Heterogeneity of melts in impact deposits and implications for their origin (Ries suevite, Germany)

Abstract

Impact melt‐bearing clastic deposits (suevites) are one of the most important records of the impact cratering process. A deeper understanding of their composition and formation is therefore essential. This study focuses on impact melt particles in suevite at Ries, Germany. Textures and chemical evidence indicate that the suevite contains three melt types that originate from different shock levels in the target. The most abundant melt type (“melt type 1”) represents well‐mixed whole‐rock melting of crystalline basement and includes incompletely mixed mafic melt schlieren (“melt type 1 mafic”). Polymineralic melt type 2 comprises mixes between monomineralic melt types 3 and melt type 1. Melt types 2 and 3 are located within melt type 1 as small patches or schlieren but also isolated within the suevite matrix. The main melt type 1 is heterogeneous with respect to trace elements, varying geographically around the crater: in the western sector, it has lower values in trace elements, e.g., Ba, Zr, Th, and Ce, than in the eastern sector. The west–east zoning likely reflects the heterogeneous nature of crystalline basement target rocks with lower trace element contents, e.g., Ba, Zr, Th, and Ce, in the west compared to the east. The chemical zoning pattern of suevite melt type 1 indicates that mixing during ejection and emplacement occurred only on a local (hundreds of meters) scale. The incomplete larger scale mixing indicated by the preservation of these local chemical signatures, and schlieren corroborate the assumption that mixing, ejection, and quenching were very rapid, short‐lived processes.

Compositional Heterogeneity of Impact Melt Rocks at the Haughton Impact Structure, Canada: Implications for Planetary Processes and Remote Sensing

“…While clasts are an important tracer, the groundmass is equally or more important, as the melt is entirely contained within the groundmass. We use the same definition of groundmass as Osinski, Spray, and Lee (2005): “the fine‐grained material that encloses fragments of shocked and unshocked target material.” Other studies have found heterogeneities within the groundmass of impact melt rock at the Ries impact structure (Siegert & Hecht, 2018) and in crystalline targets (Marion & Sylvester, 2010), though the nature of the groundmass differs in crystalline targets.…”

“…We use the same definition of groundmass as : "the fine-grained material that encloses fragments of shocked and unshocked target material." Other studies have found heterogeneities within the groundmass of impact melt rock at the Ries impact structure (Siegert & Hecht, 2018) and in crystalline targets (Marion & Sylvester, 2010), though the nature of the groundmass differs in crystalline targets. Table 3 to determine the presence or absence of each lithology, and the displayed values and grayscale intensities shown are the depth of a key absorption feature from each lithology (see Table 2 for formulas) where the given mineral indicator is positive (*BD1760 refers to the maximum of BD1760_narrow_field and BD1760_field, and BD2200_illite refers to the maximum of BD2200_illite and BD2200_illite2).…”

“…There is no hand sample that can be collected with each lithology present in the proportions of the outcrop and thus representative of the outcrop. Siegert and Hecht (2018) aggregated compositions of thousands of clasts from the Ries structure previously analyzed by von Engelhardt (1997) and Schmitt et al (2017) and found compositional heterogeneities in suevite, but the abundances of each lithology by location differ for each of the two previous studies (see Tab. 6 of Siegert & Hecht, 2018).…”

Through the impact glass: Insight into the evolution of melt at the Mistastin Lake impact structure

“…Recently, radial outward flowing impactoclastic density currents have been proposed for the formation of the Ries “suevite” deposits (Siegert et al. ; Siegert and Hecht ). This mechanism is similar to previously proposed models of a turbulent, radially expanding gas cloud (Hörz ) and lateral transport of pyroclastic‐like flows (Newsom et al.…”

“…; Osinski , ; Siegert et al. ; Siegert and Hecht ) making it a suitable candidate for a textural comparison. Given the difference in target lithologies, no chemical comparison was made, as it would not provide additional insight.…”