Transformation textures, mechanisms of formation of high‐pressure minerals in shock melt veins of L6 chondrites, and pressure‐temperature conditions of the shock events

Abstract

Abstract-The high-pressure polymorphs of olivine, pyroxene, and plagioclase in or adjacent to shock melt veins (SMVs) in two L6 chondrites (Sahara 98222 and Yamato 74445) were investigated to clarify the related transformation mechanisms and to estimate the pressure-temperature conditions of the shock events. Wadsleyite and jadeite were identified in Sahara 98222. Wadsleyite, ringwoodite, majorite, akimotoite, jadeite, and lingunite (NaAlSi 3 O 8 -hollandite) were identified in Yamato 74445. Wadsleyite nucleated along the grain boundaries and fractures of original olivine. The nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine, and as intracrystalline ringwoodite lamellae within original olivine. The nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite. Jadeite-containing assemblages have complicated textures containing "particle-like," "stringer-like," and "polycrystalline-like" phases. Coexistence of lingunite and jadeite-containing assemblages shows a vein-like texture. We discuss these transformation mechanisms based on our textural observations and chemical composition analyses. The shock pressure and temperature conditions in the SMVs of these meteorites were also estimated based on the mineral assemblages in the SMVs and in comparison with static high-pressure experimental results as follows: 13-16 GPa, >1900 °C for Sahara 98222 and 17-24 GPa, >2100 °C for Yamato 74445.

Natural dissociation of olivine to (Mg,Fe)SiO 3 perovskite and magnesiowüstite in a shocked Martian meteorite

“…The chemical composition of the poorly crystallized material is identical to ðMg;FeÞSiO 3 . Although clinopyroxene, majorite, and akimotoite are candidates for such a composition, they occur in shocked meteorites usually in the crystalline state (4,14,22). Hence, the glass with the ðMg;FeÞSiO 3 composition is unlikely any of the three mentioned above.…”

Jadeite in shocked meteorites and its textural variations

“…Jadeite crystallite assemblages contain many 'particle-like' materials as shown in Figure 3B. Ozawa et al (2009) described that the 'particle-like' material was silica-rich amorphous material, and proposed that original albite feldspar transformed to jadeite and the amorphous material. Similar jadeite crystallite assemblages were also observed in the albite feldspar (Ab 83-85 An 10 Or 5-7 ) grains entrained in the shock-melt veins of Sahara 98222 L6 ordinary chondrite .…”

“…Their typical textures are shown in Figure 4. Ozawa et al (2009) suggested that the particle-like and stringer-like materials may be composed of silica-rich amorphous materials. Further detail investigation on jadeite in shocked ordinary chondrites was conducted using focused ion beam (FIB)-assisted TEM and XRD patterns together with FE-SEM and Raman spectroscopy.…”

“…Identification of jadeite in albite feldspar grains using a laser micro-Raman spectroscopy has been reported in many shocked H, L, and LL ordinary chondrites (e.g., Ohtani et al, 2004;Ozawa et al, 2009;Miyahara et al, 2013). Detail investigations of jadeite in shocked ordinary chondrites were conducted by Ozawa et al (2009) using a high-resolution field-emission (FE)-SEM and laser micro-Raman spectroscopy after the first discovery of jadeite in Y-75100 H6 ordinary chondrite by Kimura et al (2000). Figure 3A shows a lamellar texture composed of jadeite and lingunite assemblage observed in the shockmelt veins of Y-74445 L6 ordinary chondrite .…”

“…On the other hand, many L-type chondrites and some H-type chondrites (Sahara 98222 L6, Y-74445 L6, Y-791384 L6, and Y-75100 H6) show clear evidence for the solid-state transformation of jadeite. These ordinary chondrites include other high-pressure polymorphs such as ringwoodite, wadsleyite, majorite and akimotoite together with jadeite (Ohtani et al, 2004;Ozawa et al, 2009;Miyahara et al, 2010;Fudge et al 2015). The parent-body sizes of these chondrites estimated by the transformation kinetics are also small (~10 km) although the impact velocity (~2.0 km/s) (Ohtani et al, 2004) is larger than that recorded in LL-and H-type chondrites described above.…”