Abstract: The chemical weathering of tropical volcanic island/arc plays an important role in global carbon cycle and Sr isotope evolution of seawater. In this paper, a comprehensive study of river water, groundwater, rainwater, bedrock and sediment was carried out in some small basaltic and granitic watersheds in Hainan Island, China. The element contents and 87Sr/86Sr ratios of water and solid samples were analyzed. The results show that there is a certain relationship between the chemical alteration index (CIA) and ⁸⁷Sr/⁸⁶Sr, which is mainly attributed to the different stages of weathering: in the early stage dominated by biotite weathering, ⁸⁷Sr/⁸⁶Sr is relatively high. After that, plagioclase begins to be weathered and ⁸⁷Sr/⁸⁶Sr decreases gradually. In the middle weathering stage, after a large amount of plagioclases are decomposed, the weathering of various K-rich minerals is intensified, and the ⁸⁷Sr/⁸⁶Sr value rises to the highest point. In the stage of high weathering degree, the stable weathering residual materials are dominant as the decrease of K-rich minerals, and the ⁸⁷Sr/⁸⁶Sr value declines gradually. Compared with the young active volcanic islands, for example, the Lesser Antilles Islands in the Caribbean Sea and the Reunion Island in the Indian Ocean (the subsurface chemical weathering rate is 2-5 times higher than the surface weathering rate), the underground water flow and total dissolved solids (TDS) in Hainan Island are much lower due to the relatively low rainfall and the low porosity of the old inactive volcano. As a result, the subsurface chemical weathering rate of Hainan Island is lower than that of the surface weathering, only close to the Kamchatka active volcanic island in high latitude, 6%-25% of the Lesser Antilles and Reunion Islands in tropical zone, and is one of the regions with the lowest contribution of subsurface weathering in the world. The surface chemical weathering and CO₂ consumption rates of basalt area in Hainan Island are higher than those of Massif Central, France and Siberia, slightly lower than those of Hawaii and Deccan Trap, and significantly lower than those of Java and Luzon Island in Southeast Asia. Under the condition of similar temperature, runoff has an obvious control over the chemical weathering rate. The CO₂ consumption capacity of Hainan Island in the tropical region is only in a global average range due to the low annual runoff.

Key words: Hainan Island, Sr isotope, incongruent weathering, surface chemical weathering, subsurface chemical weathering;
CO2 consumption