From ridge to subduction zone: Dual-temperature serpentinization of the ophiolitic mantle revealed by triple oxygen isotopes and magnetite formation

Mantle sections in ophiolites are almost always serpentinized. However, it often remains unclear whether the serpentinization occurs near the ridge axis during lithospheric formation, or later within the subduction zone during ophiolite obduction and emplacement. To constrain serpentinization in ophiolites, we combine a triple-oxygen-isotope technique with rock magnetic and major-element analyses to establish an integrated framework linking fluid sources and temperatures to magnetite formation and iron behavior in serpentinite. We apply this framework to the Troodos ophiolite in Cyprus, where representative serpentinized samples were collected from ultramafic zones along a fossil ridge-transform intersection preserved within the ophiolite. Our analyses reveal fluid sources and distinct temperature regimes for ridge-axis and subduction mantle wedge serpentinization. Ridge-axis serpentinization is characterized by ~200−310 °C seawater circulation and strongly magnetic serpentinites, whereas mantle wedge serpentinization is associated with ~150−220 °C slab-derived fluids and weakly magnetic serpentinites. We further identify a temperature-dependent relationship linking serpentine iron content, magnetite formation, and magnetic properties. This relationship indicates the contrasting magnetic properties of serpentinite result from the preferential formation of magnetite over iron-rich serpentine at temperatures >200−220 °C.