Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Abstract

Abstract-Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10 −6 SI) and natural remanent magnetization (10 −3 -10 −1 A/m) are generally weak, but locally higher values up to 10.6 × 10 −3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C).

The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

“…Therefore, the amplitude and frequency components of these anomalies are not directly comparable to the marine and airborne magnetic data sets. Petrophysical scanning of the cores is generally consistent with the borehole logs and shows sporadic susceptibility and remanence enhancements Kontny et al 2007;Morris et al 2007). However, these susceptibility values are one order of magnitude lower that the values required by the Plado et al (2000) model.…”

“…Although the drilling yielded unequivocal evidence of the three anticipated upper layers (Coney et al 2007;Ferrière et al 2007), it did not penetrate the hypothesized highly magnetic body in the structure. Instead, drilling went through a weakly magnetic breccia with only minor amounts of impact-melt fragments (Coney et al 2007;Ferrière et al 2007;Elbra et al 2007;Kontny et al 2007;Morris et al 2007). The borehole magnetic logging revealed distinct shortwavelength magnetic anomalies in the bottom part (Morris et al 2007).…”

The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB‐08A litho‐log, related ejecta, and shock recovery experiments

“…Kontny et al (2007) deduced peak shock temperatures on the order of 320 °C for the target lithologies in core LB-08A from the magnetic mineralogy data. According to their investigation, pyrrhotite, the major carrier of the magnetic signal, is a pre-impact phase that was crushed and fractured by the shock.…”

“…It is commonly understood that such decorated PDFs originate by post-shock annealing and alteration (e.g., Cordier and Doukhan 1989). In the case of the young Bosumtwi impact crater, annealing is estimated to be of minor importance due to the documented low peak-shock pressures, as a corollary of which annealing was only at low temperatures and for a restricted time interval (e.g., Kontny et al 2007). The widespread presence of decorated PDFs may be related to the high fluid content in the metasediments (up to 22 wt% loss on ignition [LOI] [forthcoming data]; <13 wt% LOI in metasediments of core LB-07A) (Coney et al 2007b) forming the target at the Bosumtwi impact site.…”

Archaeabacterial lipids in drill core samples from the Bosumtwi impact structure, Ghana

“…However, temperature effects on GDGT stability should also be taken into account. There is evidence from pyrrhotite deformation in the impact rocks of Bosumtwi suggesting peak shock temperatures at the drilling site around 250 °C (Kontny et al 2007). There is also evidence of a moderately high-temperature post-impact hydrothermal alteration event near the central uplift, with calculated temperatures not higher than 300-350 °C (Petersen et al 2007), although geochemical analysis suggests that this did not produce a particularly severe alteration or involved a limited volume of fluid percolating through the impacted breccias (Ferrière et al 2007).…”