REE geochemistry of marine oil shale from the Changshe Mountain area, northern Tibet, China

The Shengli River–Changshe Mountain oil shale zone, located in the North Qiangtang depression, northern Tibet plateau, represents a potentially large marine oil shale resource in China. The contents and distribution patterns of rare earth elements (REEs) in selected oil shale and micritic limestone samples from the Changshe Mountain area were studied by inductively-coupled plasma mass spectrometer (ICP-MS). Analyzed oil shale samples from the Changshe Mountain area are characterized by high total organic carbon (TOC) contents (7.02–16.32%) and ash yields (53.22–82.12%) with shale oil contents from 3.85% to 11.76%. The total rare earth element (∑ REE) contents in oil shale samples are 68.19 μg/g, close to those of US coals, and higher than those of micritic limestone samples (37.69 μg/g) from the Changshe Mountain area, but lower than those of world-wide black shales, common Chinese coals, and North American Shale Composite. There are two types, A and B, of distribution patterns of REEs in the Changshe Mountain oil shale samples. Type A shows negligible Ce anomalies (0.97–1.01), with slightly higher ∑ REE concentrations (36.87–118.38 μg/g) and LREE/HREE (6.79–10.74) and (La/Yb)n (6.68–8.36) ratios, whereas type B exhibits a slightly negative Ce anomaly (0.84–0.88), with slightly lower ∑ REE concentrations (13.73–15.31 μg/g) and LREE/HREE (5.54–6.34) and (La/Yb)n (5.03–6.65) ratios. Both types A and B oil shales are characterized by distinctly sloping LREE trends (Lan/Smn = 3.60–5.44) accompanied by flat HREE trends, with distinct Eu negative anomalies (0.51–0.69). The vertical variations of ∑ REE contents are similar to those of ash, Si, Al, K, Na, Ti and Fe, and show a negative correlation with organic sulfur and organic carbon, indicating that the REE contents in oil shale seams are mainly controlled by land-derived detritus.