Protracted hydrothermal alteration recorded at the microscale in the Chenaillet ophicarbonates (Western Alps): Insights from in situ δ18O thermometry in serpentine, carbonate and magnetite

Abstract

The study of serpentinites and ophicarbonates from ophiolitic terrains provides a three-dimensional perspective on the hydration and carbonation processes affecting modern oceanic lithosphere. The Chenaillet ophiolite (western Alps) is interpreted as a fragment of an oceanic core complex that resembles a modern slow spreading center, and it was weakly affected by Alpine metamorphism. Ophicarbonates from the Chenaillet ophiolite were targeted in this study for in situ analysis by Secondary Ion Mass Spectrometry (SIMS) of oxygen and carbon isotopes in serpentine, calcite, dolomite and magnetite. The high spatial resolution of SIMS allowed us to target different serpentine, carbonate and magnetite generations intergrown at scales ≤ 50 μm, and reveal systematic zoning in δ18O with a range of 5.8‰ in serpentine (from 3.0 to 8.8‰, V-SMOW), 21.2‰ in carbonate (9.4 to 30.6‰), and 5.6‰ in magnetite (–5.0 to –10.6‰). Coupled analysis of oxygen isotopes in seven different touching-pairs of co-crystallized serpentine + carbonate and serpentine + magnetite provides independent constraints on both the temperatures and δ18O(water) values during serpentinization and carbonation responsible for the formation of the Chenaillet ophicarbonates. The new stable isotope data and thermometric estimates can be directly linked to textural and petrographic observations. These new results identify at least four different stages of hydrothermal alteration in the Chenaillet ophicarbonates: (1) peridotite hydration during seafloor exhumation at temperatures down to 200–130 °C and water δ18O values varying from 5 to 2‰, as documented by serpentine + magnetite in mesh textures; (2) carbonation during exhumation near the seafloor at temperatures as low as 10 °C assuming water δ18O values of –1‰, as documented by the highest oxygen isotope ratios in texturally older calcite; (3) serpentinization and carbonation at temperatures up to 240 °C and water δ18O values of 2-3‰, as documented by serpentine + magnetite in veins crosscutting mesh textures (T = 192 ± 66 °C, δ18O(water) = 2 ± 1‰, 2 standard deviation), serpentine + magnetite (T = 182 ± 32 °C, δ18O(water) = 2 ± 1‰) and serpentine + dolomite (T = 243 ± 79 °C, δ18O(water) = 3 ± 2‰) in recrystallized hourglass domains within serpentinite clasts, serpentine + dolomite (T = 229 ± 50 °C, δ18O(water) = 3 ± 1‰) and serpentine + calcite (T = 208 ± 40 °C, δ18O(water) = 2 ± 1‰) within the fine-grained calcite matrix surrounding serpentinite clasts; (4) late stage carbonation at temperatures down to 70–40 °C assuming water δ18O values of 3 to –1‰, as documented by the highest oxygen isotope ratios in a large calcite vein crosscutting both serpentinite clasts and fine-grained carbonate matrix. We suggest that the textural and isotopic observations are consistent with a protracted serpentinization and carbonation of the lithospheric mantle that started during progressive exhumation to the seafloor and continued due to interaction with hot and isotopically shifted seawater, which circulated at depth in the oceanic crust and was then discharged near the seafloor, similar to modern mid-ocean ridge venting systems.