Sulfur isotope composition

Sulfur is one of the important components of mantle fluid. Sulfur in mantle rocks often exists in the form of sulfide, and mineral components are mostly pyrrhotite, nickel pyrite and chalcopyrite. The main occurrence forms are (Xu Jiuhua et al., 2000): ① Early mechanical inclusions (sulfide particles captured by host minerals); ② Fracture fillings, distributed in grain boundaries or secondary fractures of minerals such as olivine; (3) Sulfide inclusions are different from mechanical inclusions, and are the products captured by host minerals after the molten fluid phase cools. The analysis objects of S isotope composition in the mantle mainly focus on the whole mantle rocks, sulfur-bearing minerals (mechanical inclusions and fracture fillings) and sulfides in sulfide inclusions (ion probe analysis). A large number of analytical data show that the S isotopic composition of meteorites, fresh basic rock deposits and ocean floor basalts is relatively uniform, and has no correlation with S content, and its δ34SCDT values are concentrated in the range of-1 ‰ ~+ 1 ‰ (Figure 1-27), such as mid-ocean ridges, famous areas and the Gaiman trough. The S content of submarine basalt is (800 100) × 10-6, and the δ34SCDT value is concentrated in (+0.8 0.5) ‰ (Sakai et al., 1984). Therefore, the δ34SCDT value of about 0‰ is considered to be the S isotopic composition of the mantle.

Fig. 65438 Histogram of +0-27 S isotopic composition of meteorites, basic rock deposits and basalts.

(quoted from Daines, 1989)

In recent years, more and more analysis results show that the S isotopic composition of basalts (especially continental basalts and island arc basalts) and their mantle xenoliths deviates from that of the mantle (Figure 1-28, Figure 1-29), and even the S isotopic composition of diamond sulfide inclusions. Eldridge et al. (199 1) systematically analyzed the S isotopic composition of diamond sulfide inclusions in eight kimberlite pipes in South Africa, and the δ34SCDT values ranged from-12‰~+ 14‰. At the same time, it is found that the S isotopic composition of diamond sulfide inclusions in kimberlite pipes is obviously restricted by the types of rocks produced. The Ni content of sulfide in P-type diamond sulfide inclusions is more than 8%, and its δ34SCDT value is concentrated around 0‰ (-5 ‰ ~+5 ‰), which is similar to the S isotopic composition of the mantle, while the Ni content of sulfide in E-type diamond sulfide inclusions is less than 8%, and its δ34SCDT value ranges from-12 ‰ to+14 ‰. The contamination of crustal materials is often used to explain the deviation of S isotopic composition of basalts (and mantle xenoliths). The negative deviation of δ34SCDT value can be regarded as the result of biological sulfide sediment pollution, and the positive deviation can be regarded as the result of marine sulfate sediment pollution. Therefore, the S isotopic composition of basalt and mantle can be used to indicate the recycling of crustal sediments, which provides geochemical evidence for plate subduction. Magmatic degassing can also explain the deviation of S isotopic composition of basalt. The theoretical results of Zheng et al. (1996) show that the sulfide loss in rocks caused by SO2 degassing of magma is 34S, and its δ34SCDT value can vary from 0 ‰ to 8 ‰. Due to the degassing of magma H2S, sulfide in rocks is relatively enriched for 34S, and its δ34SCDT value can vary from 0 ‰ to+6 ‰.

Fig. 65438 +0-28 S isotopic composition of mantle xenoliths and mantle-derived rocks

Figure 1-29 Comparison of Sulfur Isotopic Composition and Sulfur Content of Mantle-derived Rocks

(quoted from Dehong, 200 1)

It is worth noting that the existing form and mineral composition of S in carbonate rocks are quite different from those in other mantle rocks, and most of them exist in the form of sulfide and sulfate. Sulfides are commonly chalcopyrite, galena, pyrite and pyrrhotite, and sulfates are mainly barite. Sulfide and sulfate are often distributed in the grain boundaries or secondary cracks of carbonate minerals in the form of aggregates. The statistical results of Deines( 1989) show that the S isotopic composition of carbonate rocks is obviously different from other mantle-derived rocks. The δ34SCDT values of sulfates are all greater than 0‰, mainly between+4 ‰ ~+10 ‰, while the δ34SCDT values of sulfides vary widely between-23 ‰ and+5 ‰. Deines( 1989) also noticed that the S isotopic composition of carbonate rocks in different areas is obviously different, and the S isotopic composition of the same carbonate complex has different characteristics at different stages. In this regard, Daines (1989) thinks that the mantle is characterized by the heterogeneity of S isotope composition, the contamination of crustal materials during the evolution of carbonate rocks, and sulfide and sulfate may be the products of different stages of magmatic process (or post-magmatic process).

Figure 1-30 Histogram of Sulfur Isotope Composition of Carbonate Rock

(quoted from Daines, 1989)