PETROGRAPHIC ANALYSIS OF THE MAGNETIC SPHERULES FROM THE SEDIMENTS OF KARSTIC DOBRA RIVER, CROATIA

Detection of an unusual magnetic susceptibility (χ) in the sediments at some locations of karstic Dobra River by the Croatian co-authors, initiated the search for source of magnetic particles in sediments of upper course of the river and anticipated an exceptional origin (volcanic or meteorite impact) of the magnetic particles based on chemical signature (i.e. ratio of Ni:Fe vs. Cr:Fe). In the second phase of the study in collaboration with Virginia Commonwealth University (VCU), the sediment samples were collected from locations with higher χ. Magnetic particles were manually separated from thoroughly washed and dried sediment samples by a rare-earth magnet. Then the magnetic particles were carefully sorted via stereo-microscopy to avoid erosional interference. Phase identification was performed by x-ray diffraction (XRD) analysis using a Cu anode (Cu K_α ≈ 1.54 Ǻ) over a scan range from 5 to 70 degrees 2θ. Multiple phases were identified and estimated that include anorthite (51%), hypersthene (25%), clinopyroxene (11%), ferrous diopside (10%) and magnetite (3%). Polished thin sections were primarily investigated with optical microscopy in polarized transmitted and reflected light revealing that the magnetic spherules are mostly constituted of vesicular glass with quench texture and schlieren. Ballen textures were also observed in the lechatelierite and spectrums were acquired for the ballen textures by Raman microscopy giving the typical peaks for α-cristobalite at 111–113, 227–229, 413–417, and 780–782 cm^-1. Further detailed analyses were performed with FIB-SEM (focused-ion beam) equipped with energy dispersive spectroscopy (EDS) and an electron back-scatter diffraction capabilities (EBSD) that revealed an immiscible phases of silicates and devitrification texture in the samples along with elongated laths of clinopyroxene embedded in anorthitic melts. The presence of clinopyroxene was ascertained by electron backscatter diffraction (EBSD). Furthermore, the presence of micro-spherules of iron sulfide with trace concentrations of Cr, Ni and Co along with iron sulfide spherules having compositional variability and Fe rich angular grains, likely troilite, suggest an meteorite impact origin of the magnetic spherules from the sediments of the karstic Dobra River.