Shock‐induced microtextures in lunar apatite and merrillite

Abstract

Apatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock‐loading. In this work, we analyzed a set of well‐studied lunar highlands samples (Apollo 17 Mg‐suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock‐pressure barometry using optical microscopy, Raman spectroscopy, and SEM‐based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM‐CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock‐loading. An early shock‐stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal‐plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal‐plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock‐loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages.

Apatite from NWA 10153 and NWA 10645—The Key to Deciphering Magmatic and Fluid Evolution History in Nakhlites

“…Similar crystal-plastic deformations were reported from relatively low-shocked Martian shergottites and lunar highlands samples (stages S2-S3 in the six-stages scale) [62,92,93]. Moreover, the Raman spectra collected from lunar apatites indicates that the υ 1 band weakened during the progressive increase of shock degree and disappearance of the υ 2 and υ 4 bands in stage S5 [94]. In the ACP spectrum (Figure 8), both bands are visible, which may indicate a stage lower than S5.…”

“…Moreover, the presence of clearly distinguishable Raman bands at~285,~486, and~506 cm −1 in different plagioclase clusters in NWA 10645 in comparison to the literature data [94] clearly indicate the S1-S4 shock stage. All signs indicate a lack of significant structural disorder and low-shock effects, which are most likely not sufficiently strong to cause recrystallization that was reported in high-shock stages (S5-S6) [62,94] and possibly not sufficiently powerful to cause the amorphization of apatite in either meteorite.…”

“…Shock-induced microtextures were observed not only in apatite but also in plagioclase. The lack of twinned plagioclase crystals in lunar apatite is related to the S1-stage shock [94]. Moreover, the presence of clearly distinguishable Raman bands at~285,~486, and~506 cm −1 in different plagioclase clusters in NWA 10645 in comparison to the literature data [94] clearly indicate the S1-S4 shock stage.…”

High‐resolution microstructural and compositional analyses of shock deformed apatite from the peak ring of the Chicxulub impact crater

“…2019; Černok et al. 2019). PFs consist of multiple sets of parallel, planar features that cut across the host apatite grain and are typically spaced 5–10 µm apart (Cavosie and Centeno 2014; Li and Hsu 2018; McGregor et al.…”

“…2018), crystal–plastic deformation (Černok et al. 2019; Kenny et al. 2020), and cataclastically deformed zones (Birski et al.…”