Serpentinization of mantle xenoliths in Kerguelen archipelago: A first petrographic and geochemical study

The spinel harzburgites xenoliths from the Lac Michèle outcrop, Kerguelen archipelago (South Indian Ocean), have the particularity to show unique and unusual evidences of fluids circulation and especially of serpentinization. In this paper, in situ and whole-rock petrographic and geochemical data on a selection of variously serpentinized samples are presented and attest that several episodes of fluid-rock interaction occurred. Serpentinization processes affected notably and with variable degrees the spinel harzburgites leading to changes in their mineralogy and chemical compositions. Degree of serpentinization ranges from very slight (LOI < 3 wt% and Fe3+/Fetot < 0.1) to moderate (4 wt% < LOI < 6 wt% and 0.21 < Fe3+/Fetot < 0.33). Most serpentinized samples show a preferential petrographic direction of serpentinization, forming a subparallel serpentine network, which is sometimes intersected by serpentine veins. As a result, at least two serpentinization episodes were identified within the Lac Michèle samples, without any chemical differences between the two generations except for Cl content. This suggests very few chemical evolution of the system during serpentinization. Due to the significant amount of Cl measured in serpentine minerals (601 ppm on average), the nature of the fluids that interacted with the spinel harzburgites xenoliths during the serpentinization events can be hypothesized to be seawater-like or seawater-derived fluids.