Abstract
AbstractThe 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 phosphorus‐bearing 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 strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich 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 P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III 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.
Highlights
Phosphorus in Meteorites: Knowns and UnknownsMeteorites provide direct samples of some of the most primitive solid materials found in the solar system, yielding insights into early disk processes including chemical partitioning in the protoplanetary disk, the assembly of dust into planets, and subsequent dynamical evolution (e.g., Scott 2007)
Phosphate grain boundaries truncate against other phases sharply except in the case of olivine, where contacts are often irregular and complex/embayed (Fig. 2A)
Our observations suggest that plagioclase in Chelyabinsk was extensively mobilized under S4 conditions, in the light lithology, but that this metal/sulfide mobilization was increasingly extensive under the S5 conditions of the dark lithology
Summary
Phosphorus in Meteorites: Knowns and UnknownsMeteorites provide direct samples of some of the most primitive solid materials found in the solar system, yielding insights into early disk processes including chemical partitioning in the protoplanetary disk, the assembly of dust into planets, and subsequent dynamical evolution (e.g., Scott 2007). A large body of evidence shows that impact events continued to disturb these objects beyond the end of thermal metamorphism, resetting a number of mineral geochronometers in the process (e.g., Wittmann et al 2010). Both radiogenic and impact-induced heating are thought to mobilize fluids within chondrites (Lewis and Jones 2016; Zhang et al 2016), resulting in redistribution of volatile components and growth of secondary phases. Volatile-bearing phases that retain such information include halides (Jones et al 2016), sulfides (Visser et al 2019), and phosphates (Jones et al 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
