Genetical Relationship Between Ignimbrites and Suevites? — Implications from the Suevitic Ries/Steinheim Impact Lithologies
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Abstract:
Introduction: Construed as the result of a collapsing ejecta plume since 1977 [1], the formation and emplacement of the Ries suevite was recently reinterpreted as a result of (a) ground-hugging impact melt flows [2], or (b) ‘phreatomagmatic eruptions‘ that were caused by the interaction of surficial water with an impact melt pool [3,4]. Furthermore, [5] pointed out a striking similarity between structural features in suvites and ignimbrites (compare Figs. 1 and 2). Ignimbrites are deposits of pyroclastic density currents (pyroclastic flows), a hot suspension of particles and gas driven by the collaps of an eruptive column. The deposits are composed of a poorly-sorted mixture of volcanic ash and pumice, commonly with scattered lithic fragments; various stages of welding and reomorphic flow structures can be observed [6]. They usually exhibit a fine-grained, often nonerosive, basis (surge), followed by ash layers that contain inversely graded rock fragments. Bottom-up, ignimbrites are dominated by pumice-rich ash layers, overlain by very fine-grained fall-back ashes [7]; elutriation (degassing) pipes are frequently developed at the top.Keywords:
Phreatomagmatic eruption
Pumice
Lapilli
Pyroclastic fall
Peléan eruption
Caldera
Lithology
Volcanic ash
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Recent drilling has produced the samples of impact melt breccias examined here. All samples examined here are from the YAX-1 borehole [7]. Two main types of impact breccias have been studied. The first type is a green altered impact meltrock found in the lower portion of the impact sequence. The texture of the rock is microcrystalline and is composed of pyroxene, plagioclase, and alkali feldspars. Its composition is consistent with continental margin rocks. It is generally massive with some flow structure. The rock was brecciated and altered after solidfication and contains small amounts of both shocked and unshocked clasts of the impacted lithologies. These lithologies include lithic quartzite, and isolated feldspar crystals. The compositions of these rocks are similar to those seen in meltrocks sampled by the Yucatan-6 borehole [8-9]. Our study includes samples YAX-1_861.4, YAX-1_863.51, and YAX- 1_876.46, which represent both the top and lower portion of the green impact meltrock. The middle sample in the sequence has the least amount of (mineralogical) alteration [10]. The second type of melt breccia under study is a brown altered impact meltrock. It also has a microcrystalline texture and both shocked and unshocked clasts of the target material. Even though this rock type has been altered, remnant schleiren, metaquartzite, and micritic calcite have been identified. Sample YAX-1_841.32 is representative of this type of rock. It was recovered from a polymict breccia in the middle of the impact sequence.
Breccia
Pyroxene
Lithology
Impact structure
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This study explores the petrogenesis of Shişr 161, an immature lunar regolith breccia meteorite with low abundances of incompatible elements, a feldspathic affinity, and a significant magnesian component. Our approach was to identify all clasts >0.5 mm in size in a thin section, characterize their mineral and melt components, and reconstruct their bulk major and minor element compositions. Trace element concentrations in representative clasts of different textural and compositional types indicate that the clast inventory of Shişr 161 is dominated by impact melts that include slowly cooled cumulate melt rocks with mafic magnesian mineral assemblages. Minor exotic components are incompatible-element-rich melt spherules and glass fragments, and a gas-associated spheroidal precipitate. Our hypothesis for the petrologic setting of Shişr 161 is that the crystallized melt clasts originate from the upper ~1 km of the melt sheet of a 300 to 500 km diameter lunar impact basin in the Moon’s feldspathic highlands. This hypothesis is based on size requirements for cumulate impact melts and the incorporation of magnesian components that we interpret to be mantle-derived. The glassy melts likely formed during the excavation of the melt sheet assemblage, by an impact that produced a >15 km diameter crater. The assembly of Shişr 161 occurred in a proximal ejecta deposit of this excavation event. A later impact into this ejecta deposit then launched Shişr 161 from the Moon. Our geochemical modeling of remote sensing data combined with the petrographic and chemical characterization of Shişr 161 reveals a preferred provenance on the Moon’s surface that is close to pre-Nectarian Riemann-Fabry basin.
Breccia
Petrogenesis
Regolith
Trace element
Lithic fragment
Rare-earth element
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Abstract– Melt‐bearing impactites dominated by suevite, and with a minor content of clast‐rich impact melt rock, are found within the central part of the Gardnos structure. They are preserved as the eroded remnants in the relatively small complex impact structure with a present diameter of 5 km. These rocks have been mapped in the field and in the Branden drill core, and described according to mineralogy/petrology, including matrix, litho clast, and melt content, as well as geochemistry. Based on our extensive field mapping, a simple 3‐D model of the original crater was constructed to estimate tentative volumes for the melt‐bearing impactites. The variations in lithic and melt fragment content and chemistry of suevite matrix can mostly be explained by incorporation of mafic rocks into a dominant mixture of granitic, gneissic, and quartzitic target rocks, reflecting mixing of material from different parts of the crater. Melt fragments within suevite occur with a variety of shapes and textures, probably related to different original target rock composition, to the various temperatures the individual fragments were subjected to during the impact event and deposition processes. This study discusses the impact‐related deposits based on a sedimentological approach. Their overall composition and structures indicate dominating gravity flow processes in the final transportation and deposition of the suevite.
