Geochemistry of a confirmed Precambrian impact ejecta deposit: The Grænsesø spherule layer, South Greenland

Abstract

Samples from a single outcrop of the Graenseso spherule layer, Midternaes, South Greenland, consist of a spherule‐bearing dolomixtite with matrix‐supported intraclasts up to 1 m in size. In addition to field observations, we performed mineralogical and whole rock geochemical analysis, including electron microprobe, neutron activation analysis, X‐ray fluorescence, and mass spectrometry of the horizon and the overlying and underlying strata. We show that the spherules are petrographically similar to those in the Zaonega spherule layer, Karelia, Russia. Our petrographic and chemical results are consistent with the previous suggestion that the Grænsesø layer correlates with the Zaonega layer, and it is possible that both layers are related to the Vredefort impact event. The samples containing spherules, as well as the overlying rocks, have elevated REEs compared to the underlying pre‐impact layer, suggestive of a new continental source of sediment that may be coincident with the impact event. Zircons separated from the lower part of the Grænsesø spherule layer display complex age patterns suggesting that they have genetically different origins based on distinctly different Th/U ratios. Crystallization ages of all groups are ≥ 2.8 Ga, with ~2.8 Ga representing a time of major crustal growth globally. Therefore, we cannot conclusively determine in this study if the zircons are locally derived or if they are transported with the ejecta. The spherule layer was deposited by a high‐energy, subaqueous debris flow, an origin that is consistent with the hypothesis that the layer was deposited by impact‐induced waves and/or currents.

Micro–X-ray fluorescence (µXRF) analysis of proximal impactites: High-resolution element mapping, digital image analysis, and quantifications

“…Overall, we demonstrate that µXRF is more than only a screening tool for heterogeneous impactites, because it rapidly produces bulk and phase-specific geochemical data sets that are suitable for various applications within the earth sciences. Huber et al, 2019) and Cenozoic ejecta materials, including Australasian tektites (e.g., Goderis et al, 2017). In this study, we explored the potential of µXRF as a fast, inexpensive, and nondestructive technique particularly suitable to the study of proximal impactites.…”

Lunar Impact Glasses: Probing the Moon's Surface and Constraining its Impact History

“…They determined that asteroids with D > 10 km are required if spherules are deposited in a global layer. As more terrestrial impact spherule layers are found, characterized, and connected to craters of various sizes (e.g., Davatzes et al, 2015;Huber et al, 2019;Keller et al, 2003), production models can be refined and applied to lunar impact glasses. The lunar glasses are most likely dominated by the high-velocity formation of melt droplet spherules that are morphologically identical to the terrestrial vapor condensate spherules.…”