The substance of the Chelyabinsk meteorite: Results of geochemical and thermomagnetic studies

Abstract

The fragments of the Chelyabinsk meteorite studied are represented by light-gray granular rock of chondritic structure. The chondrules and their cementing matter are mainly constituted by olivine and orthopyroxene. The matrix consists of a pyroxene-olivine aggregate with plagioclase, apatite, melted glass, and the inclusions of ore minerals: taenite, kamacite, troilite, pyrrhotite and pentlandite (more rarely), and individual grains of chromite and ilmenite. The comparison of the composition of the Chelyabinsk meteorite to the average composition of LL chondrites had shown their complete convergence. The concentrations of sidero- and chalcophile rare elements in the meteorite, normalized to CI chondrites, are much close to the values for LL chondrites and almost reproduce the character of their distribution in the spider diagram. However, some high-charged and lithophile elements (Nb, Zr, Hf, Sr, Ba, Th, and U) not belonging to the mentioned groups are characterized by somewhat increased contents. The enrichment of the samples of the Chelyabinsk meteorite in rare-earth elements compared to LL chondrite (5.18 against 3.58 ppm) is also revealed. This is related to the higher concentrations of light lanthanides in the meteorite samples, which is seen from the increased La/Yb ratio compared to the value for LL chondrite (1.9–2.3 and 1.4, respectively). Iron-nickel alloys are the main magnetism carriers in the Chelyabinsk meteorite. The compositions of kamacite, taenite, chromite, and Fe-sulfides are not much different. The optical and microprobe data are confirmed by the thermomagnetic parameters as well: (1) The specific magnetization of 4–6 Am2/kg points to small variations in the concentrations of magnetic minerals. (2) The M(T) curves for all the samples nearly repeat each other, and the Curie temperatures of 490–520 and 740–770°C are registered in the curves of the first and second heating, hence, these curves correspond to kamacite of various composition, right up to pure iron. (3) The monocline ferrimagnetic pyrrhotite of TC = 320–340°C is registered in the treated fragments in both the M(T) curves of heating and cooling. (4) The concentrations by thermomagnetic analysis amount to 0.6–1.6% (0.9% average) for kamacite, 0.7–1.5% (1.1% average) for taenite, and 0–1.5% (0.4% average) for monocline pyrrhotite. (5) No magnetite was found in the M(T) curve during the first heating of the samples. Hence, the content of magnetite is much below 0.1.