Petrography, mineralogy, and transformation texture of high-pressure phases in Bori chondrite: Constraints on classification, shock condition, and peak metamorphic temperature

Abstract We present a detailed petrographic, mineralogical and spectroscopic investigation of the Bori meteorite, which is classified as an L6 ordinary chondrite, to constrain its shock stage, pressure-temperature conditions, formation mechanism of maskelynite and peak metamorphic temperature. The host matrix consists of olivine (Fa25) and low-Ca pyroxene (En78Fs21Wo1), consistent with the compositional range of L-group chondrites and shows a recrystallized chondritic texture. Other phases include plagioclase glass (An18-32), Fe-Ni metal, troilite, chromite, apatite and merrillite. A shock-melt vein (<280 μm) and several melt pockets (<200 μm) contain entrained host matrix fragments and quenched high-pressure mineral assemblages, including wadsleyite, ringwoodite, majorite, majorite-pyrope solid solution, akimotoite, albitic jadeite, lingunite, and tuite. These assemblages constrain the peak shock pressure and temperature to ∼18-24 GPa and >2000 oC, respectively, while the presence of albitic jadeite in SMV and MPs indicates localized pressure conditions of ∼12 GPa. The pervasive transformation of plagioclase to maskelynite throughout the host matrix indicates strong, heterogeneous shock effects, with localized pressures reaching ∼30 GPa. Orthopyroxene-clinopyroxene Fe-Mg exchange thermometer yields an equilibration temperature of ∼855 oC, indicating thermal equilibration during parent-body metamorphism prior to shock. Bori meteorite experienced bulk shock stage of S5 followed by rapid cooling that preserved the metastable high-pressure assemblages, with localized regions approach transitional S6 conditions.