TRACES OF METEORITE IMPACT IN THE SEDIMENTS OF KARSTIC DOBRA RIVER (CROATIA)

Some years ago an unusual and unexpected anomaly of magnetic susceptibility (MS) was observed in sediments of the upper course of the karstic Dobra River (Croatia) during preliminary measurements of MS throughout the Kupa River drainage basin [1, 2]. In that study, following techniques have been used: magnetic susceptibility and Curie temperature, isothermal remnant magnetism (IRM), mineralogical analysis (using XRD) and chemical analysis (using ICP-MS). Thermomagnetic curves showed a distinctive Curie-point of magnetite at 580 degrees C, while additional transformation observed at 520-560 degrees C derives from titanomagnetite. Magnetic particles from the Dobra River sediments contain pyroxene, plagioclase, hematite and quartz, in addition to magnetite. The ratio Ni/Fe versus Cr/Fe suggests that the magnetic spherules are most probably impactites. New and exciting finding of magnetic spherules in sediments of this karstic river deserved further field and laboratory research, which continued in collaboration with Virginia Commonwealth University (VCU), USA. Petrographic analysis of polished thin sections in polarized transmitted light revealed that the magnetic particles are mostly made of vesicular glass with quench texture. Ballen textures are also observed in the lechatelierite and consisting of alpha-cristobalite. Scanning electron microscopy and elemental mapping revealed immiscible phase of silicates and devitrification texture in the glasses. Presence of micro-spherules of iron sulfide with traces of Cr, Ni and Co, along with iron sulfide spherule with immiscible compositional variability and Fe rich angular grains, probably Troilite, further supports already suggested impact origin of the magnetic spherules from the sediments of the karstic Dobra River.