Distribution, speciation and mobility of metals in sediments of the Tianxiu hydrothermal field, Carlsberg Ridge, Northwest Indian Ocean

Tianxiu is the only ultramafic-hosted hydrothermal field that has been discovered on the slow-spreading Carlsberg Ridge (CR), northwest Indian Ocean. Sequential extraction has been performed on the surface sediments from the near-vent and far-field of Tianxiu to characterize the compositions of metalliferous sediments, the speciation of selected metals and the potential risks of metal mobility. The near-vent sediments of Tianxiu are mainly derived from hydrothermal inputs and are enriched in Fe, Zn, P, Pb, Mo and especially Cu, Mg, Co and Ni with significantly higher contents compared to metalliferous sediments formed in basalt-hosted hydrothermal systems. Most of the ore-forming elements (Fe, Cu, Zn, Mn, Co and Ni) and rare-earth elements (REEs) exist in the residual fraction and FeMn oxide fraction, followed by the carbonate fraction and organic fraction. The chondrite-normalized REE pattern in each phase is characterized by relative enrichment in light (L)REEs and positive Eu anomalies, consistent with the REE patterns of hydrothermal fluids. The far-field sediments of Tianxiu are mainly derived from background pelagic sedimentation while the sediments at some sites are influenced by a small amount of hydrothermal plume fallout. The investigated elements mainly exist in the FeMn oxide fraction, residual fraction and carbonate fraction. In particular, ore-forming elements Cu, Pb, Mn, Co and Ni are mostly enriched in the FeMn oxide fraction, which is attributed to their adsorption by hydrothermal particulates in the plumes. The potential mobility of each metal is evaluated in terms of its total content in the unstable phases, which reveals that most of the heavy metals (e.g., Cu, Zn, Mn and Pb) in the metalliferous sediments have high potential mobility (> 50%).