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Deposition
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Abstract The term “suevite” has been applied to various impact melt‐bearing breccias found in different stratigraphic settings within terrestrial impact craters. Suevite was coined initially for impact glass‐bearing breccias from the Ries impact structure, Germany, which is the type locality. Various working hypotheses have been proposed to account for the formation of the Ries suevite deposits over the past several decades, with the most recent being molten‐fuel‐coolant interaction ( MFCI ) between an impact melt pool and water. This mechanism is also the working hypothesis for the origin of the bulk of the Onaping Formation at the Sudbury impact structure, Canada. In this study, the key characteristics of the Ries suevite, the Onaping Formation and MFCI deposits from phreatomagmatic volcanic eruptions are compared. The conclusion is that there are clear and significant lithological, stratigraphic, and petrographic observational differences between the Onaping Formation and the Ries suevite. The Onaping Formation, however, shares many key similarities with MFCI deposits, including the presence of layering, their well‐sorted and fine‐grained nature, and the predominance of vitric particles with similar shapes and lacking included mineral and lithic clasts. These differences argue against the viability of MFCI as a working hypothesis for genesis of the Ries suevite and for a required alternative mechanism for its formation.
Breccia
Phreatomagmatic eruption
Impact structure
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Molasse
Radiogenic nuclide
TRACER
Denudation
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Breccia
Shock metamorphism
Hypervelocity
Impact structure
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One major objective of our Sudbury project was to define origin and age of the huge breccia units below and above the Sudbury Igneous Complex (SIC). The heterolithic Footwall Breccia (FB) represents a part of the uplifted crater floor. It contains subrounded fragments up to several meters in size and lithic fragments with shock features (greater than 10 GPa) embedded into a fine- to medium-grained matrix. Epsilon(sub Nd)-epsilon(sub Sr) relationships point to almost exclusively parautochthonous precursor lithologies. The different textures of the matrix reflect the metamorphic history of the breccia layer; thermal annealing by the overlying hot impact melt sheet (SIC) at temperatures greater than 1000 C resulted in melting of the fine crushed material, followed by an episode of metasomatic K-feldspar growth and, finally, formation of low-grade minerals such as actinolite and chlorite. Isotope relationships in the Onaping breccias (Gray and Green Member) are much more complex. All attempts to date the breccia formation failed: Zircons are entirely derived from country rocks and lack the pronounced Pb loss caused by the heat of the slowly cooling impact melt sheet (SIC). Rb-Sr techniques using either lithic fragments of different shock stages or the thin slab method, set time limits for the apparently pervasive alkali mobility in these suevitic breccias. The data array and the intercept in the plots point to a major Rb-Sr fractionation around 1.54 Ga ago. This model age is in the same range as the age obtained for the metasomatic matrix of the FB. Rb-Sr dating of a shock event in impact-related breccias seems to be possible only if their matrix had suffered total melting by the hot melt sheet (FB) or if they contain a high fraction of impact melt (suevitic Onaping breccias), whereas the degree of shock metamorphism in rock or lithic fragments plays a minor role. In the Sudbury case, however, the impact melt in the seuvitic breccias is devitrified and recrystallized, which changed Rb/Sr ratios quite drastically. Therefore, the Onaping breccias give only age limits for alteration and low-grade metamorphism. The Sm-Nd system was not reset during the Sudbury event; clasts as well as the matrix in the FB and in the Onaping breccias show preimpact 'Archean' Nd isotope signatures.
Breccia
Metasomatism
Impact structure
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Abstract A variety of quantitative petrographic methods, such as the study of clast size distribution ( CSD ), have been used in the study of melt rocks and have, for example, led to validation of the melting origin of pseudotachylites in paleoseismic faults, the estimate of the energy involved in melting and crushing processes, the distinction between seismic and aseismic faults, and understanding the crystallization process in volcanic rocks. Recently, quantitative petrography was applied to distinguish impact melts from pristine mare basalts on the Moon. Here, we apply this approach to impact lithologies formed in a volcanic target. The El'gygytgyn structure, an 18 km‐diameter and 3.58 Ma old impact crater in north‐eastern Chukotka, Arctic Russia, represents the only known impact crater on Earth mainly excavated in siliceous volcanic rocks. The structure was recently drilled in the framework of an ICDP project, providing fresh samples of suevite and other impact breccias, which can be compared with samples from the unshocked target. As the target is mostly composed of rhyolitic‐dacitic ignimbrites and tuffs, impact melt clasts are almost indistinguishable from the unshocked volcanic clasts in the absence of shock evidence. We show here that geometric characterization provides a reproducible technique for quantitative description of impact lithologies. Although the studied suevite reveals a high local variability in the evaluated geometric parameters, an overall homogenization of these parameters occurs. Furthermore, quantitative petrography allows the classification of unshocked to slightly shocked volcanic clasts included in the suevite.
Breccia
Lithology
Impact structure
Shock metamorphism
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Breccia
Impact structure
Coesite
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