Dual Magnetite Formation Pathways in Abyssal Serpentinites From the Mid-Atlantic Ridge

Abstract

Serpentinization, which involves hydrothermal alteration of olivine and pyroxene in ultramafic rocks, generates magnetite and strongly modifies the magnetic properties of oceanic lithosphere. However, the microstructural pathways of magnetite formation and their bulk magnetic expression remain insufficiently constrained. Here we integrate petrography, mineral chemistry, rock magnetism and electron microscopy on abyssal peridotites from the Mid-Atlantic Ridge (7–8°N) to resolve two end-member pathways controlled by protolith mineralogy. In moderately serpentinized samples, olivine alteration produces lizardite and magnetite. Magnetite first nucleates as nanocrystalline clusters within the lizardite matrix and grows by coalescence into submicron dendritic and clustered aggregates that are dominated by vortex domain states. With increasing alteration, progressive oxidation and cation substitution drive magnetite away from stoichiometry, expressed by a suppressed Verwey transition and elevated Curie temperatures. In contrast, orthopyroxene alteration yields serpentine/chlorite mixtures, together with systematic modification of Mg-Al spinel. With increased alteration, Mg-Al spinel is replaced sequentially by Cr-Fe chromite and then by Cr-rich magnetite, accompanied by coherent chemical and magnetic changes and the development of crystallographically oriented Cr-bearing magnetite grains. These divergent olivine- and pyroxene-derived pathways produce complementary magnetic carriers and fabrics within the same serpentinite body. Our results demonstrate that protolith mineralogy exerts a first-order control on serpentinite magnetism and provide a process-based framework for interpreting serpentinization-related magnetic anomalies and mantle-derived magnetic signatures at slow spreading ridges.