Systematic investigations of high‐pressure polymorphs in shocked ordinary chondrites

Abstract

Shock-induced melting textures and high-pressure polymorphs in 178 ordinary chondrites of all chemical groups and petrologic types were investigated. The shock-induced melting modes were classified into three types, namely pocket, line, and network. The type of shock-induced melting depends on the petrologic type. The width of the shock-induced melt increased with increasing the petrologic type number. The approximate estimated shock-pressure ranges recorded in and around the shock-induced melts of the H-group ordinary chondrites based on the identified high-pressure polymorphs were as follows: H3, less than 2 GPa; H4-H6, 2-6 GPa. For ordinary chondrites of the L/LL group, the values were as follows: L/LL3, 2-6 GPa; L/LL4, 2-14 GPa; L5: 14-20 GPa; LL5, 2-14 GPa; L6, 17-23 GPa; and LL6, 14-18 GPa. After adopting the estimated shock pressures into the onion shell-structured parent body model, the shock pressure on the surface was much lower than in the interior. One possibility is that the apparent lower shock pressure on the surface is due to spallation during the impact. Considering the features of the high-pressure polymorphs, the major disruption history of the parent bodies is different in each chemical group, although the L/LL chondrite parent bodies may have a similar major disruption history.

Apollo 15 regolith breccia provides first natural evidence for olivine incongruent melting

“…In ordinary chondrites, wadsleyite and ringwoodite aggregates have been typically found within and/or in the surrounding of the shock features (Chen et al 2004;Yin et al 2018;Miyahara et al 2020), and ringwoodite has been also found in association with ferropericlase (Chen et al 1996).…”

Chondrule Flattening by Shock Recovery Experiments on Unequilibrated Chondrites

“…Typical whole-area BSE images of investigated H/L/LL3 ordinary chondrites are shown in Figure S10. The modes of melting are shown in Tables S2-S4 and the definition of melting followed Miyahara et al (2020), that is, pocket, line, and network. The textures and constituents of shock-induced melts were similar to those of shocked ALH-78084 H3 and Y-793375 L3 chondrites (Figures 5a and 5b).…”

“…Miyahara et al. (2020) conducted systematic investigations of shock‐induced melts and high‐pressure minerals in ordinary chondrites and reported that some petrologic type 3 ordinary chondrites have shock‐induced melts and high‐pressure minerals such as coesite and Na‐pyroxene. Also, some petrologic type 3 ordinary chondrites with shock‐induced melts include flattened chondrules.…”

“…The average size of chondrules in H3 chondrites differs from those of L/LL3 chondrites: the mean diameters of chondrules in H3 and L/LL3 are respectively about 450 and 500–550 μm (Friedrich et al., 2015). The flattening degree of chondrules may depend on the size distribution of chondrules because L/LL chondrites with melts seem to be abundant in flattened chondrules (Miyahara et al., 2020). There was little information about shock metamorphism recorded in the outermost layers (composed of type 3 chondrites) of asteroids.…”

“…By contrast, few works report on shock features recorded in petrologic type 3 and 4 ordinary chondrites derived from outside the parent body (Ruzicka, Grossman, et al, 2015;Ruzicka, Huston, et al, 2015), because, in general, the shock features in these chondrites are not distinct from petrologic type 5 and 6 ordinary chondrites. Miyahara et al (2020) conducted systematic investigations of shock-induced melts and high-pressure minerals in ordinary chondrites and reported that some petrologic type 3 ordinary chondrites have shock-induced melts and high-pressure minerals such as coesite and Na-pyroxene. Also, some petrologic type 3 ordinary chondrites with shock-induced melts include flattened chondrules.…”