Magnetic properties of variably serpentinized abyssal peridotites

Abstract

We have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven Deep Sea Drilling Project and Ocean Drilling Program sites. The magnetic susceptibility (K) of these samples does not increase linearly with the degree of serpentinization (S). Instead, K remains modest in partially serpentinized samples (S < 75%) and then increases rapidly. The rate of formation of magnetite during serpentinization increases as the iron content of serpentine minerals decreases: ∼6% FeO in the first stages of serpentinization and only 2–3% FeO as serpentinization proceeds beyond S ∼ 75%. Contrasting remanent behaviors observed in extensively serpentinized samples (some samples have natural remanent magnetizations (NRM) comparable to those of basalts, while other samples have low NRM values, for similarly high K values) are due to differences in the effective magnetic grain size and appear related to serpentine textures. Samples with small effective magnetic grain sizes have a well‐developed serpentine meshwork, outlined by thin vein‐like magnetite concentrations. Most serpentinized abyssal peridotites have this well‐developed serpentine mesh texture. We thus propose that high NRM (4–10 A/m on average) and K (∼0.07 SI on average) are the most likely signature for extensively serpentinized (S > 75%) peridotites in the oceanic crust. Moderately serpentinized peridotites (S < 75%) have NRM values <5 A/m and K < 0.05 SI. Low‐temperature oxidation of magnetite is found to lower the NRM and K values of seafloor samples and of cataclastically deformed drilled samples.