (> 8–9). Low δ 11B values of fixed boron in secondary smectite in fall-out suevites suggests that B was incorporated into the mineral structure during the precipitation of authigenic smectite; whereas the smectite precipitated in equilibrium with the boron and hydrogen isotopic composition in the meteoric water that percolated through the system. Our study provides new constraints on the origin and evolution of the (geothermal) fluids that were involved in the formation of altered minerals at the Ries crater and suggests that boron isotope composition studies of the secondary clay phases associated with post-impact cooling may have important implications for the changes in fluid flow and changes in geochemical signatures of the fluids, and provide new constraints on fluid/rock interaction and mass transport during the cooling of the impact crater.

Drill core LB‐08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift

Abstract— The 1.07 Myr old Bosumtwi impact structure in Ghana (West Africa), which measures 10.5 km in diameter and is largely filled by Lake Bosumtwi, is associated with one of four currently known tektite strewn fields. Two boreholes were drilled to acquire hard‐rock samples of the deep crater moat and from the flank of the central uplift (LB‐07A and LB‐08A, respectively) during a recent ICDP‐sponsored drilling project.Here we present results of major and trace element analysis of 112 samples from drill core LB‐08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta‐graywacke (from fine‐grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium‐grained meta‐graywacke.Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks.Suevite from core LB‐08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite‐normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other.Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work.

Geochemistry of impactites and basement lithologies from ICDP borehole LB‐07A, Bosumtwi impact structure, Ghana

Abstract— In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB‐07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB‐07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so‐called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta‐graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major‐ and trace‐element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB‐07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB‐07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major‐ and trace‐element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB‐07A suevites, but broader ranges in MgO and CaO contents are observed for the LB‐07A suevites.

Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics

Abstract— An attempt was made to detect a meteoritic component in both crater‐fill (fallback) impact breccias and fallout suevites (outside the crater rim) at the Bosumtwi impact structure in Ghana. Thus far, the only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic signature in Ivory Coast tektites, which formed during the Bosumtwi impact event. Earlier work at Bosumtwi indicated unusually high levels of elements that are commonly used for the identification of meteoritic contamination (i.e., siderophile elements, including the platinum group elements [PGE]) in both target rocks and impact breccias from surface exposures around the crater structure, which does not allow unambiguous verification of an extraterrestrial signature. The present work, involving PGE abundance determinations and Os isotope measurements on drill core samples from inside and outside the crater rim, arrives at the same conclusion. Despite the potential of the Os isotope system to detect even small amounts of extraterrestrial contribution, the wide range in PGE concentrations and Os isotope composition observed in the target rocks makes the interpretation of unradiogenic, high‐concentration samples as an impact signature ambiguous.

Lithostratigraphic and petrographic analysis of ICDP drill core LB‐07A, Bosumtwi impact structure, Ghana

Abstract— Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB‐07A in the Bosumtwi impact structure revealed two sequences of impactites below the post‐impact crater sediments and above coherent basement rock. The upper impactites (333.38–415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67–470.55 m depth) consists essentially of monomict impact breccia formed from meta‐graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55–545.08 m depth) is composed of lower greenschist‐facies metapelites (shale, schist and minor phyllite), meta‐graywacke, and minor meta‐sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric‐textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre‐impact granitoid complexes.Impact melt fragments are not as prevalent in LB‐07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within‐crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB‐07A appears to have been derived from the <35 GPa shock zone of the transient crater.

First petrographic results on impactites from the Yaxcopoil‐1 borehole, Chicxulub structure, Mexico

Abstract— The ICDP Yaxcopoil‐1 (Yax‐1) borehole located 60 km south‐southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795–895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795–922 m) comprises an apparently reworked, poorly sorted and graded, fine‐grained, clast‐supported, melt fragment‐bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823–846 m) and 3 (846–861 m) are groundmass‐supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine‐grained calcite, altered alkali element‐, Ca‐, and Si‐rich cement, as well as occasional lithic fragments. Unit 4 (861–885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885–895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine‐grained, carbonate‐dominated groundmass. This groundmass could represent either a secondary hydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well‐preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts.Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K‐metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground‐surge deposit.

