The noble gas isotope geochemical composition of chert at the bottom of Cambrian in Tarim Basin, China

Abstract

In the Tarim Basin, black shale series at the bottom of Cambrian is one of the important marine facies hydrocarbon source rocks. This research focuses on the analysis of the isotope of noble gas of 11 cherts. The R/R a ratio of chert in the Keping area is 0.032-0.319, and 40 Ar/ 36 Ar is 338-430. In Quruqtagh the R/R a ratio is 0.44-10.21, and 40 Ar/ 36 Ar is 360-765. The R/R a ratio of chert increases with 40 Ar/ 36 Ar from the west to the east accordingly. They have evolved from the crust source area to the mantle source area in a direct proportion. Surplus argon 40 Ar E in chert is in direct proportion to the R/R a ratio, indicating that it has the same origin of excess argon as in fluid and mantle source helium. Comparison of the R/R a ratios between the west and the east shows that the chert in the eastern part formed from the activity system of the bottom hydrothermal venting driven by the mantle source, where the material and energy of crust and mantle had a strong interaction in exchange; whereas in the western part, chert deposited from the floating of hydrothermal plume undersea bottom, which is far away from the centre of activities of the hydrothermal fluid of ocean bottom. In addition, from noble gas isotope composition of chert, it is suggested that the ocean anoxia incident happened at the black shale of the Cambrian bottom probably because of the large-scaled ocean volcanoes and the following hydrothermal activities.Tarim Basin, Cambrian, chert, rare gas isotope Chert is widely spread in a long time span. It may appear under various circumstances from the Archean to modern ocean bottom hydrothermal venting area and the terrestrial hot spring area. It seems that from the formation cause, the older the chert is, the more advantageous the chemical cause of formation would be. However, the biological cause of formation gradually gained advantage since Mesozoic [1] . The cause of formation of marine facies black shale is always being concerned by geologists, but opinions differ, such as the cause of the accident of anoxia. Silica is one of the important rock of black shales [2][3][4][5][6] . It can reflect the ancient environment, the ancient structure, the source of mineralization and other information, etc., because it had a strong resistance against later reconstruction and formed under certain geochemical conditions. So it attracts a wide range of attention from geologists. This paper tries to probe into the construction activities and the interaction between crust and mantle at that time through the research on the rare gas isotope of chert from the black shale at the bottom of Cambrian in the Tarim Basin.

Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin, Northwest China

“…This interpretation is corroborated by geochemical evidence from this bedded chert horizon in which trace element (i.e. barium and vanadium) enrichments, positive Eu anomalies, and surplus 3 He and 40 Ar isotopes suggested significant contributions from hydrothermal fluids during deposition (Sun et al 2009;Yu et al 2009).…”

“…These include the Yurtus Formation in the Aksu area, northwest Tarim Basin (Yu et al 2009) and the equivalent Xishanbulake Formation in the Quruqtagh area, northeast Tarim Basin (Yao et al 2005). Geochemical studies on the Yurtus and Xishanbulake bedded cherts suggest significant silica contributions from submarine hydrothermal venting (Yang et al 2006;Sun et al 2009;Yu et al 2009). However, direct geological evidence for such a submarine hydrothermal system has not been reported yet in this area.…”

Zircon geochronology, geochemistry and stable isotopes of the Wang’ershan gold deposit, Jiaodong Peninsula, China

“…The standard sample (air) for the experiments was from the summit of Gaolan Mountain, Lanzhou, China. The analytical precision for the noble gas isotopic measurements is better than 10%, adopting procedures reported in earlier studies (Ye et al, 2007;Sun et al, 2009). …”