Per. Mineral. (2006), 75, 2-3, 11-24 http://go.to/permin An International Journal of O PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Determination of the origin of unusual glass with metallic spherule inclusions found in the area between Inzingkofen and Sigmaringen (Bavaria, Germany), South-West of the Steinheim-Ries craters Matteo Ardit,1 Anna Maria Fioretti,2* Gianmario Molin,3 Emilio Ramous4 and Ulf-Christian Bauer5 1 Centro di Ateneo per i Musei, via Orto Botanico, 15 – 35122 Padova (Italy) 2 Istituto Geoscienze e Georisorse - CNR, c.so Garibaldi, 37 – 35137 Padova (Italy) 3 Dipartimento di Mineralogia e Petrologia, Università di Padova, c.so Garibaldi, 37 – 35137 Padova (Italy) 4 Dipartimento di Innovazione Meccanica e Gestionale, Università di Padova, via Venezia, 1 – 35139 Padova (Italy) 5 Seestrasse, 46a – 83727 Schliarsee (Germany) Abstract. — This paper reports the characteristics Riassunto. — In questo lavoro vengono studiati of a suite of pale green glass fragments and their diversi frammenti di vetro verde chiaro aventi metallic spherule inclusions (5 to 400 μm) found la particolarità di contenere inclusioni di sferule over a relatively wide area between Inzigkofen and metalliche (di dimensioni variabili tra 5 e 400 Sigmaringen (Bavaria, Germany), about 150 km SW μm), ritrovati in un’area relativamente vasta tra of the Steinheim-Ries impact craters. The distribution Inzigkofen e Sigmaringen (Baviera, Germania), a area of these glasses and their close macroscopic ~150 km SW dai crateri di impatto di Steinheim- similarity (apart from the spherules) with moldavite Ries. La distribuzione areale di questi vetri e la loro suggested us that they may shed new light on the nature stretta somiglianza macroscopica con le moldaviti of the binary asteroid which caused the Steinheim- (ad eccezione che per le sferule) ci suggerirono Ries impact craters. However, our petrographic and che essi potessero contribuire a chiarire la natura chemical data rule out an extra-terrestrial origin. dell’asteroide binario che generò i crateri di Metallographic microstructure, lack of Ni and high P Steinheim-Ries. I nostri dati chimici e petrografici contents (up to 9 wt.%) reveal the metallic inclusions escludono tuttavia un’origine extraterrestre dei vetri. as cast iron spherules. The chemical composition of La microstruttura metallografica, l’assenza di Ni e the host glass consistently differs from both moldavite l’elevato contenuto in P (anche superiore al 9 wt.%) and suevite compositions. Our data indicate that the indicano che le inclusioni metalliche sono sferule di investigated samples are by-products of metallurgic ghisa. La composizione chimica dei vetri in studio activity. This conclusion is consistent with the differisce sistematicamente da quella delle moldaviti presence in the Sigmaringen area of important cast e della suevite. I nostri dati indicano che i campioni iron furnaces which have been operating since the th studiati rappresentano pertanto un sottoprodotto di 18 century. attività siderurgica. Questa conclusione è avallata dal fatto che nelle vicinanze di Sigmaringen sono attivi Key Words: slag, cast iron, metallic spherules, altoforni operanti sin dagli inizi del XVIII secolo. moldavite, Ries crater. Parole Chiave: scoria di fusione, ghisa, sferule * Corresponding author, E-mail: [email protected] metalliche, moldavite e cratere di Ries. 12 M. Ardit, A.M. Fioretti, G. Molin, E. Ramous and U.-C. Bauer Introduction A wide variety of distinctive rock types (tektites, glass, breccias, melts, shock-metamorphosed In the early 1960s, one of the authors (UCB) target rocks) are actually produced during impacts. found several fragments of pale green glass with Glass and melted meteoritic metal spherules, metallic spherule inclusions (5 to 400 μm) in formed as a consequence of terrestrial impacts, a relatively wide area between Inzigkofen and have been found in association with young, well- Sigmaringen, near the Danube river, about 150 km preserved craters (e.g. Wabar, Saudi Arabia). SW of the Steinheim-Ries impact craters (Fig. 1). However, these spherules are not commonly Except for their inclusions, macroscopically the observed, perhaps because such ejected molten glass closely resembles moldavite (see later), a particles do not survive the disruptive effects of the group of tektites which were generated during the atmosphere, or because they are quickly destroyed Steinheim-Ries impact event. Although the area by weathering and erosion (French, 1998). One of where the glass fragments were found is exactly the main interests in studying these materials is opposite (with respect to the Steinheim-Ries that they may provide information on the nature craters) the moldavite distribution area (Fig. 1), and composition of the impactor. the close similarity of the new glasses with tektite The Steinheim-Ries craters originated about prevented us from excluding a priori that these 14.34 Ma ago (Laurenzi et al., 2003) in an materials were totally unrelated. area where minimal later erosion allowed the Fig. 1 – Map of Central Europe, showing Ries and Steinheim craters and moldavite strewn field (modified after Lange, 1996). Dashed lines (9 to 25.5°) define fan within which coherent melt patches are observed on inner slope of Ries crater rim. Moldavite strewn field extends about 450 km from the centre of the Ries crater to ENE forming a fan with an angle of about 57° (Stöffler et al., 2002). Determination of the origin of unusual glass with metallic spherule inclusions found ... 