Toward classifying magnetic properties of impact melts — An example from the Boltysh astrobleme

Impact melts form as a result of large-scale, hypervelocity strikes of cosmic bodies on the surfaces of rocky planets and moons. Cratering has been a major agent forming planetary surfaces in the early history of the Solar system. On the Earth, however, due to its geological activity, only about 200 impact structures have been confirmed and for a few hundred more the impact origin has been suspected (compare with ~2 million craters on the Moon) [1]. Furthermore, determining the impact origin of a geological structure requires extensive field and laboratory work, which are often severely limited by remote location and poor exposure.

Magnetic surveys are potentially a powerful tool for characterizing (suspected) impact structures. However, to develop feasible geological models based on survey data, a thorough knowledge of magnetic properties of impact-related rocks (melts, breccia, etc.) is necessary. At present, these data are available only for a handful of structures, and are not summarized into a more or less general classification relating magnetic properties to the genesis of impact rocks.

Toward this end, we have carried out a detailed study of the magnetism of a ~300-m long core of impact melt recovered from borehole 11475, which was drilled through the central uplift of the Boltysh impact structure (Ukraine, diameter 24 km, age 65.39 ± 0.14 Ma [2]). The experiments included hysteresis and FORC measurements, and thermomagnetic analysis at high (up to 700 °C) and cryogenic temperatures. The magnetic mineralogy of the studied samples appears complex. NRM is carried by phases with Curie temperatures ranging from 250 °C to ca. 550-570 °C. This implies in most cases a relatively high degree of iron substitution in the magnetite lattice. However, even the samples with the highest Curie temperatures lack the Verwey phase transition characteristic of stoichiometric magnetite. This can be explained by a simultaneous presence of foreign cations and vacancies in magnetite lattice [3]. Room-temperature coercivity varies widely, from <5 mT to >100 mT. FORC diagrams reveal that the grain size of the NRM carriers varies significantly along the core. At the same time, a rise of figurative points density is observed at near-zero coercivity, suggesting the presence of a significant amount of ultrafine superparamagnetic (SP) grains. The signal from SP grains is also clearly seen in LT-SIRM warming curves where at least 40-50%, and in some cases up to 90%, of remanence acquired at 2 K is lost below 30 K.

The high amount of SP particles, including those with extremely small blocking volumes, may be interpreted as evidence for extremely fast initial crystallization of the impact melt, which could have lasted only ~10²-10³ s. At the same time, grains carrying NRM, which are several orders of magnitude larger, must have formed at a later stage, during much slower cooling. At this stage, at least some of the already formed SP grains have apparently been protected from further change by the surrounding glassy matrix.