Experimental shock metamorphism of the L4 ordinary chondrite Saratov induced by spherical shock waves up to 400 GPa

Abstract

Abstract-We carried out shock experiments on macroscopic spherical samples of the L4 ordinary chondrite Saratov (natural shock stages S2-S3), using explosively generated spherical shock waves with maximum peak pressures of 400 GPa and shock-induced temperatures >800°C (up to several thousands°C). The evolution of shock metamorphism within a radius of the spherical samples was investigated using optical and scanning electron microscopy, microprobe and magnetic analyses as well as Mo¨ssbauer spectroscopy and X-ray diffraction techniques. Petrographic analyses revealed a shock-induced formation of three different concentric petrographic zones within the shocked samples: zone of total melting (I), zone of partial melting (II), and zone of solid-state shock features (III). We found a progressive pressure-induced oxidation of Fe-Ni metal, whose degree increased with increasing shock peak pressure. The amount of FeO within zone I increased the factor of 1.4 with respect to its amount in the unshocked Saratov sample. This suggests that within zone I about 70 wt% of the initial metallic iron was oxidized, whereas magnetic analyses showed that about 10 wt% of it remained intact. This strongly supports the hypothesis that, in addition to oxidation, a migration of metallic iron from the central heavily shocked zone I toward less shocked peripheral zone took place as well (likely through shock veins where metallic droplets were observed). Magnetic analyses also showed a shock-induced transformation of tetrataenite to taenite within all shocked subsamples, resulting in magnetic softening of these subsamples (decrease in remanent coercivity). These results have important implications for extraterrestrial paleomagnetism suggesting that due to natural impact processes, the buried crustal rocks of heavily cratered solid solar system bodies can have stronger remanent magnetism than the corresponding surface rocks.

Experimental shock metamorphism of terrestrial basalts: Agglutinate‐like particle formation, petrology, and magnetism

“…Shock‐induced magnetic softening (observed on the 4th lithology of tholeiitic basalt, shot 25) was also previously reported in the literature (Bezaeva et al. ; Kohout et al. ).…”

“…; preshock bulk densities: ~2.1 g cm −3 ) or shock‐induced transformations of ferromagnetic phases (e.g., disorder of tetrataenite into taenite in FeNi‐bearing samples due to shock‐induced heating in Bezaeva et al. ). The 4th lithology of tholeiitic basalt, which demonstrates the effect of shock‐induced magnetic softening, was not particularly porous (moreover, its density is the highest of all basalts, see Table ) and we do not expect any irreversible phase transformations in Tmt due to shock.…”

The effects of 10 to >160 GPa shock on the magnetic properties of basalt and diabase

“…Two opposed shock effects have been reported in the literature in rocks with a range of magnetic mineralogies: shock‐induced magnetic hardening (i.e., an increase in bulk coercivity) [ Cisowski and Fuller , ; Pesonen et al ., ; Langenhorst et al ., ; Gattacceca et al ., ; Louzada et al ., ; Bezaeva et al ., ; Mang et al ., ; Reznik et al ., ] and shock‐induced magnetic softening (i.e., a decrease in bulk coercivity) [ Bezaeva et al ., ; Kohout et al ., ]. Halls [] observed that target rocks from the most shocked central zone of the Slate Islands impact structure had somewhat higher remanent coercivities than target rocks from the periphery of the exposed central uplift (see Figure 13c of their paper).…”

“…The “high‐intensity” regime, used for sample og‐2 , is similar to that implemented by Bezaeva et al . [] and Kozlov and Sazonova []. The “low‐intensity” regime, used for sample og‐1 , was previously implemented by Dobromyslov et al .…”

“…One way to experimentally assess the effects of shock on magnetic minerals is to conduct shock experiments wherein rocks are briefly taken to high pressures as an analog for the effect of the passage of shock waves during a hypervelocity impact. In spherical shock experiments, explosively generated shock waves travel through a prepared spherical rock sample [ Bezaeva et al ., ]. The spherical shock approach has several advantages relative to other types of shock experiments.…”