Search for a meteoritic component in impact-melt rocks from the Lonar crater, India - Evidence from osmium isotope systematics

Introduction: The Lonar crater in western India (Maharashtra) is a bowl-shaped simple impact structure of 1830 m diameter and a depth of 120 m below the rim crest. The crater formed 0.656 × 0.081 Ma ago on the 65 Ma old basaltic lava flows of the Deccan Traps (Jourdan et al. 2010) and is one of the few terrestrial impact structures to have formed in basaltic host-rocks. In the absence of actual meteorite fragments, the impact origin of this structure was supported by the identification of a variety of shock metamorphic features (e.g. Fredriksson et al. 1973). However, clear indications of an extraterrestrial component in impactites based on geochemical studies are absent or remained ambiguous so far (e.g. Osae et al. 2005). As the Os isotope tool has the potential to provide firm constraints on the presence or absence of even very small (<<1%) contributions of meteoritic matter to impactite lithologies (e.g. Koeberl et al. 2002), we conduct a detailed Os isotope study of a variety of unshocked host-basalts (target rocks) and impactites (impact-melt rocks) from the Lonar crater. Samples and Method: All samples analyzed in this study were collected in 2000 and 2001 and were geochemically characterized by Osae et al. (2005). Osmium (and additional PGE) analyses were performed on about 2 g whole rock powders, which were spiked with a mixed 190Os,185Re,191Ir,194Pt tracer, and digested via high pressure Asher using inverse aqua regia. Osmium solvent extraction and microdistillation were performed as described by Cohen and Waters (1996). Osmium isotopic compositions were measured using a TRITON N-TIMS at the Department of Lithospheric Research in Vienna. Results and Discussion: Osmium data on seven target and nine impact melt rocks reveal 187Os/188Os ratios ranging from ~0.38 to ~2.23 for the target rocks and from ~0.22 to ~0.59 for the nine analyzed impact melt rocks, whereas Os concentrations range from ~7.1 to ~31.6 ppt and ~7.2 to ~134 ppt, respectively. Although in the typical range for basalts from the Deccan Traps (e.g., Allegre et al. 1999), our data clearly point toward an enrichment of Os in impact melt rocks compared to the host basalt. Detailed petrographic as well as geochemical investigations (Osae et al. 2005) found no evidence for a possible mantle contamination that could also account for the elevated Os concentrations and low 187Os/188Os signatures in the impact-melt rocks. Instead, the data for the impact-melt rocks show a good fit to a calculated curve generated for mixing between chondritic and basaltic end-members. However, further information about the type of a meteoritic component in the impact melt rocks form the Lonar structure has to await detailed PGE (Re, Ir, Pt) characteristics that will be presented at the conference. References: Allègre et al. (1999) EPSL 170, 197-204; Cohen and Waters (1996) Analytica Chimica Acta, 332, 269-275; Fredriksson et al. (1973) Science 180, 862-848 ; Jourdan et al. (2010) 41st LPSC 1661; Koeberl et al. (2002) GSA Special Paper 356, 607-617; Osae et al. (2005) MAPS 40, 1473-1492