Magnetic Properties and Redox State of Impact Glasses: A Review and New Case Studies from Siberia

Abstract

High velocity impacts produce melts that solidify as ejected or in-situ glasses. We provide a review of their peculiar magnetic properties, as well as a new detailed study of four glasses from Siberia: El’gygytgyn, Popigai, urengoites, and South-Ural glass (on a total of 24 different craters or strewn-fields). Two types of behavior appear: 1) purely paramagnetic with ferromagnetic impurities at most of the order of 10 ppm; this corresponds to the five tektite strewn-fields (including the new one from Belize), urengoites, and Darwin glass. Oxidation state, based in particular on X-ray spectroscopy, is mostly restricted to Fe2+; 2) variable and up to strong ferromagnetic component, up to the 1 wt % range, mostly due to substituted magnetite often in superparamagnetic state. Accordingly, bulk oxidation state is intermediate between Fe2+ and Fe3+, although metallic iron, hematite, and pyrrhotite are sometimes encountered. Various applications of these magnetic properties are reviewed in the field of paleomagnetism, magnetic anomalies, recognition of glass origin, and formation processes.

Impact glasses from Belize represent tektites from the Pleistocene Pantasma impact crater in Nicaragua

“…The porosity of most samples is low (<1% based on bubble counting) but a few samples are bubble-rich (one zone with up to 20% porosity). The magnetic susceptibility, measured on circa 4000 samples, is distributed in a very narrow range 23 with a mean of 125 ± 4 × 10 −9 m 3 kg −1 excluding 31 more magnetic outliers (>200 × 10 −9 m 3 kg −1 ) that will be discussed later. Such a narrow distribution is typical of tektites 12 , 23 , as it indicates both the lack of Fe 3+ and homogeneous iron content.…”

“…The magnetic susceptibility, measured on circa 4000 samples, is distributed in a very narrow range 23 with a mean of 125 ± 4 × 10 −9 m 3 kg −1 excluding 31 more magnetic outliers (>200 × 10 −9 m 3 kg −1 ) that will be discussed later. Such a narrow distribution is typical of tektites 12 , 23 , as it indicates both the lack of Fe 3+ and homogeneous iron content. Negligible Fe 3+ is confirmed by synchrotron X-ray absorption near-edge structure spectra of 8 samples 24 .…”

“…Pantasma impact glasses are rich in melted inclusions of Fe–Ti oxides with peculiar granular texture 19 , 23 . For the 31 belizite samples that are anomalously magnetic, one-third of them have χ > 2000 10 −9 m 3 kg −1 .…”

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

“…They proposed that the presence of metallic iron could be another reason for the magnetic highs of pseudotachylytes. Similarly, studies on high-pressure high-temperature effects in other disciplines, such as impact glasses (e.g., Rochette et al, 2015Rochette et al, , 2019, fulgurite (e.g., Essene & Fisher, 1986), shocked Martian meteorites (e.g., Van de Moortèle et al, 2007), have also documented that metallic iron was formed from reduction of more oxidized material. It is hard to identify the metal nanoparticles with conventional mineralogical techniques (e.g., scanning electron microscopy [SEM], Raman spectrometry, and Mössbauer spectroscopy; cf.…”

57Fe Mössbauer Analysis of Meteorites and Tektites

“…Dunlap [10] published the first Mössbauer analyses of a Tibetan tektite, which are consistent with those of indochinites from Cambodia. Devouard et al [11] identify a potentially new tektite (atacamaite) strewn field around the Atacama Desert, Chile, although later work [12] indicates this may be an impact-glass strewn field. Povenmire and Cornec [13] postulate another new strewn field in and around Belize.…”