The Chelyabinsk meteorite: Thermal history and variable shock effects recorded by the ⁴⁰Ar‐³⁹Ar system

Abstract

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the 40 Ar-39 Ar dating technique, we identified low-and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite-feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable 40 Ar/ 36 Ar ratios even within lowand high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event~1.7 AE 0.1 Ga ago formed the light-dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with 40 Ar/ 36 Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.

The lunar Dhofar 1436 meteorite: 40Ar‐39Ar chronology and volatiles, revealed by stepwise combustion and crushing methods

“…In contrast, shocked chondrites release K-derived 39 Ar at much higher temperatures of >1100°C, often as high as 1400°C (Bogard and Hirsch 1980;Kunz et al 1997;Trieloff et al 2018), most likely due to shockinduced phases (shock glass, jadeite, etc.). This increase of the release temperature of K-bearing phases is also observed for the more calcic plagioclase of shergottites (Korochantseva et al 2009), where plagioclase (maskelynite) separates were found to release both Kderived 39 Ar and Ca-derived 37 Ar at 1300-1400°C, indistinguishable from the 37 Ar release from pyroxene.…”

“…Note that also the majority of the Ne and Ar is released at this temperature. High temperature degassing of minerals is obviously the consequence of shock metamorphism (e.g., Trieloff et al 2018). A small release peak of carbon is also observed at this temperature (Fig.…”

“…Such values must contain a redistributed in situ radiogenic 40 Ar component, as these values are far above reasonable solar wind mixtures of 36 Ar and 40 Ar captured from the lunar atmosphere. Such trapped argon components are also known from non-lunar meteorites that originated from Mars, Vesta, and other asteroids (Korochantseva et al 2017;Trieloff et al 2018).…”

Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions

“…The extensive troilite melting, however, requires pressures within the shock stage C-S5 (Moreau et al 2019). The "impact melt" lithology appears either as melt pockets or shock veins, dominating fragments of light-colored lithology, or as a more massive body containing floating light-colored and darkcolored clasts (Trieloff et al 2018). Its shock stage corresponds to C-S6 (veins, pockets-dominated material) or C-S7 (massive melt body).…”

A structural insight into the Chelyabinsk meteorite: neutron diffraction, tomography and Raman spectroscopy study

“…After that, the meteorite body destructed into thousands of small meteorite fragments and was accompanied by the spread of shock waves. The collision explosion was so powerful that the shock wave may twice round the Earth [4,6,7]. The particular attention should be paid to the fact that this meteorite collision led to the great destruction of buildings and injuries to thousands of people by broken glass mostly [7,8].…”