Proposal for a Unified Classification System of Shock Metamorphosed Planetary Silicate Rocks – Call for Comments

Introduction: Shocked rocks are known from various planetary bodies of the solar system including Earth, Moon, Mars, and several types of asteroids. It was first recognized on Earth that stages of progressive shock metamorphism can be defined [1,2,3,4] and this concept was later on confirmed for lunar rocks [5] and meteorites [6,7,8]. Initiated by the IUGS “Subcommission on the Classification of Metamorphic Rocks” these classification systems were updated in a chapter on “Impactites” [9] which is part of a book on metamorphic rocks [10]. Problems with the present classification system of shocked rocks: The presently used systematics for shock metamorphosed planetary silicate rocks suffers from the fact that separate classifications are used for terrestrial, lunar, Martian, and asteroidal rocks although many essential rock types such as plutonic and volcanic rocks are common on all four types of planetary bodies. This leads often to unnecessary and confusing duplications for rock types present on more than one planetary body such as basalts, gabbros, dunites or anorthosites. Therefore a reorganisation of the presently existing classification systems of shocked silicate rocks is recommended. The reorganised systematics of shocked rocks should be based exclusively on rock types and the critical shock effects of their major mineral constituents independently of their source planets or planetoids. Basic principles for the definition of a specific stage of shock metamorphism: At least one or two major mineral constituents diagnostic of specific ranges of peak shock pressure should be present in any sample, e.g. quartz for sandstone or quartz and plagioclase for granite. Only well-defined, experimentally calibrated shock effects to be recognized in a polarizing microscope should be used for the shock classification systems in order to provide a relatively easy and fast mode to determine the degree or stage of progressive shock metamorphism of a rock. Shock effects which can only be detected by more sophisticated techniques such as X-ray diffraction or Raman spectroscopy should not be used. Consequently, the polarizing microscope is the main tool for shock classification of a rock or sediment although it may have to be supported by electron microprobe techniques (SEM and EMPA) for a safe identification of a mineral phase in question that exhibits shock effects. Identification of major classes of rocks and definition of 7 different shock classification systems: The rock classes for which different shock classifications will be defined, can be summarized in three major groups: (I) Crystalline rocks comprising igneous rocks, metamorphic rocks, and crystalline impact melt rocks, (II) Accretionary rocks (chondrites), and (III) Sedimentary deposits. Group (I) comprises 3 different classification systems: Felsic (quartzo-feldspathic) (F), Mafic (M), and Ultramafic rocks (U). Group (II) is defined by one system (C) and Group (III) by 3 different systems: Sedimentary rocks (SR), clastic sediments (S), and regoliths (R). Adapting the widely accepted denomination system for progressive stages of shock metamorphism of chondrites [7] it is proposed to use the abbreviations S1, S2, S3, S4 etc. for the stages of increasing shock and to add in parenthesis the rock class, e.g. S3 (F) for a felsic crystalline rock. The previously used shock stage 0 [9] for the stage of lowest shock should be dismissed and denominated stage S1. The classification system is summarized in the following Table: Rock type Crystalline rocks Accretionary rocks Sedimentary deposits (rocks and unconsolidated sediments) Classification system Felsic Mafic Ultramafic Chondrites Sedimentary rocks, quartz-bearing Sediments, quartz-bearing Regoliths