Revising the shock classification of meteorites

Abstract

The current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock-forming minerals such as olivine and plagioclase. The S6 stage (55-90 GPa; 850-1750°C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high-pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high-pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high-pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable highpressure phases. High-pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s À1 ) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.

Shock-Induced Olivine-Ringwoodite Transformation in the Shock Vein of Chondrite GRV053584

“…However, the formation mechanism of maskelynite is still a matter of debate. Many researchers hold that maskelynite is a diaplectic plagioclase glass formed via solid-state transformation [3,[40][41][42][43]. However, Chen et al [44] and El Goresy et al [45] suggested that maskelynite could be formed by melting of plagioclase and is a dense quenched glass.…”

“…According to the latest shock classifications of meteorites by Fritz et al [3], the occurrence of maskelynite and ringwoodite in local melt zones of meteorites needs a shock pressure >15 GPa. But this shock classification can't provide the precision formation pressure of maskelynite.…”

“…Therefore, high-pressure phases within and adjacent to shock veins can only represent shock pressures in these places. The most widely used calibration is firstly established by Stöffler et al [11], then it is revised by Stöffler and Grieve [10] and Fritz et al [3].…”

“…The parent body of meteorites undergoes one or more impact events in space, and the minerals in the meteorites are modified by shock-induced high pressures and high temperatures. Therefore, shock effects seen in these minerals are records of the shock conditions [2,3]. Understanding the processes involved in formation of these shock effects is fundamental for any interpretations of the impact events.…”

“…Because of the hypervelocity impact on the parent body of the meteorite, olivine in the meteorites always has been modified by shock waves and they usually display shock metamorphic features, such as undulatory extinction, mosaicism, planar fractures, planar deformation features, phase transformation, melting, and decomposition [2,[6][7][8][9]. Olivine provides an index for a meteorite shock classification scheme, i.e., shock levels of S1 (unshocked) to S6 (very strongly shocked) [3,10,11]. Wadsleyite and ringwoodite are high pressure phases of olivine.…”