Magma degassing of ore-metals into submarine hydrothermal systems: a case study from the Xunmei hydrothermal field, South Mid-Atlantic Ridge

The Xunmei hydrothermal field, located at 26°S along the South Mid-Atlantic Ridge, is an active submarine hydrothermal system underlain by a basaltic substrate. This field comprises two distinct types of basalts: massive basalts, characterized by aphyric to moderately porphyritic textures without large vesicles, and vesicular basalts, known for their highly vesicular nature. Olivine-hosted melt inclusions within the massive basalts exhibit a diverse range of chemical compositions. Type-A melt inclusions are distinguished by lower levels of K₂O, Rb, Ba and U, but higher concentrations of S, Co, Ni, and Cu. Conversely, Type-B melt inclusions exhibit higher levels of K₂O, Rb, Ba and U, but lower concentrations of S, Co, Ni, and Cu. Although both types of melt inclusions show similar ranges of La/Sm, La/Yb, Sr/Yb, and Zr/Nb, the significant differences in K₂O/TiO₂ and Nb/U indicate that the massive basalts likely originate from the mixing of two distinct melts derived from different source regions. Data from melt inclusions and quenched basaltic glasses, combined with theoretical calculations, indicate that Type-I melts, represented by the Type-A melt inclusions, were sulfide-saturated during the crystallization of olivine at depth, evolving into sulfide-unsaturated melts as they ascended towards the seafloor. Approximately 50% of the Cu in the Type-I melts transitioned to the gas phase and were eventually released from the magma to the overlying hydrothermal system. Conversely, Type-II melts, represented by the Type-B melt inclusions, did not reach sulfide saturation. The presence of magmatic sulfides within or attached to vesicles, occupying voids in the primocryst frameworks, and lining the walls of vapor bubbles in melt inclusions, may suggest a volatile-driven transport of magmatic sulfides from the magma system as compound drops during magma degassing. This mechanism likely plays a crucial role in the supply ore-metals during the formation of seafloor massive sulfides in the Xunmei and possibly other hydrothermal fields along mid-ocean ridges.