Shock metamorphism of planetary silicate rocks and sediments: Proposal for an updated classification system

TLDR

Shock‑metamorphosed rocks appear as lithic clasts or melt particles mainly in proximal impactites and span a continuum from unshocked to shock‑melted, with the degree of metamorphism depending on mineralogy and porosity, so different planetary rocks and sediments require distinct classification systems. The study reevaluates the systematics and geological setting of terrestrial, lunar, Martian, and asteroidal impactites and proposes a unified, updated classification system for progressive shock metamorphism of silicate rocks and sediments. The authors introduce eight rock and sediment classes—F, M, A, U, C, SR, SE, RE—grouped into crystalline, chondritic, and sedimentary categories, each with progressive shock stages S1 to Sx (up to S7) and associated pressure ranges, providing a standardized framework for shock classification.

Abstract

Abstract We reevaluate 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, we propose 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. They 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. We define 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 GP a) in parentheses.