13 preservation of the impact products. The target to 7 wt.%), and H2O (up to 17 wt.%), consistent rocks consist of a sequence of sedimentary rocks with the melting of distinct protoliths. (limestone, shale, sandstone) (Tab. 1) underlain by In spite of the great amount of melted material crystalline rocks (gneiss, granite, and metabasite) produced during the Steinheim-Ries impact event, (Hüttner & Schmidt-Kaler, 1999; Stöffler et al., direct evidence for the composition of the projectile 2002). The impact produced melting of distinct is at present restricted to sub-micrometric Fe-Cr- target rocks generating moldavite, suevite (a Ni particles found in intragranular micro-fractures polymict breccia with clastic matrix and melts), in the crystalline basement underlying the crater and a variety of breccias and glass (Engelhardt, (El Goresy & Chao, 1976). On the basis of textural 1997; Osinski, 2003). relationships and particles composition, the above Moldavites have a typical green colour and vary authors suggested that the Ries impacting body in size from less to 1 mm (micro-tektites) to about was a stony meteorite. Subsequent studies (Morgan 20 cm. They have persilicic composition that is et al., 1979; Pernicka et al., 1987; Schmidt & constant, and characteristic for the strewn field Pernicka, 1994; Vennemann et al., 2001) were (French, 1998). To the best of our knowledge, unable to better specify the nature of the impactor. no metallic spherules have been reported within In this framework, the finding of the unusual moldavite. association of green glass including metallic The fall-out suevite is an impact breccia spherules, never before reported in the area, characterised by the presence of glass fragments may represent a new discovery of impact-related (Stoffler, 1974; Engelhardt, 1997). Four glass material, of paramount importance in shedding types are distinguished within suevite by Osinski new light on the nature of the impactor generating (2003) on the basis of their composition and the Steinheim-Ries craters, and may also indicate microtextures. They show wide ranges in SiO2 (50 the presence of a possible, still unidentified, third to 100 wt.%), Al2O3 (up to 21 wt.%), high CaO (up impact crater. Table 1 – Stratigraphy and lithologies of Ries pre-impact target (modified from Hüttner & Schmidt-Kaler, 1999). Stratigraphic Sections Thickness Subdivisions and Lithologies and Thickness (m) (m) Tertiary Middle Miocene = sands with minor clay and limestone 0-50 0-50 ε = massive limestone ∼20 δ = massive limestone 70-100 Malmian γ = bedded limestone with some marly limestone, marl and some reef limestone ∼30 150-200 β = mainly bedded limestone ∼20 α = marly limestone, marl and bedded limestone 40-50 γ-ζ = limestone, calcareous limestone and shale ∼10 Dogger β = sandstone, in part iron-rich ∼40 140-150 α = grey shale 90-100 Liassic Black shale with some sandstone and marl ∼30 30 Upper Keuper = red shale ∼30 Triassic Middle Keuper = sandstone with some shale ∼200 250-300 Muschelkalk = calcareous sandstone 0-50 Buntsandstein ∼15 several Hercynian Basement Various gneiss, granites and amphibolites kilometers 14 M. Ardit, A.M. Fioretti, G. Molin, E. Ramous and U.-C. Bauer An alternative possibility is that the glass derives IGG laboratory (Padova) on a CAMECA Camebax from human activity. Sigmaringen has been known Microbeam electron microprobe (EMP) fitted with for its metallurgic activity since the 18th century, four wavelength-dispersive spectrometers and one and vitreous materials are common by-products energy-dispersive spectrometer. For glass analyses, of this industrial process. Historical data report the system was operated at an accelerating voltage that in 1707, Meinrad II, Prince of Hohenzollern- of 15 kV and a beam current of 15 nA. A series of Sigmaringen, built a blast-furnace for iron natural (wollastonite for Si and Ca; corundum for extraction in Laucherthal (near Sigmaringen), Al; orthoclase for K; albite for Na) and synthetic after abandoning the methods for iron forging standards (Fe2O3 for Fe; MnTiO2 for Ti and Mn; which had long been used by his predecessors. In MgO for Mg; Cr2O3 for Cr) were employed. For the following years, iron production expanded and micro-spherule analyses, the system was operated another blast-furnace was built on the outskirts of at an accelerating voltage of 20 kV and a beam the town. These factories were both closed in 1878, current of 20 nA. A series of natural pure element because of a financial crisis. The Hohenzollern standards was employed.