Shock metamorphic effects in Itokawa phosphates: A comparison with megaregolith‐derived meteorites

Abstract Returned regolith samples from the asteroid Itokawa provide a unique opportunity to compare shock metamorphic effects in unconsolidated regolith materials with those preserved in lithified meteorites, that is, megaregolith. We analyzed four Itokawa particles ( Ueda —RA‐QD02‐0519, Narahara —RA‐QD02‐0573, Domon —RA‐QD02‐0588, Ishiuchi —RX‐MD03‐0212) containing phosphates (merrillite and apatite) to assess their impact history. To place these observations in context, we also describe the associated mineral assemblages (silicates and chromite). While both space weathering effects, irradiation and impact, are present, the primary focus of this study is on impact‐related modifications. We identified microcratering with a density comparable to that measured for Murchison, rare comminution effects in subsurface regolith materials, localized melting and vaporization, and partial decomposition of chromite into a high‐pressure Fe 2 Cr 2 O 5 phase (modified ludwigite‐type). The two apatite crystals analyzed lack any brittle deformation; however, one shows strong submicron‐scale chlorine heterogeneity and porosity that are consistent with partial melting and volatile redistribution. In contrast, the two merrillite grains, identified in two different particles, contain dislocations. Their microstructures indicate distinct shock histories: one particle preserves only limited, localized deformation probably induced by micrometeoroid impacts, whereas the other shows extensive brittle deformation features consistent with a more pervasive shock event. The combination of ductile and brittle deformation, along with melting and comminution, reflects a more intense and spatially extensive shock metamorphic process. Dislocation densities are comparable to those observed in ordinary chondrites (OCs) of shock stage S2 (5–10 GPa). This study shows that phosphates in Itokawa regolith record highly localized and heterogeneous shock metamorphic overprints, in contrast to the more uniform relationship between shock metamorphic stage and phosphate deformation described in megaregolith OCs. Phosphates are sensitive shock metamorphic tracers in asteroidal regolith, but meteorite‐based calibrations must be applied cautiously to unconsolidated materials.