Understanding the heterogenic magnetic properties of oceanic upper mantle and lower crustal materials

Abstract:

Serpentinization of peridotites and gabbroic rock is widespread on the seafloor with slow-spreading mid-ocean ridges, and subduction-related systems. It has been extensively investigated as the process significantly changes the physical properties of the oceanic lithosphere. As serpentinization processes are often associated with production of magnetite, magnetic properties canprovide valuable insights for better understanding the processes. However, the quantitative relationship between the magnetite abundance and degree of serpentinization remains poorly constrained. Nonlinear relationship is often observed for natural serpentinized peridotite, it varies from site to site with great scatters (e.g., Oufi et al., 2002; Fujii et al., 2016). In addition, simple laboratory experiments by Malvoisin et al. (2012) shows that magnetite amount grows linearly as serpentinization advances, which is different from the trend observed from natural samples. To understand the relationship between magnetic properties and serpentinization processes, we are currently conducting comprehensive magnetic studies on serpentinized peridotites and gabbroic rocks with a variety of serpentinization degree in global scale. The samples were collected from several area during onland surveys and Japanese research expeditions of the R/V Yokosuka, R/V Hakuho-maru, and R/V Kairei; 1. Yokoniwa Rise, Central Indian Ridge; 2, Vulcan Fracture Zone, Mid Atlantic Ridge; 3, 15°20’N Fracture Zone, Mid Atlantic Ridge; 4. Atlantis Bank, Southwest Indian Ridge; 5, Mado Megamullion, Phillipne Sea; 6, Kamuikotan Complex (serpentinite diapir), Hokkaido, Japan; 7, Mariana Forearc slopes close to the Challenger Deep; 8, Shinkai Seep Field, Mariana Forearc. In this presentation, we introduce our preliminary results of rock magnetic properties (susceptibility, natural remanent magnetization, magnetic domain state, magnetite abundance, temperature dependence), grain density, reflected light microscopy, scanning electron microscopy, and microscopic magnetic mapping along with presented geophysical background and petrological signatures. It is widely shown that the relationship between magnetite abundance and serpentinization degree may sensitively depend on the reaction pathway of serpentinization.