Mineralogy and chemistry of cored sediments from active margin off southwestern Taiwan

Abstract

The cored sediments sampled by R/V Marion Dufrense are mostly muds consisting mainly of quartz, feldspar, illite, chlorite + kaolinite and calcite. Authigenic carbonates mainly composed of aragonite, calcite, dolomite and associated with Fe-montmorillonite and pyrite occur in core samples collected from station MD-052911 (22°15.6'N, 119°51.0'E) at 2137-2140 cm and 2237-2240 cm depth which may be formed via sulfate reduction by CH₄. In general Illite/Quartz intensity ratios of the sediments show little variation with core depth indicating the relatively constant abundance of illite in the source rock on Taiwan. The cored sediments have higher average Al₂O₃, ΣFeO and MgO but lower SiO₂, Na₂O, and CaO contents when compared with upper continental crust (UCC). High field strength elements (Zr, Hf, Y, Nb and Ta) are also depleted in the cored sediments. CaO, Sr, Mn and Pb in the core samples collected from station MD-052912 (22°21.5'N, 119°48.5'E) tend to decrease with depth which may be essentially related to the decrease of biogenic CaCO₃ with core depth. The La/Th, La/Sc, Th/Sc ratios of the cored sediments are similar to those of UCC, however the (La/Yb)_N ratios of the cored sediments are higher. The cored sediments display similar REE patterns with LREE enrichment and negative Eu anomaly reflecting a felsic nature of the source rock which can also be identified in the La-Th-Sc plot. The chemistry of the sediments can be deduced using a mixing model involved four end members i.e., shale, greywacke, quartzite and limestone.