High-pressure clinopyroxene formation in L6 chondrites (Ozerki, Chug-Chug-011): Implications for impact processes

IntroductionSodic plagioclase is common in Earth’s crust and in many differentiated and undifferentiated meteorites. Under high temperature (HT) and high pressure (HP) conditions in asteroidal collisions, sodic plagioclase may transform into either hollandite-structured lingunite [1] or the recently discovered albitic jadeite [2]. When stoichiometric jadeite forms by decomposition of albite, the excess silica forms an SiO2 polymorph, often stishovite [3]. Albitic jadeite, by contrast, a Na-rich analogue of tissintite [2], is super-silicic, vacancy-rich pyroxene with excess Si coordinated in the octahedral M1 site. Searching for albitic jadeite alongside other P-sensitive mineral assemblages is therefore potentially important for expanding the list of pressure constraints available for impact events.We report preliminary results on the occurrence of albitic jadeite within shock veins in the L6 ordinary chondrites Ozerki and Chug-Chug-011 (Fig. 1). Ozerki (fell 21st June 2018 in Russia) is moderately shocked (S4/5) and un-weathered (W0); it was recovered quickly (25th June 2018) after its fall. Chug-Chug-011 is a find, recovered in 2018 in Antofagasta, Chile; it is weakly shocked (S2), with minor weathering (W1).Materials and MethodsPolished thin sections of Ozerki and Chug-Chug-011 were carefully examined for shock indicators and HP polymorphs, with intensive focus on the melt veins (MVs). We used optical microscopy, a JEOL JSM-IT300LV scanning electron microscope, a JEOL JXA 8900 electron probe micro-analyzer, and a dispersive confocal Renishaw inVia Reflex Raman microscope (514 nm laser).Petrography & mineral chemistryThe thin section of Ozerki displays two discrete areas (Fig. 1A); light-colored chondritic and dark-colored impact melt-rich area. We focused on a network of shock veins intruding the light-colored area. The MVs are dark, variable width (40-850 μm), and locally disrupted by angular to sub-rounded clasts. Clasts are more abundant in wider MVs; jigsaw-fit breccia textures are widespread. Clasts, mostly silicate, concentrate in the center of each MV, whereas the margins are rich in metallic segregations and sulfides.In Chug-Chug-011, three different MVs (~100 μm wide) crosscut the meteorite matrix (Fig. 1B). Elongated silicate clasts oriented parallel to the veins are common in their central domains.In Ozerki, albitic jadeite forms acicular to dendritic crystallites aggregates (≤ 2 μm) associated with feldspathic glass (Fig. 2A). In Chug-Chug-011, albitic jadeite is found within a composite clast: low Ca-pyroxene surrounds sodic plagioclase (Fig. 2B). Crystallites near the core of the plagioclase show brighter backscatter than those near the rim.Albitic jadeite in Ozerki yields an empirical formula (Na0.70Ca0.15K0.05□0.14)(Al0.82Si0.10Fe0.04)Si2O6 whereas that from Chug-Chug-011 is variable: (Na0.57-0.64Ca0.07-0.07K0.03-0.05Mg0.01-0.07□0.16-0.29)(Al0.78-0.86Si0.10-0.18Fe0-0.05Mg0-0.13)Si2O6, with Ca# [100×Ca/(Ca+Na)] from 10 to 13.Pyroxene Raman spectroscopyRaman spectra of the albitic jadeite in Ozerki display five distinct peaks at 376, 526, 698, 986 and 1036 cm-1 (Fig. 3A). In Chug-Chug-011, the predominant peak is at 698 cm-1, but there is a noteworthy 1016 cm-1 peak in addition to the “typical jadeite” 1038 peak. This may be associated either with a diopside-related structure or another high-P clinopyroxene (Fig. 3B).Discussion and ConclusionsIn Ozerki, albitic jadeite was found in the middle of ~70 μm and ~300 μm wide MVs. The presence of equant idiomorphic crystals with 120° triple junctions suggests that these MVs reached peak HT above the liquidus of the matrix. From such conditions, a ~300 μm wide vein surrounded by cold matrix conductively cools and solidifies in ~6.5 ms, which is an upper limit for growth time of minerals in the MV. Albitic jadeite is less dense than lingunite, implying formation from sodic plagioclase at lower pressures. The absence of lingunite suggests maximum pressures below 21 GPa. According to experiments [4] in jadeite-rich compositions (Jd70-80), jadeite + stishovite + garnet is stable at 13.5-21.5 GPa. However, the absence of stishovite and garnet in our MV may only reflect sluggish nucleation of these phases rather than an insufficient peak P