Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125
Comunicações Geológicas (2012) 99, 2, 27-34 ISSN: 0873-948X; e-ISSN: 1647-581X
Iridium anomaly and extraterrestrial component in the clays at the Cretaceous-Paleogene boundary in Denmark, Spain and New Zealand Anomalia de irídio e componente extraterrestre das argilas do limite Cretácico – Paleogénico na Dinamarca, Espanha e Nova Zelândia P. I. Premović1*, B. S. Ilić2 Artigo original Original article
Recebido em 28/09/2011 / Aceite em 11/12/2011 Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012 © 2012 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The Cretaceous-Paleogene boundary clays at Højerup (Denmark), Caravaca (Spain) and Woodside Creek (New Zealand) show anomalous enrichments of iridium compared with the marine sedimentary rocks. For the average iridium content of 465 ppb of CI carbonaceous chondrite the estimate of CI chondrite proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is about 25 % at Højerup, 15 % at Caravaca and > 30 % at Woodside Creek. These proportions are most likely too high due to a significant Ir influx from the nearby marine or continental site to these sections. Keywords: Fish Clay, Carbonaceous chondrite.
Caravaca,
Woodside
Creek,
Iridium,
Resumo: As argilas do limite Cretácico – Paleogénico em Højerup (Dinamarca), Caravaca (Espanha) e Woodside Creek (Nova Zelândia) mostram enriquecimentos anómalos de irídio comparados com rochas sedimentares marinhas. Para o conteúdo médio de irídio de 465 ppb do condrito carbonáceo CI, a estimativa das proporções condríticas CI nas camadas descarbonatadas ricas em irídio da zona de limite, baseado nas fluências integradas de irídio, é cerca de 25% em Højerup, 15 % em Caravaca e > 30% em Woodside Creek. Estas proporções demasiado elevadas são provavelmente devidas a influxo significativo de Ir a partir das regiões marinhas ou continentais próximas destas secções. Palavras-chave: Argila, Caravaca, Woodside Creek, irídio, condrito carbonáceo. 1
Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia. 2 Department of Pharmacy, Faculty of Medicine, University of Niš, 18000 Niš, Serbia. *Corresponding author / Autor correspondente:
[email protected]
1. Introduction In the original paper Alvarez et al. (1980) have first reported anomalously high Ir concentrations in the Cretaceous-Paleogene (KPB) boundary clays at Gubbio (central Italy, Fig. 1), Højerup (the eastern Denmark, Fig. 1) and at Woodside Creek (the northern part of the South Island of the New Zealand, Fig. 1). They proposed an impact of extraterrestrial bolide to explain the elevated Ir content at the KPB. Alvarez et al. also suggested that approximately 60 times the mass of the impactor would have been ejected into the atmosphere as impact dust. Almost simultaneously with Alvarez et al., Smit & Hertogen (1980) reported an anomalous Ir in the boundary clay at Caravaca (southeastern Spain, Fig. 1). Since the Alvarez et al. discovery, the KPB has
been identified and studied at about 345 sites worldwide. Of these, 85 sites, representing all depositional environments, contain an Ir anomaly. In general, Ir and other platinum-group elements (PGE: collectively the elements Ru, Rh, Pd, Os, Ir and Pt) are invariably enriched in the prominent boundary clays. Other trace elements (e. g. heavy metals) are also relatively abundant in these clays. Many researchers consider that the KPB impactor formed the ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig. 1). It has been suggested that the impactor was a carbonaceous chondrite projectile - probably type CI (Kyte, 1998; Shukolyukov & Lugmair, 1998; Frei & Frei, 2002; Quitté et al., 2003; Trinquier et al., 2006), though it is still unclear whether it was a carbonaceous chondrite or a comet. Indeed, comets are believed to be primitive bodies with a composition like that of carbonaceous chondrites (Gelinas et al., 2004). The anomalous Ir associated, however, with the prominent boundary clay is consistent with the high Ir content typical of most chondritic meteorites and inconsistent with the general proposition of comets. Indeed, a simple calculation shows that in the case of the ice-rich (>70 %) comets the amount of Ir produced by an impact energy for a crater of the Chicxulub size could be less than 0.001 % then that of an asteroid. In the past many researchers used the iridium concentration and/or integrated amount of Ir to estimate the proportion of chondritic component in the renowned