Meteoritic component in impact breccias of the Jeokjung-Chogye structure, South Korea: Evidence from the HSE abundances and ReOs isotopic systematics

doi:10.1016/j.chemgeo.2022.121145

Chemical Geology

Volume 613, 20 December 2022, 121145

https://doi.org/10.1016/j.chemgeo.2022.121145 Get rights and content

Highlights

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We report HSE and ReOs isotope data for impactites from the JC structure in Korea.
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The impactites have 5 times higher abundances of PGE compared to the target rocks.
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The impactites have ¹⁸⁷Os/¹⁸⁸Os ratios relatively lower than the target rocks.
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Endogenic or chondritic relative HSE abundances argue against irons as impactor.
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Chondritic contribution to the impact breccias is estimated to be ∼0.05% or less.

Abstract

Geochemical compositions, including highly siderophile element (HSE) abundances and Re-Os isotope compositions, of nine polymict impact breccias in two drill cores and three target rocks from the Jeokjung-Chogye structure, South Korea, have been investigated to test the impact origin of the structure, and to infer the potential projectile type for the formation of the crater. The impactites have around 5 times higher abundances of platinum-group elements (PGE) compared to target rocks, and two orders of magnitude higher Re abundances. The weighted mean for ¹⁸⁷Os/¹⁸⁸Os ratios of impactites is 0.806, which is lower than this value for the target rocks (1.049), suggesting the presence of a meteoritic component. The higher PGE abundances and a broad negative correlation between Os contents and ¹⁸⁷Os/¹⁸⁸Os ratios show that the impactites incorporated ∼0.05 wt% of a chondritic component. The endogenic or chondritic relative abundances of the HSE argue against iron meteorites as a potential impactor for the Jeokjung-Chogye structure. Yet, the present data do not allow to unambiguously identify the projectile type (i.e., chondrite class) present in the impactites because of the high proportion of the indigenous component.

Introduction

Interplanetary bodies, such as asteroids and comets, are responsible for the formation of impact structures. About 190 impact structures have so far been identified on Earth (https://impact.uwo.ca/map/), with one or more new impact structures are being discovered every year. Impact crater can be recognized via remote sensing or geophysical investigations, but only petrographic and geochemical studies provide compelling evidence for an impact event (e.g., Koeberl, 2014).

The Jeokjung-Chogye Basin in South Korea has a 7 km-diameter bowl-shaped geomorphology, and its rim rises ∼200–600 m above the floor (Fig. 1a, b). Based on its geomorphology, lithology, and gravity survey, Choi et al. (2001) raised the possibility of its origin via a meteorite impact. Recently, Lim et al. (2021) reported the first direct evidence of impact cratering, including shatter cones and decorated planar deformation features in quartz grains from the impact breccia, found at 86–142 m depth in the drill core drilled within the basin. They argued that the impact likely occurred during the last glacial period (∼50 Ka) based on the radiocarbon dates of charcoals in the lacustrine sediments overlying the impact breccia. In addition, Choi et al. (2022) proposed that the Jeokjung-Chogye structure was formed by a steeply inclined impact (∼50-60^o) from the east based on the interpretation of the gravity field. However, meteorite fragments have not been found within the structure yet. Extraterrestrial fragments are found within impact structures only in very rare cases (e.g., Goderis et al., 2012). In addition, considering that there is a greater tendency for the impactor to be vaporized for a larger (> ∼ 1.2 km in diameter) crater (Grieve, 1991), and that the amount of projectile vaporization tends to increase with the impact angle (Pierazzo and Melosh, 2000), it seems unlikely that the remnants of the impactor will ever be found in the Jeokjung-Chogye structure.

