Traces of an H chondrite in the impact‐melt rocks from the Lappajärvi impact structure, Finland

Abstract

Abstract-Here we present the results of a geochemical study of the projectile component in impactmelt rocks from the Lappajärvi impact structure, Finland. Main-and trace-element analyses, including platinum group elements (PGEs), were carried out on twenty impact-melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajärvi impact structure. The PGE composition of the highly diluted projectile component (~0.05 and 0.7 wt% in the impact-melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.

Projectile Identification in Terrestrial Impact Structures and Ejecta Material

“…Earlier studies used the 54 Cr/ 52 Cr chondrite for Lappajärvi (Finland; Tagle et al, 2007). The 25 cm ordinary chondrite fragment discovered within the impact melt sheet of the Morokweng impact structure (Maier et al, 2006) conclusively supports the PGE-based projectile identification carried out by McDonald et al (2001).…”

“…Nevertheless, over the last few years the situation has improved significantly (e.g. McDonald et al, 2001;McDonald 2002;Tagle and Claeys, 2005;Koeberl et al, 2007;Tagle et al, 2007;Goderis et al, 2009;Simonson et al, 2009). …”

“…Generally between 7 and 14 ratios are used, supplemented with four moderately siderophile element ratios (Ni/Ir Cr/Ir, Co/Ir and Ni/Cr) and Au/Ir (e.g. McDonald, 2002;McDonald et al, 2001McDonald et al, ,2009Claeys, 2004,2005;Tagle et al, 2007Tagle et al, ,2009Goderis et al, 2009). Figure 15.5a-c illustrates the discriminative value of the different ratios according to Tagle and Berlin (2008).…”

“…15. 4a-c (McDonald et al, 2001;Tagle and Claeys, 2005;Tagle et al, 2007) and compared with the mean ratios of the different types of meteorites, compiled in siderophile element databases (e.g. McDonald et al, 2001;Tagle and Berlin, 2008;Tagle et al, 2009;Fischer-Gödde et al, 2010).…”

“…Morgan This method has been applied to identify the projectiles responsible for the formation of the Morokweng, East Clearwater, Popigai, Lappajärvi, Wanapitei, Rochechouart, Sääksjärvi and Gardnos impact structures, as well as the 2.63-2.49 Ga impact spherule layers in Western Australia and South Africa (e.g. McDonald et al, 2001;McDonald, 2002;Tagle and Claeys, 2005;Tagle et al, ,2007Tagle et al, ,2009Goderis et al, 2009;Simonson et al, 2009). The identification of the projectiles of the Morokweng, Popigai and Lappajärvi impact structures are shown in Fig.…”