ISSN 0016-7029, Geochemistry International, 2016, Vol. 54, No. 4, pp. 378–385. © Pleiades Publishing, Ltd., 2016.
Silver and Silver Microparticles of the Cretaceous-Paleogene Boundary Sediment of Fish Clay Højerup (Stevns Klint, Denmark)1 P. I. Premovića, N. R. Đorđevića, and M. S. Miljkovićb a
Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia bFaculty of Medicine, University of Niš, 18000 Niš, Serbia e-mail:
[email protected] Received February 20, 2015; in final form, April 6, 2015
Abstract—We report the occurrence of micrometer-sized silver particles in the red (“impact” or “ejecta”) layer of the Fish Clay. These appear to be embedded into the biogenic/abiogenic calcite matrix. Energy dispersive X-ray analysis indicates that these microparticles are composed of pure silver (>99% of the total weight). Scanning electron microscopy images reveal irregular and predominantly rounded shapes with rugged surfaces. Numerous silver microparticles are spherical, and some exhibit dendritic textures. These microparticles were probably originated during the Chicxulub asteroid impact event, or immediately afterwards, and dispersed globally. Keywords: silver, microparticles, Fish Clay, Cretaceous-Paleogene DOI: 10.1134/S001670291604008X
INTRODUCTION Alvarez et al. [1] explained the presence of anomalous iridium (Ir) in prominent marine CretaceousPaleogene (KPB) clays at three localities (Gubbio in Italy, Stevns Klint and Woodside Creek in New Zealand, Fig. 1a) by proposing a late Cretaceous asteroid impact. This suggestion was followed by reports of the Ir anomaly in many other marine/continental boundary clays worldwide [2]. These clays mark one of the most significant impact events in the Phanerozoic, one that appears responsible for one of the greatest extinctions in Earth history. It has been suggested that the impact site is at Chicxulub (a village at Yucatan Penninsula, southern Mexico, Fig. 1a) [3] and the impactor was a CI carbonaceous chondrite-type body [4]. Fish Clay: Location and Lithology The Fish Clay (of earliest Danian age) at Højerup (Fig. 1b) belongs to one of the classic CretaceousPaleogene boundary (KPB) deposits at Stevns Klint, Fig. 2a. The detailed lithology of the Fish Clay constituent of the Rødvig Formation near the village of Højerup was described by Surlyk et al. [5]. The authors distinguished four distinctive layers within this boundary section: the latest Maastrichtian chalk, a 2–5 cmthick dark marl, brown-to-grey marl, and grey marl overlain by the lower Danian Cerithium limestone member (Figs. 2a/b). Dark and brown-to-grey marls 1 The article was translated by the authors.
are here considered to constitute the main part of the Fish Clay [6, 7]. Schmitz [6] and Elliott [7] subdivided the dark marl into the red (“impact” or “ejecta”) layer (maximum ca. 2–4 mm) (hereinafter the red layer) overlain by the black marl (Figs. 2a, 2b). Wendler and Willems [8] proposed that these two layers represent the first decades or centuries following the KPB impact event. The red layer is underlain by the latest Maastrichtian chalk and is in sharp contact with both this chalk and overlying marl. Smectite is a predominant (>90%) constituent of the red layer and is probably detrital and redeposited from adjacent coastal areas, with a small portion being authigenic and derived from ejecta fallout [9]. Wellpreserved impact-related goethite-rich microspherules (between about 100–800 μm in diameter) in the smectite matrix are relatively abundant in the red layer [6, 10, 11]. Bauluz et al. [11] interpreted these grains as altered meteorite fragments. The red layer contains altered nano-size impact-derived glasses and nano-size goethite grains [11, 12], a few shocked quartz grains are also identified in this layer [13, and references therein]. The microspherules and nanosize glasses of the red layer are likely reworked and redeposited at or near the KPB simultaneously with smectite [9]. Beside Ir, this layer also contains geochemical indicators of the impact event, including the other platinum group of elements (Pt, Pd and Ru) probably also of extraterrestrial origin as Ir [14] and some siderophile metals partly cosmogenic, such as Ni and Cr [see 6, and references therein]. Most research-
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Dogie Creek
Stevns Klint
Asia Hess Rise
Europe Lattengebirge
North America Sabinas Cantarell Chicxulub
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ATLANTIC OCEAN Haiti
Africa
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South America Australia Woodside Creek
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Fig. 1. (a) Geographic locations of Chicxulub, Cantarell, Sabinas, Gubbio, Stevns Klint, Woodside Creek, Hess Rise, Lattengebirge and Dogie Creek. (b) Geographic locations of Stevns Klint in Denmark. GEOCHEMISTRY INTERNATIONAL
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(a) Cerithium limestone Grey marl cm 10
Brown-to-marl Black marl Red layer The latest Maastrichtian bryozoan-rich chalk
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(b)
Black marl Red layer 0
Fig. 2. (a) Expanded lithological log of the Fish Clay at Højerup. These logs are based on Surlyk et al. (5) and (b) the red layer specimen studied enriched with AgMPs.
ers consider that the above impact markers are probably directly related to the Chicxulub event. RESULTS AND DISCUSSION Sample and Methodology Dr. Helle Schummel collected bulk rock samples of the Fish Clay from an outcrop 200 m south of the old church of Højerup. Powdered black marl sample (1 g) was treated (12 hrs) with acetate buffer: acetic acid (1 M)/sodium acetate (1 M) solution at pH 5.0 to remove most of the carbonates. The insoluble material constitutes the carbonate-free fraction. The whole marl sample and its demineralized (carbonate-free) fraction were analyzed for silver (Ag) by Instrumental Neutron Activation Analysis (INAA). INAA was carried out by Activation Laboratories Ltd. (Toronto, Canada). The reproducibility of the results from this analytical method is good. Analytical precision and accuracy were better than 30%. The red layer sample was demineralized with 6 M HCl at room temperature for about 5 min. This treatment also removes most carbonates (about 10%). Individual Ag microparticles (AgMPs) in the demineralized fraction were handpicked under a binocular stereomicroscope. Selection criteria were their distinctive forms and brightness. All Scanning Electron Microscopy/Energy Dispersive Spectrometry work was carried out using a Jeol JSM-35 electron microscope equipped with a Tracor
TN-2000 energy dispersive X-ray spectrometer. Operating conditions for energy-dispersive analyses were at a 25-keV accelerating voltage, 0.1-μA beam current and a beam spot diameter of approximately 3 μm. Ag and the Boundary Clays and Glasses Alvarez et al. [1] determined the Ag concentrations in the whole Fish Clay (1.2 ppm), while Elliott [7] reported the Ag concentration of the Fish Clay as 90% (the red layer).
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Graup et al. [18] reported that glassy microtektites in the KPB section in the Lattengebirge (Fig. 1a, Germany) contained about two hundreds (~15 ppm) up to three hundred times (~55 ppm) higher Ag than chondrites. In addition, Koeberl [19] reported that the boundary black and yellow glasses from Haiti (Fig. 1a) contain
