Geochemistry Of Modern Seafloor Hydrothermal System Of The Southwest Indian Ridge

Submarine hydrothermal activity, hydrothermal sulfide minerals and biological resources are a major research area in the current domestic and international marine science, which not only have important scientific significance, but also have important economic value. But the degree of current human development and utilization of marine resources is still low, the development of metallic minerals on the seabed is almost none. This situation attracts human’s attention on speeding up research and development of marine resources.Samples used in this research are from the Fifth Leg, China’s Ocean Voyage 20th and the 21th, in the Southwest Indian Ocean Ridge. Ultra-slow spreaded Southwest Indian Ridge（SWIR）is the boundary of Antarctic plate and Africa plate. It is from the Rodrigues Triple Junction （RTJ） to Bouvet Triple Junction（BTJ）. Select basalts, peridotite, sulfide smoker （black smokers） and opal smokers （white smokers） to do the analysis of elements （major elements and trace elements）, fluid inclusions, stable isotopes （hydrogen and oxygen isotopes）.According to major elements, basalts are calc-alkaline basalts, specifically is low potassium tholeiitic series. According to the Rare Earth Elements（REE）, basalt samples have obvious characteristics of oceanic tholeiite, REE curves of smokers are the same or similar with peridotite, basalt samples and seawater, and have the same elements of loss trends and characteristics.Fluid inclusions were found in opal. Gas-liquid inclusion is the main type and pure liquid inclusions could be seen. Fluid inclusions are generally small, most of them are <5μm and the maximum size is up to 30μm. Liquid ratio is > 60%. Range of freezing points is -6.9℃ -2.3℃and the average is -4.78℃. Correspondingly, salinity range is 3.87% 10.36% and the average salinity is 7.49%. Homogenization temperature range is 61.8℃ 192℃. High salinity suggests characteristic of the magma. Liquid compositions of fluid inclusions show that Na⁺, K⁺ are main in ore-forming fluid and Ca²⁺, Mg²⁺ are little. Anions are Cl^- and SO₄^2- and SO₄^2- is main, followed by Cl^-; F^- can not be detected in the results. Overall total anions SO4_<sup>2- content account for 93.6 96.7%, which is rich in sulfur-based solution. High SO4_<sup>2- content is due to the dissolution of oxidation of sulfide minerals and anhydrite. According to w （Na⁺） / w （K⁺） and w （Na⁺） / w (Ca²⁺ + Mg²⁺) values of samples and the Cl^- concentration, it can be detected that magmatic water and seawater are end-members.δ¹8O_V-SMOW‰values of samples are within the range of magmatic water.δD_V-SMOW‰value of DY115-20V-TVG2 is within the range of magmatic water, butδD_V-SMOW‰values of DY115-20V-TVG7 and DY115-20V-TVG13 are very low, which may be due to magma degassing and interaction of organic matter and the fluid.It is inferred that magma degasses and black smokers and opal smokers eruptted through the deep-sea sediments and reacted with organic matters in the process.