[B-PT27_28AM2] Phanerozoic biodiversity change: Extinction and diversification

[BPT27-02] Platinum group element anomalies in the Triassic-Jurassic deep-sea sediments

*Wataru FUJISAKI¹, Yusuke SAWAKI¹, Shinji YAMAMOTO², Tetsuya YOKOYAMA¹, Shigenori MARUYAMA³ (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.The University of Tokyo, 3.Earth-Life Science Institute, Tokyo Institute of Technology)

Keywords:T-J boundary mass extinction, deep-sea sediments, platinum group element

One of the biggest mass extinctions in the Phanerozoic occurred at the Triassic-Jurassic (T-J) boundary. The large magmatic activity associated with the breakup of Pangaea (CAMP event) or a bolide impact attract interests as causes of the mass extinction at the T-J boundary. However, the cause of the mass extinction is still controversial because of insufficient geological evidences. PGE abundances and radiogenic Os isotope ratios are powerful tracers that potentially distinguish ancient basaltic magmatism from the effect from extraterrestrial. We conducted detailed geological survey at the Inuyama area, where Triassic to Jurassic deep-sea sediments well crop out. We developed detailed a geological map of the study area and reconstructed ocean plate stratigraphy. We collected ca. 70 siliceous shale samples bed-by-bed were also collected to measure PGEs concentration and Os isotopes with a high spatial resolution. The rock powder was spiked with ¹⁹⁰Os, ¹⁸⁵Re, ¹⁹¹Ir, ⁹⁹Ru, ¹⁹⁴Rt, and ¹⁰⁵Pd and digested by 2:1 mixture of HNO₃ and HCl in a sealed Carious tube at 240oC for 48 hours. After chemical separation using an anion exchange resin, the isotope ratios of PGE were measured by a quadrupole type ICP-MS at Tokyo Tech. The Os isotope ratios were determined by N-TIMS (Triton plus) at Tokyo Tech. PGEs concentrations and Os isotope composition are determined from 28 siliceous shale samples across the T-J boundary. Re and Os contents varies from 14.7 to 128.6 pg/g and from 4.9 to 99.2 pg/g, respectively. ¹⁸⁷Os/¹⁸⁸Os decreases from 0.77 to 0.34 before the T-J boundary. The ¹⁸⁷Os/¹⁸⁸Os values in the Jurassic siliceous shales fluctuated around ca. 0.5. The highest Os concentration and negative Os isotope anomaly corresponds to the first occurrence of Jurassic type radiolarian. Also, Ir/Pt vs Pd/Pt cross plot and C1 chondrite-normalized PGE patterns of siliceous shales across the T-J boundary show similar trend to CAMP and upper continental crust (UCC). This indicates that the origin of PGEs detected from siliceous shales are the mixture of CAMP and UCC, and that extraterrestrial influence at the T-J boundary was minor.