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

Introduction

Sodic 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 21^st June 2018 in Russia) is moderately shocked (S4/5) and un-weathered (W0); it was recovered quickly (25^th 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 Methods

Polished 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 chemistry

The 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 (Na_0.70Ca_0.15K_0.05□_0.14)(Al_0.82Si_0.10Fe_0.04)Si₂O₆ whereas that from Chug-Chug-011 is variable: (Na_0.57-0.64Ca_0.07-0.07K_0.03-0.05Mg_0.01-0.07□_0.16-0.29)(Al_0.78-0.86Si_0.10-0.18Fe_0-0.05Mg_0-0.13)Si₂O₆, with Ca# [100×Ca/(Ca+Na)] from 10 to 13.

Pyroxene Raman spectroscopy

Raman 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 Conclusions

In 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 (Jd_70-80), 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<13.5 GPa [5]. The presence of albitic jadeite, by itself, therefore yields only an upper limit and not a fully quantitative P constraint.

In Chug-Chug-011, high-pressure Na-clinopyroxene [(Na_0.49Ca_0.15K_0.03Mg_0.24□_0.09)(Al_0.62Si_0.04Fe_0.13Mg_0.21)Si₂O_6] is enclosed in a melt pocket included in pyroxene that is in turn entrained in a MV. The bright crystallites near the center of the pocket yield compositions and spectra similar to the HP-sodic clinopyroxene identified by [6]. The backscatter-dark crystallites closer to the pocket margins better match albitic jadeite. Neither phase is yet calibrated for shock pressure. However, the presence of a mixed xieite-chromite spectrum at the rim of another MV in the section suggests higher P conditions, 18-23 GPa (Fig. 3C). The same MV shows minor wadsleyite peaks near its center, requiring gradients over space or time in preserved P and T conditions across the MV.

Acknowledgements

This research received support from European Social Funds and the Greek State (call code EDBM103).

References

[1] Gillet, P., et al. 2000. Science, 287(5458), 1633-1636; [2] Ma, C., et al. 2020. 51^st LPSC, #1712; [3] Liu, L.G. 1978.EPSL, 37(3), 438-444; [4] Bobrov, A.V. et al. 2008. GCA, 72, 2392-2408, 2008; [5] Kubo, T., et al. 2009. Nature Geoscience, 3, 41-45, 2009; [6] Baziotis, I., et al. 2018. Scientific Reports, 88, 9851, 2018.

Fig. 1: Transmitted-light mosaics of (A) Ozerki and (B) Chug-Chug-011; rectangles indicate the areas hosting HP polymorphs (Figs. 2, 3).

 

Fig. 2: A) BSE image of MV in Ozerki showing albitic jadeite crystals (spectra #31 and #51 in Fig. 3A) in a partly crystallized melt area. B) BSE image of MV in Chug-Chug-011 with albitic jadeite (spectra in Fig. 3B). Bright core near C3 may be HP sodic clinopyroxene (see text).

 

Fig. 3: A) Ozerki Raman spectra: typical jadeite peaks at ~698, 986, and ~1036 cm^–1 in spectra #3_31, #3_51. Spectrum #5_11 shows the 698 cm^-1 peak but the two higher wavenumber peaks are not clearly resolved. B) Chug-Chug-011 Raman data: jadeite peak at 698 cm^-1 is apparent. The peak at 960 cm^-1 in spectrum C12 is apatite. C) Chug-Chug-011 MVA spectra: rim point MVA3_91 is mixture of chromite and xieite with olivine. Center point MVA1_151 shows wadsleyite peaks at 720 and 915 cm^-1.