Formation and origin of Fe–Si oxyhydroxide deposits at the ultra-slow spreading Southwest Indian Ridge

Abstract

Low-temperature hydrothermal systems are generally dominated by Fe–Si oxyhydroxide deposits. However, the formation process and mechanism of modern hydrothermal Fe–Si oxyhydroxides at ultra-slow spreading centers remain poorly understood. This study focused on six Fe–Si oxyhydroxide deposits collected from different sites at a typical ultra-slow spreading center, the Southwest Indian Ridge (SWIR). The mineralogical and geochemical evidence showed significant characteristics of a low-temperature hydrothermal origin. Sr and Nd isotope compositions of Fe–Si oxyhydroxide deposits at the SWIR probably reflected a combined signature of the hydrothermal fluids and seawater. Pb in the Fe–Si oxyhydroxides exhibited a close association with the substrate rocks and seawater. The Mössbauer spectra and iron speciation data further provided insights into iron-bearing phases in all deposits. Two different types of biomineralized forms were also discovered in these deposits by scanning electron microscopy (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) and nanoscale secondary ion mass spectrometry (nanoSIMS) revealed that distinct biogenic structures were mainly composed of Fe, Si, and O, together with some trace elements. Based on these findings, we propose that microbial activity plays a significant role in the formation of Fe–Si oxyhydroxides at the ultra-slow spreading SWIR.