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

Stöffler D., Hamann C., and Metzler K.

Abstract

We reevaluate the systematics and geologic setting of terrestrial, lunar, Martian,and asteroidal "impactites" resulting from single or multiple impacts. For impactites derivedfrom silicate rocks and sediments, we propose a unified and updated system of progressiveshock metamorphism. "Shock-metamorphosed rocks" occur as lithic clasts or melt particlesin proximal impactites at impact craters, and rarely in distal impactites. They represent awide range of metamorphism, typically ranging from unshocked to shock melted. As thedegree of shock metamorphism, at a given shock pressure, depends primarily on themineralogical composition and the porosity of a rock or sediment sample, different shockclassification systems are required for different types of planetary rocks and sediments. Wedefine shock classification systems for eight rock and sediment classes which are assigned tothree 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 progressiveshock stages are introduced as: S1-S7 for F, S1-S7 for M, S1-S6 for A, S1-S7 for U, S1-S7for 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 ofsymbols characterizing the degree of shock metamorphism of a sample, i.e., F-S1 to F-S7with the option to add the tabulated pressure ranges (in GPa) in parentheses.