Search for Projectile Traces in Melt Rocks of the Charlevoix and Dellen Impact Structures
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Bibliographic record
Abstract
Introduction: The identification of projectile components is an important goal of impact crater research. The first studies that addressed this issue in the 1970s involved lunar impact melt (IM) rocks, e.g., [1, 2]; only later work focused on terrestrial impact structures. It is remarkable, however, that despite almost 40 years of projectile research, the number of craters for which a projectile is precisely known remains small [3 and ref. therein]. Precise identification of projectile components by geochemical techniques has only been accomplished in the past 10 years. Projectile identification in lunar materials is, in principle, easier than in terrestrial rocks because the projectile contamination (reflected by Ir concentration), is usually significantly higher than in most impact melt rocks of terrestrial craters, where the proportion of projectile is usually less than 1%. Here, we present the results of investigations into two terrestrial impact structures: Charlevoix, Canada, and Dellen, Sweden. Charlevoix: The impact structure (47°32 N/70°18 W) is located ~100 km northeast of Quebec City. The ~54 km complex crater structure is topographically well expressed. The impact event was dated by K/Ar to 357 ± 15 Ma [5], but newer Ar-Ar laser fusion suggests later ages between 460470 Ma [4]. However, further studies are required for a better age constraint of this structure. For this study, a set of 22 impact melt rock samples was selected for analysis. The samples were collected at three IM localities. One sample was collected near to an in-situ outcrop Im1 (± 30 m), a set of 9 samples from an outcrop of glacier reworked material ~ 1 km from the first location (outcrop Im1 and Im2 described by [5]), and 12 samples from an outcrop near to the central peak at Mt. des Eboulements. Dellen: This 19 km diameter impact structure is located at N 61°48'/E16°48'. Dating by Ar-Ar and Rb-Sr give an age of 89.2±2.7 Ma [6]. A set of 15 IM samples and one Suevite of glacially reworked material from two localities were studied for projectile traces. Methods: Major, minor and selected trace elements of the IM samples were determined by XRF. Ni, Co and Cr were measured by ICP-MS after acid dissolution to obtain a better resolution. PGE were analyzed after the procedure of [7] by NiS-fire assay in combination with ICP-MS. In the case of Charlevoix, only 3 samples from Im2 were analyzed for PGE because of the homogenous composition of these samples indicated by the XRF and ICP-MS data. A total of 16 PGE analyses of the Charlevoix samples were performed. For Dellen, all samples were used several times for a total of 20 analyses. For each sample, relatively large (40 and 80 g) masses were used in order to minimize background effects. Results: The XRF and ICP-MS analyses reveal a homogenous composition of the IM rocks for the three localities sampled within the Charlevoix crater, as well as a relative low concentration of Co, Ni and Cr. The same is observed for the Dellen IM rocks (Table 1). This low concentration of Ni and Cr suggests that, if present in the IM, the proportion of extraterrestrial material is extremely low. Table 1. Composition of the impact melt rocks
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Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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