Impact glasses in fallout suevites from the Ries impact structure, Germany: An analytical SEM study

Abstract— t‐Impact‐generated glasses from fallout suevite deposits at the Ries impact structure have been investigated using analytical scanning electron microscopy. Approximately 320 analyses of glass clasts were obtained. Four glass types are distinguished on the basis of composition and microtextures. Type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers. Major oxide concentrations indicate the involvement of granitic rocks, amphibolites, and minor Al‐rich gneisses during melting. Type 2 glasses are chemically heterogeneous, even within individual clasts, with variations of several wt% in most of the major oxides (e.g., 57–70 wt% SiO2). This suggests incomplete mixing of: 1) mineral‐derived melts or 2) whole rock melts from a wide range of lithologies. Aluminium‐rich clinopyroxene and Fe‐Mg‐rich plagioclase quench crystals are present in type 1 and 2 glasses, respectively. Type 3 glasses contain substantial amounts of H2O (˜12–17 wt%), low SiO2 (50–53 wt%), high Al2O3 (17–21 wt%), and high CaO (5–7 wt%) contents. This suggests an origin due to shock melting of part of the sedimentary cover. Type 4 glasses form a ubiquitous component of the suevites. Based on their high SiO2 content (˜85–100 wt%), the only possible protolith are sandstones in the lowermost part of the sedimentary succession. Calcite forms globules within type 1 glasses, with which it develops microtextures indicative of liquid immiscibility. Unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1, 2, and 4 glasses, indicating that montmorillonite was originally an impact melt glass. Clearly, the melt zone at the Ries must have incorporated a substantial fraction of the sedimentary cover, as well as the underlying crystalline basement rocks. Impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases (type 2, 3, and 4 glasses and carbonate melts).

Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana

Abstract— The 1.07 Ma well‐preserved Bosumtwi impact structure in Ghana (10.5 km in diameter) formed in 2 Ga‐old metamorphosed and crystalline rocks of the Birimian system. The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glassrich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke‐phyllite, shale, and granite). Graywacke‐phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ≤15 m and to occupy an area of ˜1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well‐preserved impact structure.

Stable isotope composition of impact glasses from the Nördlinger Ries impact crater, Germany

The Ries impact, which today is represented by a 24-km-diameter complex crater, occurred at 15 Ma. It is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2.8 km depth. Impact melt glasses investigated are found within the suevite, the breccia that forms the uppermost layers of the ejecta blanket around the crater. The glasses are considered to represent quenched melts produced as a result of the impact. The oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with δ 18O values in the range of 6.7 to 7.4‰ and δ 17O of 3.3 to 3.7‰. With increasing devitrification and alteration the δ 18O values increase up to 15.8‰, δ 17O up to 8.1‰. Water content also increases with devitrification from fresh glasses with about 1.3 wt.% to 3.8 wt.% in devitrified glass. δD values decrease with increasing water content from about −87 to −127‰. Whole-rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27‰, and crystalline basement rocks range from 8.8 to 13.5‰ for granites and gneisses, whereas amphibolites have δ 18O values of 5.2 and 6.1‰. Models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence (Engelhardt, 1997) do not agree with the oxygen isotope composition of the glass. The simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only. High Fe and Ni contents in the glass, Cr/Ni and Cr/Co ratios, and relatively low δ 18O values indicate involvement of amphibolites during melting. The balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare-earth element geochemistry. The best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite.

Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts

Abstract— Clasts of deep‐seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty‐three clasts, representing these lithologies, were analyzed for major element bulk composition.The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stöffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock‐fusion of a similarly composed basement.The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nördlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies.The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep‐burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.

Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany—a review

The lithologies and distribution of the various ejecta formations from the Ries crater, Germany, are described. They may be subdivided into: (1) low shock level (< 10 GPa), represented by the Bunte Breccia (sediments from the upper 600 m of the target area), megablocks of sedimentary rocks, and megablocks and monomict breccias derived from the crystalline basement, and (2) high shock level (> 10 GPa) represented by polymict crystalline breccias, fall-out suevites, crater suevite, and tektites (moldavites). The ejecta from the Mesozoic sedimentary column do not show effects of shock pressures exceeding 10 GPa, and only basement gneisses were shock melted to yield suevite glasses. It is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level, where a complex of gneisses underlies shallower granitic intrusions. The estimated depth of the transient crater was about 2700 m and its diameter was 12–13 km. Bunte Breccia, megablocks of sedimentary rocks, and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits, extending up to a radius of 22 km. Fall-out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta. In contrast, the crater suevite never left the crater cavity; it represents the main mass of suevite. Moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised Tertiary sands, covering the surface of the target area.

Suevite of the ries crater, Germany: Source rocks and implications for cratering mechanics

Suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamorphic rocks.