Chemical Composition and Iron Oxidation State of the Amorphous Silicate Matrix in Acfer 094

Introduction: The unique ungrouped carbonaceous chondrite Acfer 094 is to date one of the most pristine meteorites available for study. The major component in this primitive chondrite is the fine-grained matrix (grain size << 10 μm). This matrix consists of an unequilibrated mixture of anhydrous silicates, sulfides, metal grains, and amorphous silicate material [1]. This amorphous silicate material is the major component of the finegrained matrix and indicates very low thermal and aqueous alteration effects, which would have led to recrystallization of the amorphous material. Furthermore, the high reactivity of this amorphous phase makes it a sensitive indicator for small-scale aqueous alteration effects, which cannot be observed in bulk sample analysis. The origin of the amorphous silicate matrix is not yet clear. The main scenarios include either primary processes before accretion of the parent body, e.g. disequilibrium condensation from a fractionated solar gas, or secondary processes on the parent body, e.g. aqueous alteration of a precursor material [1]. Linking changes in the chemical composition and the oxidation states of Fe in the amorphous silicate matrix to petrographic observations could reveal information about alteration effects and possible formation scenarios of amorphous silicate matrix. Here we report on analytical transmission electron microscopy (TEM) investigations of six electron-transparent Acfer 094 matrix samples to put constraints onto these issues. Samples and methods: The six TEM lamellae of the amorphous silicate matrix in Acfer 094 have been prepared from a polished thin-section using focused ion beam techniques (FIB) for previous investigations on presolar grains in the matrix [2]. The chemical compositions in the three samples 3113, 2520, and 2106 were determined using a Jeol JEM-3010 TEM (300 kV) with an Oxford Isis EDX system. Experimentally determined and published Cliff-Lorimer-k-factors were used to measure the chemical composition of 25 to 30 spots in each of the three samples [3]. The oxidation states of Fe in the six samples were measured using a Zeiss Libra 200FE TEM (200 kV) with an in-column Ω-energy filter for Electron Energy Loss Spectroscopy (EELS). For each sample, 14 EEL spectra at the Fe-L23 edge were acquired at different sample regions to improve the reproducibility and accuracy. The Fe 3+ /ΣFe ratios were calculated using the mod ified integral intensity ratio technique [4]. In both approaches, the amorphous areas in the samples were carefully selected using the TEM diffraction mode prior to EDX/EELS analyses to exclude contribution from crystalline grains, i.e., silicates or nanosulfides. The texture of each sample was also examined by Z-contrast imaging (high angle annular darkfield, HAADF) in scanning TEM mode. Results: Mineralogical and textural differences in the samples can be observed on a sub-μm scale (Fig. 1). The amount of sulfide and metal grains vary in the samples. Moreover, differences in the porosity of the samples are visible. The sample 3113 represents an unequilibrated mineral assemblage with widespread porosity (Fig. 1B). In contrast, sample 2106 has nearly no porosity and very low amounts of sulfide and metal grains. Within the amorphous matrix of 2106 small bright veins indicate secondary recrystallization effects (Fig. 1A).