Shock metamorphism of minerals
F. Langenhorst, Bayerisches Geoinstitut, Universität Bayreuth,
D-95440 Bayreuth, Germany; Falko.Langenhorst@uni-bayreuth.de
Shock metamorphism is a fundamental and common process in our Solar System (Deutsch and Langenhorst 1998). It is caused by the passage of strong shock waves, which occur exclusively in the context of natural impact events, nuclear or chemical explosions, and laboratory-scale shock experiments. A shock wave is a degenerated, short-living compression wave, which is accompanied by high pressures and temperatures and a material transport behind the shock front. With regard to the uncompressed material it propagates with supersonic velocity. At the scale of rocks, natural impacts and the resulting shock waves cause the formation of breccias and large impact melt volumes. At the scale minerals, a great diversity of natural shock effects is known to occur. They have been reproduced, to a large extent, in laboratory experiments, as well.
Minerals either undergo shock damage and transformation in solid-state or can newly form from melt and vapour (Langenhorst and Deutsch 1999). The latter category of minerals is, in a strict sense, not shocked, i.e., the minerals do not contain defects caused by the shock wave. The formation of secondary minerals such as high-pressure phases is, however, often unique and indicative of impact, and therefore represents a clear sign of shock metamorphism. Both the shocked primary minerals and the newly formed (high-pressure) phases are not only unequivocal indicators of impact events but also serve for shock barometry and thermometry.
A list of shock-metamorphic features is provided in Tab. 1. The physical nature and occurrence of the submicroscopic shock effects has been recognised only during the last decade using transmission electron microscopy.
Shock deformation results in the formation of one (dislocations)- to three (mosaicism)-dimensional lattice defects. Shock-induced activation of dislocations is mainly observed in silicate minerals in which SiO4-tetrahedra are not three-dimensionally linked. The chain and island silicates clinopyroxene and olivine, respectively, develop dislocation densities of up to 1014 m—2 , whereas shocked quartz is essentially free of dislocations. On the other hand, quartz reacts under shock compression by the formation of planar fractures (PF) and planar deformation features (PDF). PFs can basically be regarded as cleavage planes that are preferentially or only activated under high-pressures. PDFs are thin (< 200-300 nm) amorphous lamellae with the same composition as the host crystal; their orientation is crystallographically controlled and shows a pressure-dependent variation. The formation of PDFs is basically restricted to framework silicates including quartz. Mechanical twinning results from activation and subsequent high-velocity migration of partial dislocations. Clinopyroxene and calcite are instructive examples for this type of shock deformation behaviour. Kink bands are closely related to mechanical twins and are observed in minerals with only one slip plane. This is the case for sheet structures such as micas. Mosaicism can be regarded as internal fragmentation of crystals reflected in a mottled extinction behaviour under a polarising microscope and asterism in X-ray diffraction patterns.
In the context of impact events, high-pressure minerals form either by solid-state transformation or by crystallisation from high-pressure melt. The formation of diamond is the only clear case of solid-state transformation. The diamonds result from martensitic-like transformation of graphite and, as a consequence, inherit morphological and internal characteristics of the parent graphite. Crystallisation of high-pressure minerals is observed in shock-fused glasses or pseudotachylites within strongly shocked meteorites or basement rocks in impact craters. Polycrystalline coesite aggregates occur in shock-fused silica glass, e.g., at the Ries and Popigai craters. Stishovite has been identified in thin pseudotachylite veins in shocked basement rocks of the Vredefort structure, South Africa. Strongly shocked L6 ordinary chondrites are carriers of high-pressure minerals that are otherwise only known to occur in Earth´s transition zone and Lower mantle. These include wadsleyite, ringwoodite, majorite, hollandite, silicate ilmenite, and silicate perovskite.
The second type of transformation is a crystalline-amorphous transition. The amorphous phase is termed diaplectic glass or, in case of feldspar, maskelynite. Diaplectic glass retains the shape and internal features of the precursor crystal, and is densified compared to synthetic glass with the same composition. As for PDFs, diaplectic glasses are restricted to framework silicates. According to recent models, diaplectic glasses are interpreted as quenched high-pressure melts.
Shock-induced decomposition of volatile-bearing minerals, e.g., carbonates, sulfates, and hydrous silicates, is considered as critical factor for the evolution of atmosphere, climate, and life. For example, large amounts of CO2 and SO2 may have been released by the Chicxulub impact event, which occurred 65 Mio years ago in an anhydrite-rich carbonate target. This sudden perturbation of the atmosphere may, in part, be responsible for the K/T mass extinction. Recent experiments on carbonates indicate however that the shock-induced release of CO2 was previously overestimated by orders of magnitude because the main response of carbonates to shock compression is melting rather than decomposition. In general, melting and vaporisation represent the highest degree of shock metamorphism.
References
Deutsch and Langenhorst (1998) Mineralogy of astroblemes — terrestrial impact craters. In Marfunin (ed.): Advanced Mineralogy 3, Springer-Verlag, Berlin, 76-95.
Langenhorst and Deutsch (1998) Minerals in terrestrial impact structures and their characteristic features. In Marfunin (ed.): Advanced Mineralogy 3, Springer-Verlag, Berlin, 95-119.
Tab. 1 Shock-metamorphic features in minerals (modified after Langenhorst and Deutsch 1998)
Characteristic shock-metamorphic features ----- Short description
by a point symmetry element (mirror,
rotation, or inversion axis)
crystallographic relationship to the host lattice
(and gaseous species)
vapor