Platinum-group element (PGE) geochemistry of Deccan orangeites, Bastar craton, central India: implication for a non-terrestrial origin for iridium enrichment at the K–Pg boundary

Abstract

We report platinum group element (PGE) concentrations of twelve bulk-rock samples from the Behradih and Kodomali orangeite intrusions in the Mainpur field, Bastar craton, central India, which are emplaced synchronously with the Deccan flood basalts. Their palladium-group PGE (PPGE) (1.8–5.2 ppb Pt, 1.2–6.4 ppb Pd) contents are distinctly higher compared to their iridium-group PGE (IPGE) concentrations (0.8–2 ppb Os, 0.8–1.2 ppb Ir, 3.2–4.2 ppb Ru, and 0.2–0.8 ppb Rh). Their PGE contents as well as Pd/Ir ratios are either similar or even lower than those from the Mesoproterozoic and Cretaceous kimberlites and orangeites from the Kaapvaal craton (southern Africa), Cretaceous kimberlites from the Sao Fransisco craton (Brazil), Ordovician kimberlites from the North China craton and the Mesoproterozoic southern Indian kimberlites from the Eastern Dharwar craton. Anomalously elevated iridium (and other PGE) contents in sediments at the Cretaceous–Paleogene (K–Pg) boundary are commonly attributed either to a large bolide impact triggering the K–Pg mass extinction or to terrestrial causes such as volcanic eruptions (Deccan flood basalts) or even to mantle-plume derived lithospheric gaseous explosions (Verneshots). Lack of unusually high abundances of PGE in the Mainpur orangeties as well as in the co-eval Deccan flood basalts and associated alkaline rocks implies that the anomalous iridium enrichment reported at the K–Pg boundary sections was not sourced from the mantle and likely originated from an extraterrestrial source.