Major and trace elements exchanges during fluid-rock interaction at ultraslow-spreading oceanic lithosphere: Example of the South West Indian Ridge (SWIR)

Abstract

The smooth seafloor area, located between 62 and 65°E along the ultraslow-spreading South West Indian Ridge, is dominated by exhumed and altered mantle rocks, with very minor basalts and gabbros. This amagmatic region of the global ridge system is an ideal place to study long-lived fluid-rock interactions affecting peridotites and the associated mass transfers. The smooth seafloor area is composed of two corridors, west and east, perpendicular to the ridge axis allowing to establish profiles of chemical element concentrations with distance from the ridge axis. Studied serpentinites are moderately to completely serpentinized with various secondary mineralogical assemblages: predominantly serpentine (lizardite/antigorite) and magnetite associated to accessory minerals such as chlorite, tremolite, talc, carbonates, ferritchromite, sulfides, and Fe-(hydro-)oxides (hematite and goethite). LOI, serpentinization and iron oxidation degrees (Fe3+/Fetot) all increase with distance from the ridge axis in the western corridor and can thus be used as proxys to evaluate mass transfer during serpentinization. Bulk Al2O3 and CaO concentrations are inherited from partial melting and melt-rock interaction, while Fe2O3, MgO, SiO2 and Na2O contents were slightly modified during long-term low-temperature fluid-rock interactions. Concentrations in trace elements, such as Li, B, La, Pr and some FME increase with distance from the ridge and then with the serpentinization degree. During early stages of detachment faulting, a few samples interacted with high-temperature hydrothermal fluids leading to an increase in Fe2O3T, Na2O, REE and FME contents. However, the absence of sulfides in these samples, likely replaced by hematite and goethite, suggest late alteration or circulation of S-poor brines. Late fluid-rock interaction under oxidizing conditions leads to the formation of iron oxyhydroxides (IOH), hematite and goethite, in serpentinites and to a concomitant enrichment in some trace elements (La, Pr, As, Sb and Cd). Evaluation of major and trace elements mass transfer of the smooth seafloor serpentinites shed new light on the consequences of fluid-rock interactions during long-lived exhumation and hydration history.