Magnetic Characteristics of Highly Serpentinized Peridotite in the Iberia Abyssal Plain and Implications for Marine Magnetic Anomalies

Serpentinization, a consequence of water-rock interaction in mafic and ultramafic rocks, refers to the hydrothermal alteration of olivine and pyroxene with serpentine as the typical product. Magnetite is produced in variable amounts during this serpentinization process. Here, we conducted a systematic rock magnetic study on the serpentinite of Ocean Drilling Program (ODP) Hole 1070A from the Iberia Abyssal Plain. From bottom to top, three units are distinguished rock-magnetically: (a) serpentinized peridotite dominated by single-domain (SD) ± multidomain (MD) magnetite particles; (b) gabbro with SD ± vortex state magnetite; (c) breccia dominated by MD ± vortex and SD ± vortex state maghemite/hematite and magnetite. Samples of the three units are highly serpentinized with serpentinization degrees >60%. The magnetite content increases exponentially with the degree of serpentinization. Two phases of serpentinization are proposed: (a) massive serpentinization that occurred before mantle exhumation and (b) maghemitization-serpentinization that occurred near or at the seafloor after the final exhumation of mantle peridotites. The latter reduces the magnetization of the breccia unit significantly. Serpentinized peridotite associated with strong magnetization is the dominant contributor to the marine magnetic anomalies in ocean to continent transition areas.