In-situ oxygen isotope analyses in serpentine minerals: Constraints on serpentinization during tectonic exhumation at slow- and ultraslow-spreading ridges

Peridotites exhumed along detachment faults at slow- and ultraslow-spreading ridges undergo variable and complex serpentinization. Here, we present in-situ oxygen isotope analyses in serpentine minerals from a textural sequence identified in samples drilled at the Atlantis Massif (Mid-Atlantic ridge, MAR, 30°N) and dredged along the easternmost Southwest Indian ridge (SWIR, 62–65°E). The textural sequence is similar at both locations and involves: mesh texture and bastite formation after olivine and orthopyroxene, mesh texture recrystallization into chrysotile- and antigorite-dominated textures, and banded and fibrous veins. The δ18O in serpentine decreases with the textural sequence, which we interpret to record an increase in the time-integrated water-rock ratio under a nearly constant temperature. While mesh texture development starts at stoichiometric water-rock ratios and creates an isotopic variability at a scale of ~100 μm, recrystallized textures indicate serpentinization temperatures on the order of 260–290 °C (seawater-dominated fluid hypothesis) or 320–360 °C (hydrothermally altered seawater hypothesis). The subsequent banded veins record the evolution towards colder and seawater-dominated fluids potentially present in shallower levels of the detachment footwall. Serpentinization leading to the mesh textures appears faster and more heterogeneous in Atlantis Massif samples than in SWIR samples. We propose that such features are influenced by the amount of gabbros intruded in the peridotite. In addition to influencing fluid pathways, gabbros also provide a source for observed Si-enrichments in some Atlantis Massif samples and control the pH, which in turn influences the mobility of Al