Geochemistry of the New Caledonia Serpentinites: Evidences for Multiple Serpentinization Events at Various Depths

Abstract : Studies of serpentine minerals around the world have shown that different varieties can coexist depending on external conditions such as temperature, pressure and chemical exchanges. Identifying serpentine variety can thus provide significant constraints on the geodynamic environment at the time of formation. In the New Caledonia (NC) ophiolite, serpentinization is ubiquitous (>50%). The base of the ophiolite is made of a thick serpentinite sole that recorded multiple serpentinization events. This study aims at deciphering the nature and the origin of fluids involved in serpentinization processes from the characterization of primary minerals and serpentine geochemistry, including: in situ major and trace elements and stable oxygen and hydrogen isotopes. Our results show that lizardite is the main mineral species (~80% of the serpentine). In the serpentinite sole, lizardite is crosscut by multiple serpentine veins ordered as follow: lizardite 1→lizardite 2→antigorite→chrysotile→polygonal serpentine. From the trace elements analysis, we demonstrate that the transition from primary minerals to lizardite 1 occurs almost isochemically. However, serpentine composition in the sole strongly differs from lizardite 1 and show a great enrichment in fluid-mobile elements and an increase of Fe3+/FeT ratio. Stable isotopes show that serpentines display a wide range in δ18O (1.9-14‰) and a narrow range in δD (88-106‰). These results were then modeled based on Monte-Carlo simulations. Fluids in equilibrium with NC serpentines define a linear trend that extends from the meteoric water line to an area defined between 3‰ and 8‰ in δ18O and -80‰ and -60‰ in δD. These compositions are consistent with fluids derived from the dehydration of the altered oceanic crust during the subduction in the South Loyalty Basin at temperatures between 250°C and 400°C. No evidence of sedimentary contribution was observed, suggesting that the serpentinization of the NC ophiolite was complete within few million years after the initiation of the subduction. Low δ18O values indicate that the latest generation of serpentine may derive from the circulation of meteoric fluids at low temperature (<150°C), suggesting that the latest steps of serpentinization started from the beginning of the ophiolite obduction.