Stoffler and two co-authors reported in 2017 that they reevaluated the systematics and geologic setting of terrestrial, lunar, Martian, and asteroidal “impactites” resulting from single or multiple impacts. For impactites derived from silicate rocks and sediments, they proposed a unified and updated system of progressive shock metamorphism. “Shock-metamorphosed rocks” occur as lithic clasts or melt particles in proximal impactites at impact craters, and rarely in distal impactites. These rocks represent a wide range of metamorphism, typically ranging from unshocked to shock melted. As the degree of shock metamorphism, at a given shock pressure, depends primarily on the mineralogical composition and the porosity of a rock or sediment sample, different shock classification systems are required for different types of planetary rocks and sediments.
The authors defined shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments:
- 1) crystalline rocks with classes F, M, A, and U;
- 2) chondritic rocks (class C); and
- 3) sedimentary rocks and sediments with classes SR, SE, and RE.
The abbreviations stand for felsic (F), mafic (M), anorthositic (A), ultramafic (U), sedimentary rocks (SR), unconsolidated sediments (SE), and regoliths (RE). In each class, the progressive stages of shock metamorphism are denominated S1 to Sx. These progressive shock stages are introduced as: S1–S7 for F, S1–S7 for M, S1–S6 for A, S1–S7 for U, S1–S7 for C, S1–S7 for SR, S1–S5 for SE, and S1–S6 for RE. S1 stands for “unshocked” and Sx (variable between S5 and S7) stands for “whole rock melting.” We propose a sequence of symbols characterizing the degree of shock metamorphism of a sample, i.e., F-S1 to F-S7 with the option to add the tabulated pressure ranges (in GPa) in parentheses.
The authors concluded that in addition to the shock effects described in this article, some other important properties of natural rocks or sediments are affected by shock metamorphism. Besides the microscopically recognizable effects such as mechanical deformations, phase transformations, decomposition into new phases, and melting, essential chemical properties of the sample will be changed. The potential changes are:
- loss of volatile elements,
- change of the oxidation state of elements (e.g.,Fe2+/Fe3+),
- change of the isotope ratios of trace elements, and
- change of the primary isotope age.
These effects lead to a clearly defined change of the chemical state for samples of the highest stage of shock metamorphism, i.e., by melting. Consequently, the isotopically determined age of impact melts quenched to glass or crystallized impact melt rocks represents the age of the impact event by which the melt was formed.
The isotope systems of samples of intermediate stages of shock metamorphism are also affected and their ages are partially reset. These and other chemical effects have been studied in some detail on naturally and experimentally shocked silicate minerals and rocks (e.g., Jessberger and Ostertag 1982; Staudacher et al. 1982; Ostertag et al. 1984; Bogard et al. 1987, 1988; Deutsch and Sch€arer 1994; Schwenzer et al. 2008).
The general consequence of the described effects is that for any sample, especially for lunar rocks and for all types of meteorites, a clear identification of:
- the type of impactite (shocked rock, monomict breccia, polymict breccia, and impact melt rock) and
- the degree of shock metamorphism is mandatory before any detailed chemical analyses are made, and far-reaching conclusions in terms of the genetic origin of a sample are drawn in a cosmochemical or planetary context.
[The authors provided an update and addendum of corrections and additional information regarding the above cited publication. It addresses the following two topics:
1) Clarification for a correct application of the criteria for certain shock stages of chondrites, in particular stage C‐S6, and
2) Correction of a printing error in the table that contains the shock classification system of chondrites.
https://www.i2mconsultants.com/downloads/corp/Stffler_et_al_Addendum_Stffleretal_2018-2019.pdf ]
Stffler_et_al_Addendum_Stffleretal_2018-2019.pdf
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