boundary clays. This paper aims to re-examine this method using comprehensive Ir data for the boundary sections at Højerup, Caravaca and Woodside Creek which are available and published by Schmitz (1988). These stratigraphic sections are well preserved and distal (paleodistance: >7000 km) to the proposed Chicxulub impact site. An Ir analysis of the decarbonated KPB samples from Denmark, Spain and New Zealand was carried out by Schmitz using instrumental neutron activation analysis (INAA). Relative error in the precision of the analyses ranges from 5 % to 10 %. Total uncertainties (including accuracy errors) were up to 20 %. For the sake of completeness, the Ir data related to the distal KPB sections at Agost (Spain), Flaxbourne River (New Zealand), Gubbio (Italy), Bidart (France), El Kef and Aïn Settara (Tunisia), and in Ocean Drilling Program (ODP) Hole 738C (Kerguelen Plateau, southern Indian Ocean) (Fig. 1) are also briefly discussed. Although the sections studied systematically show an Ir abundance anomaly, they differ from one another because local sedimentation conditions under a strong continental influence. Throughout this paper we make five reasonable postulates: (a) Ir
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is wholly located in the non-carbonate fraction of the boundary clays studied, i. e. the carbonate fraction of this section is essentially an Ir diluents; (b) the Ir content of the carbonaceous chondrites range from 406.0 ppb to 849.4 ppb (Table 1); (c) the average content of Ir in CI chondrites is 465 ppb (Table 1); (d) all Ir found in the boundary clays originated from the carbonaceous chondrite (CC) impactor, especially of type CI; and (e) assumed density of the boundary clays is about 2 g cm-3.
P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
carbonaceous chondrites typically contain 406.0 - 849.4 ppb Ir (Table 1). So, a little addition of the CC component would be necessary to critically increase the concentration of Ir in a marine sedimentary rock. Thus, Ir is a very sensitive means of examining the CC contribution to the marine KPB clays such as the Fish Clay. Some authors suggested that Ir in the Fish Clay was sourced by the Ir from seawater (Goldberg et al., 1986). However, the average Ir concentration in the decarbonated BH (ca. 100 ppb, see below) represents an enrichment factor of about 1017 compared to seawater (ca. 2×10-15 ppb, Table 1). Moreover, according to these authors, the residence time of Ir in the oceans is about 1 million years. Thus, excess of Ir in the BH could not, therefore, have been derived from the sea reservoir. Table 1. Concentrations of Ir [ppb] for carbonaceous chondrites, marine sediments and seawater. Tabela 1. Concentrações de Ir [ppb] para os condritos carbonáceos, sedimentos marinhos e água do mar.
Fig.1. Geographic location of studied KPB clays. Fig.1. Localização geográfica das argilas KPB estudadas.
2. Results and Discussion Fish Clay. The lowermost Danian Fish clay Member of the Rødvig Formation near the village of Højerup is a classic marine KPB section. The lithology of the Fish Clay within this boundary section characterizes three distinctive layers (from bottom to top): a 3 cm thick (mainly black-to-dark) marl (hereinafter BH) with 0.5 cm-thick basal red (goethite-rich) sublayer, grey-to-brown marl and a light-grey marl (Fig. 2). The red sublayer is underlain by (the latest) Maastrichtian bryozoan-rich limestone (chalk) whereas the top marl is overlain by (early) Danian Cerithium limestone (Schmitz, 1988; Christensen et al., 1973; Schmitz, 1985; Elliott, 1993; Surlyk et al., 2006). Geochemical studies show that the Ir profile (on a whole rock basis) across the Fish Clay column is characterized by a maximum just above the base of BH with an upward gradual decrease (tailing-off) from its maximum (e. g. Schmitz, 1985). This gradual tailing off indicates that the Fish Clay represent a relatively continuous and complete section. The BH is considered to constitute the main part of the boundary section, since it contains more than 95 % of the total Ir in the Fish Clay (Premović, 2009). The mineralogy of BH is comparatively simple, smectite and authigenic (mainly biogenic) calcite being the principal components. Geochemical evidence indicates that the BH was deposited under strong anoxic conditions but the red sublayer under strong oxic sedimentation conditions (Premović, 2009). Premović et al. (2000) inferred that the BH was deposited in a shallow marine
