Mechanism of magnetic property changes of serpentinites from ODP Holes 897D and 1070A

Abstract

Serpentinites have great implications for the oceanic crust, subduction zones, island arc magmatism activity, and the formation of nickel ore deposits. To further determine the mechanism of magnetic property changes of serpentinites, samples from ODP Holes 897D and 1070A were investigated by integrating both magnetic and non-magnetic methods. Detailed rock magnetic results demonstrate that magnetite prevails in the entire serpentinite section, while maghemite is present in the upper and altered parts. The concentration of Fe in the fresh peridotite is inhomogeneous; nonetheless, the magnetic properties are generally determined by the serpentinization process. The formation and state of the magnetite depend on the fracture conditioning and fluid activities which are controlled by the serpentinization process. By comparing these two holes, we found that the production of magnetite is consistent with the serpentinization process; serpentinization is a multi-stage process which involves early high-temperature serpentinization and later low-temperature oxidation. As the serpentinization continues, the fine magnetic particles become coarser, combined with the formation of new SP particles, and the later low-temperature oxidation leads to the maghemitization of the magnetites. The duration of oxidation also contributes to the differences of remanent magnetization between these two holes. These results greatly improve our understanding of the magnetic enhancement during the serpentinization process, and provide constraints on the interpretation of the paleomagnetic and aeromagnetic anomalies in this area. magnetite, maghemite, serpentinization, ODP Holes 897D, 1070A Citation: Su K, Liu Q S, Jiang Z X, et al. 2015. Mechanism of magnetic property changes of serpentinites from ODP

Deducing Mineralogy of Serpentinized and Carbonated Ultramafic Rocks Using Physical Properties With Implications for Carbon Sequestration and Subduction Zone Dynamics

“…This would indicate that, while magnetic susceptibility may be able to accurately identify highly serpentinized harzburgitic rocks, it may not be able to do so for dunitic or pyroxenite-rich lithologies or to distinguish if the rocks are slightly carbonated (ophicarbonates rocks). We do note; however, that the reverse temperature trend has been shown for some abyssal peridotites (e.g., Su et al, 2015); although this seems to be in the minority. Additionally, this two-stage process for the generation of magnetite may only apply to ultramafic rocks originating in the oceanic crust (ophiolitic or abyssal) as highly magnetic dunitic rocks have been observed in komatiitic units and mafic-ultramafic intrusions (ter Maat et al, 2019;Williams, 2009).…”