Altered evolution sequence of abyssal peridotites from the Tianxiu hydrothermal field in the northwestern Indian Ocean: Evidence from petrochemistry and mineralogy

The alteration of ultrabasic rocks in hydrothermal fields plays a critical role in regulating regional geochemical cycles and mineral evolution. However, the detailed processes and mechanisms governing such alteration remain poorly understood. We investigated ultrabasic rocks collected from various locations in the Tianxiu hydrothermal field in the northwestern Indian Ocean. By integrating analysis results in petrography, whole-rock geochemistry, and electron micro-probing, the petrochemical and mineralogical features were clarified and the evolution of the rock alteration was reconstruct. Results indicate that the ultrabasic rocks in the hydrothermal field had undergone extensive alteration. The dominant secondary mineral assemblage includes serpentine minerals (antigorite, chrysotile, and lizardite), accompanied by magnetite, chlorite, iddingsite, calcite, and other alteration products. The alteration history could be divided into two stages. In Stage I, olivine and pyroxene experienced typical serpentinization under relatively closed-system conditions; in Stage II, olivine subsequently underwent iddingsitization in an open ad oxidizing environment. During the serpentinization, substantial Fe and Ca were mobilized and enriched in the secondary minerals such as magnetite and carbonate, suggesting that the original olivine was completely replaced by high-Mg serpentine. In the later iddingsitization stage, Fe enrichment along with Si and Mg mobilization played a catalytic role in the formation and evolution of iddingsite and associated iron oxides. These findings reveal a complex and multi-stage alteration history of deep-sea peridotites in the Tianxiu hydrothermal field and provide valuable insights into mineral transformation mechanisms and element mobility in such settings.