1 research outputs found
Microtextures in the Chelyabinsk impact breccia reveal the history of PhosphorusâOlivineâAssemblages in chondrites
Funder: European Unionâfunded Integrated ActivitiesFunder: SYNTHESYSAbstract: The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorusâbearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strainâfree merrillite populations. Grains with strainâfree subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shockâmetamorphic features. Complete melting of portions of the meteorite is recorded by the shockâmelt lithology, which contains a population of phosphorusârich olivine grains. The response of phosphorusâbearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for Pâbearing phases in Chelyabinsk involving initial phosphate growth via Pârich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains Pâenriched olivine. We generalize our findings to propose a new classification scheme for PhosphorusâOlivineâAssemblages (Type IâIII POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids