Shock-induced microtextures in lunar apatite and merrillite

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. Despite the fact that 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 analysed 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 sub-grain formation from S2 onwards, showing consistent decrease in sub-grain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2-3° of intra-grain 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.