Shock Induced Feldspar and Silica Transformation in Polymict Eucrite

Introduction: Howardite-eucrite-diogenite (HED) meteorites belong to the largest group of achondrites and originate from protoplanet 4 Vesta. The surface of Vesta has been extensively affected by impacts which brecciated and mixed eucrite and diogenite material to produce a regolith and formed large basins in the south pole region [1]. Shock features preserved in meteorites due to high-velocity impacts include deformation and fracturing, local to complete melting, and high-pressure minerals. These effects can yield constraints on the pressure-temperature-time history of impact on the parent body. Previous studies have reported variable shock features preserved in eucrites, including brecciation, mechanical twinning, transformation of feldspar to maskelynite or glass, and the local formation of melt pockets or veins cross-cutting the stone [2]. Coesite and stishovite, high-pressure polymorphs of silica, have been reported to coexist with silica glass in Béréba [3] and NWA 8003 [4]. These authors estimated a shock pressure of ~8-13 GPa and ~8-10 GPa, respectively. The purpose of this study is to explore shock metamorphic features preserved in NWA 10658 and to constrain the pressure-temperature impact conditions recorded in this sample. Sample and Methods: Regions of interest were identified in a thin section of NWA 10658 with a petrographic microscope. Back-scattered electron (BSE) images of transformation textures and mineral chemistry were acquired with a JXA-8530F electron microprobe. Wavelength-dispersive spectroscopy (WDS) analyses were performed with a 15 kV, 15 nA beam. The beam was defocused to 5-10 μm on alkali-bearing phases. High-pressure minerals were identified with Raman spectroscopy and electron backscatter diffraction (EBSD) analysis. Results: NWA 10658 is a polymict breccia composed of two populations of ≤ 4 mm clasts embedded in a fine-grained matrix. The predominant clast assemblage consists of orthopyroxenes up to 3 mm (Fs2948Wo2-4) with >10 μm exsolution lamellae of augite (Fs10-18Wo43-45) intergrown with plagioclase (Ab712An91-92Or0.1-0.6). The other clast assemblage is texturally subophitic and consists of 400 μm intergrowths of pigeonite (Fs59Wo5-6), augite (Fs28-29Wo4-42) and plagioclase (Ab12-16An82-87Or0.7-2.6) [5]. The texture of the brecciated matrix exhibits evidence of brittle-ductile deformation and local melting, including crushed and compacted fragments. Some regions have been sheared, producing glassy zones. Shock-induced melt pockets approach 700 μm and contain rounded troilite droplets and silicate fragments. Preliminary Raman data of the melt assemblage suggests the presence of pyroxene and garnet. Radial expansion cracks surround plagioclase and SiO2 fragments, crosscutting the melt and propagating into the surrounding host rock. Transformed SiO2 grains contain high contrast domains with polycrystalline texture in a glassy matrix. Raman spectra collected from the polycrystalline and homogenous regions of SiO2 confirm the presence of coesite in silica glass. Feldspars up to 4 mm have been partially to completely converted to glass. Crystalline remnants in partially maskelynitized grains exhibit mosaicism and/or planar deformation features. Plagioclases entrained in the shock-induced melt have been transformed to tissinite, the high-pressure polymorph of clinopyroxene [6]. In addition, some plagioclase has been pseudomorphically replaced by a material exhibiting a dendritic texture in BSE images (Figure 1). Raman spectra collected from this dendritic material are consistent with grossular garnet (Figure 1, inset).