Magnetic Properties of Tektite-like Impact Glasses from Zhamanshin Astrobleme, Kazakhstan

Abstract

We measured the low-field magnetic susceptibility and natural remanent magnetization (NRM) for an extensive collection of irghizites, tektite-like impact glasses from the Zhamanshin meteorite crater, Kazakhstan. For a small subset of samples, more detailed magnetic measurements, scanning electron microscopy and microprobe analysis have been carried out. In agreement with a previous study (Rochette et al. 2015), ~95% of samples showed relatively low magnetic susceptibility ranging from 45 to 300 × 10−9 m3/kg. Combining susceptibility and NRM data, we are able to classify irghizites into “normal” (susceptibility <300 × 10−9 m3/kg, NRM < 4 × 10−6 A m2/kg) and “anomalous” groups. Detailed rock magnetic experiments and microscopic observation confirm this dichotomy. Normal irghizites contain very little ferrimagnetic material which, in addition, is predominantly in superparamagnetic state, implying an extremely small, on the order of a few nm, grain size. In contrast, anomalous samples exhibit magnetic properties consistent with much larger, µm to 10s of µm, ferrimagnetic grains, which were observed microscopically as well. We propose that this difference between two groups of samples is related to the initial temperature of the impact melts from which they were formed. Normal irghizites likely originate from the spray of a strongly overheated melt with initial temperatures up to 2500–3000 °C, cooling extremely fast, while anomalous ones could have been formed from less hot and slower cooling melt.