Formation of uranium‐thorium‐rich bitumen nodules in the Lockne impact structure, Sweden: A mechanism for carbon concentration at impact sites

Abstract

Abstract-The Ordovician Lockne impact structure is located in central Sweden. The target lithology consisted of limestone and black unconsolidated shale overlaying a Precambrian crystalline basement. The Precambrian basement is uranium-rich, and the black shale is both uranium-and organic-rich. This circumstance makes Lockne a good candidate for testing the occurrence of U-Thrich bitumen nodules in an impact structure setting. U-Th-rich bitumen nodules are formed through irradiation; hence the increase in the complexity of organic matter by a radioactive (uranium-and thorium-rich) mineral phase. U-Th-rich bitumen nodules were detected in crystalline impact breccia and resurge deposits from the impact structure, but samples of non-impact-affected rocks from outside the impact structure do not contain any U-Th-rich bitumen nodules. This implies that in the Lockne impact structure, the nodules are associated with impact-related processes. U-Th-rich bitumen nodules occur throughout the geological record and are not restricted to an impact structure setting, but our studies at Lockne show that this process of irradiation can readily occur in impact structures where fracturing of rocks and a post-impact hydrothermal system enhances fluid circulation. The irradiation of organic matter by radioactive minerals has previously been proposed as a process for concentration of carbon on the early Earth. Impact structures are suggested as sites for prebiotic chemistry and primitive evolution, and irradiation by radioactive minerals could be an important mechanism for carbon concentration at impact sites.

The Proximal Ejecta Around the Marine‐Target Lockne Impact Structure, Sweden

“…The other two cores mentioned here, LOC10 and LOC11, were retrieved for this study with the use of the same type of equipment as for LOC08 and LOC09. The core log descriptions presented here are in line with the aim of the study, that is, to determine the thickness and main lithological composition of the ejecta and do, thus, not provide all additional information that would be possible to obtain from the material (e.g., Lindgren et al, 2007;Melero-Asensio et al, 2014). The LOC08 core hole was located at the center of a large solitary ejecta body that occupies the outer reach of the Tandsbyn gap.…”

“…In two instances, we use drill cores (i.e., LOC08 and LOC09) that were retrieved already in 2004 as the subject of those drillings also was to investigate the thickness and properties of the crystalline ejecta deposits. Only LOC09, however, has so far been described in publications (i.e., Lindgren et al., 2007; Melero‐Asensio et al., 2014). The other two cores mentioned here, LOC10 and LOC11, were retrieved for this study with the use of the same type of equipment as for LOC08 and LOC09.…”

A rock magnetic profile through the ejecta flap of the Lockne impact crater (central Sweden) and implications for the impact excavation process

“…Reference source not found.d). Its origin from the Cambrian alum shale is inferred from its U-Th rich bitumen nodules observed by Lindgren et al (2007). Hence, it can be assumed some of the dark matrix of the breccia has the same origin.…”

“…As a consequence of the 45º oblique impact through the layered target (Lindström et al 2005a,b), a more pronounced flap on the down-range side of the structure was developed. However, until now the only peerreviewed journal publication in addition to brief and descriptive conference abstracts (e.g., Ormö and Lindström, 2005) is that of Lindgren et al (2007) dealing with biomineralizations within certain levels of the core. This gives us reason to return to the original objective, but here with a petrophysical approach.…”