Thermal metamorphism of metal assemblage in the light lithology of the Chelyabinsk LL5 meteorite

Abstract

Research subject. Metal assemblage in the light lithology of the Chelyabinsk ordinary chondrite (LL5). Aim. The study of the structure of kamacite (α-Fe(Ni, Co)) and taenite (γ-Fe(Ni, Co)), as well as identification of the morphological features resulting from heating of chondrite matter. Materials and methods. Samples from the unaltered light lithology of the Chelyabinsk meteorite. The chemical composition of the samples was studied using optical microscopy (Zeiss Axiovert 40 MAT) and scanning electron microscopy (FE-SEM ∑IGMA VP) with an EDS unit. Results. The studied fragments can be divided into three groups depending on their structural composition: 1) metal grains with an unchanged structure; a zonal structure is observed in taenite (tetrataenite, cloudy zone); 2) metal grains with a taenite structure similar to the zonal one; 3) fragments in which no grains with a zonal structure were found; martensite-like structures formed during reheating were present. The structure of metal assemblage in the Chelyabinsk chondrite is compared with the structure of metal assemblage obtained in previous experiments with the Seymchan iron meteorite. It was established that heating to a temperature of 400°C for 6 h causes no changes in the metal grain structure. Heating to temperatures of 500 and 600°C for 6 h initiates transformation processes of the cloudy zone, which disappears completely after heated to 700°C for 6 h. Conclusions. Fragments of the light lithology of the Chelyabinsk ordinary chondrite were heating unevenly as a result of an impact event in its cosmic history. The presence of tetrataenite and a cloudy zone in one of the fragments means that temperature in this area didn’t exceed 400°C. Some areas underwent heating in the temperature range of 500–600°C and above 700°C.