Constraints for the causes of mass extinction at the Triassic-Jurassic boundary based on high precision platinum group element analyses

Abstract

The cause of mass extinctions during the Phanerozoic period has been a long standing problem in geology. The large magmatic activity (CAMP: Central Atlantic Magmatic Province) associated with the breakup of Pangaea or a bolide impact event would have led the mass extinction at the Triassic-Jurassic (T-J) boundary. However, the cause of the mass extinction is still controversial because of insufficient geological evidences. In this study, we conducted detailed geological survey of the Inuyama area where good exposure of deep-sea sediments across the T-J boundary is observed. We collected rock samples bed-by-bed to analyze the abundances of platinum group elements (PGEs) with a high spatial resolution. Twenty-eight samples were powdered and spiked with99Ru,105Pd,190Os,191Ir,194Pt. The sample was digested by a 2: 1 mixture of HNO3 and HCl in a sealed Carious tube at 240 ℃ for 48 hours. After chemical separation using an anion exchange resin, PGEs (except for Os) were measured by a quadrupole type inductively coupled plasma-mass spectrometry (ICP-MS). The PGEs concentrations were determined by the isotope dilution method. After solvent extraction with CCl4 and HBr, micro-distillation method was employed to purify Os. The Os isotopes were determined by Negative-Thermal Ionization Mass Spectrometry (N-TIMS). The relationship between Pd/Pt and Ir/Pt, as well as the PGE patterns, demonstrate that the high PGEs contents in shales can be explained by the incorporation of basaltic rocks supplied from CAMP. In addition, compared with fossil records, the highest PGE abundances (Os and Pd) are discovered from a shale just above the chert including the first appearance of Jurassic type radiolarian. These lines of evidence indicate that the CAMP volcanism would have contributed to the mass extinction at the T-J boundary.