About one-third of the over 190 impact structures on Earth contain impact melt rocks (e.g., Tagle and Hecht, 2006). Siderophile elements, including platinum group elements (PGE; Os, Ir, Ru, Rh, Pt and Pd), Ni, Co, and Cr, and Re single bond Os isotopic systematics are essential tools for the detection of meteoritic component in impact breccia and melt rocks due to significant differences in the relative abundances and isotopic compositions between most meteorites and terrestrial target rocks (e.g., Evans et al., 1993; Koeberl, 1998, Koeberl, 2014; McDonald et al., 2001; Lee et al., 2006; Tagle et al., 2009; Schulz et al., 2016; Goderis et al., 2017). Concentrations of highly siderophile elements (HSE), including Os, Ir, Ru, Pt, Pd, and Re, in impactites derived from felsic target rocks, for example, may be up to three orders of magnitude higher compared to those in the respective target rocks (e.g., Koeberl, 2014). Here, for the first time, we report HSE abundances and Re single bond Os isotope compositions, together with major and trace element concentrations, for the Jeokjung-Chogye impact melt breccia and target rocks. Our objectives were to test the hypothesis regarding the impact origin of the Jeokjung-Chogye structure, and to infer the potential projectile type of the impactor.

Section snippets

General geology

The Jeokjung-Chogye structure is located in the southeastern part of the Korean Peninsula (Fig. 1a), and was excavated in the Gyeongsang Basin (the largest Cretaceous nonmarine sedimentary basin in Korea). Based on the differences in lithology and geographical distribution, the Gyeongsang Basin is subdivided into three sub-basins (from south to north): Jinju, Uiseong, and Yeongyang. The study area belongs to the Jinju sub-basin, where the sedimentary strata dip eastward to southeastward at low

Samples

A study of the drill cores drilled and logged within the Jeokjung-Chogye structure (Lim et al., 2021) shows that it is filled with impact breccia and overlying unlithified lacustrine deposits. Thickness of the sedimentary layer of the structure has been estimated to be ∼100 m by applying the fundamental resonance frequency from microtremor measurement (Lee et al., 2019). Based on the previous studies (Lee et al., 2019; Lim et al., 2021) and personal communications (Dr. J. Lim), we selected two

Sample preparation

Rock samples were crushed in an alumina-faced jaw crusher, ground in an alumina-lined shatter box, and reground to fine powders in an alumina-faced disk mill at the Isotope Geochemistry Laboratory (IGL), University of Maryland College Park. These powders were then used for the analysis.

Major- and trace element data

Major and trace element concentrations were determined employing a lithium metaborate/tetraborate fusion and an inductively-coupled plasma mass spectrometer (ICP-MS), at Activation Laboratories Ltd. (Actlabs),

Results

The major and trace element concentrations are given in Table S1. Representative major oxide variations are shown in Fig. 2, as a function of SiO₂ content. The impactites have relatively low SiO₂ and high CaO contents compared to the three target rocks (shale and sandstone) (Fig. 2a). The CaO contents of impactites increase with decreasing SiO₂, but the Al₂O₃ and FeO* contents decrease, indicating the addition of carbonate components (Fig. 2a-c). Meanwhile, some impactite samples have higher

Evidence for the presence of an impactor component

Despite the dilution of the HSE signal by carbonates present in the impact breccias studied, the impactites have Cr, Co and Ni contents which are similar to or somewhat higher than those of the target rocks (Fig. 3a, b). They also have Cr/Ni and Co/Ni ratios between those of the target rocks and the chondrites (Fig. 3c). The ∑REE and (La/Yb)_N values of the impactites are somewhat lower than the target rocks (Fig. S3a). This may be the effect of dilution by carbonate components, because the

Conclusions

In an attempt to detect meteoritic components in impact breccias recovered in drill cores at the Jeokjung-Chogye structure in South Korea, their geochemical compositions, including HSE abundances and Os isotopic compositions, were determined. Meteoritic contribution to the impact breccias is confirmed and estimated to be ∼0.05% or less. Because of high abundances of highly and moderately siderophile elements in meteorites, this small amount of extraterrestrial contribution can account for the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (NRF-2021R1A4A5026233). Insightful reviews by Lukas Ackerman and an anonymous reviewer greatly improved the manuscript. We thank Hyemin Han, Seunghoon Lee, and Min-Kyu Oh for preparing photographs, and Yun Jin Lee, Jiyeon Kim, Sebin Oh, and Minjeong Im for preparing thin sections.