[S-CG51] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG51-P02] Effects of mineralogy and porosity on seismic velocity of fore-arc basalts and boninites in Bonin fore-arc collected by IODP Expedition 352

*Yuya Akamatsu¹, Katsuyoshi Michibayashi^2,1, Masakazu Fujii^3,4, Yumiko Harigane⁵, Yuzuru Yamamoto^6,7, Nana Kamiya⁸, Tetsuya Sakuyama⁹ (1.Research Institute for Marine Geodynamics, JAMSTEC, 2.Nagoya University, 3.National Institute of Polar Research, 4.SOKENDAI, 5.Geological Survey of Japan, AIST, 6.Kobe University, 7.Center for Mathematical Science and Advanced Technology, JAMSTEC, 8.Kyoto University, 9.Osaka Metropolitan University)

Keywords:Izu-Bonin-Mariana fore-arc, Oceanic crust, Seismic velocity, IODP

The seismic velocity structure of the igneous oceanic crust is typically divided into two layers: Layer 2, characterized by a high velocity gradient with depth, and Layer 3, which exhibits a lower velocity gradient. Since the seismic velocity of rocks is primarily controlled by the mineral composition and porosity, the velocity increase in Layer 2 is often associated with mineralogical changes due to hydrothermal alteration and/or the closure of pores with increasing pressure. To evaluate the effects of mineralogy and porosity on seismic velocities in the shallow igneous oceanic crust, here we conducted laboratory measurements of physical properties, whole-rock geochemical analyses, and microstructural observations using rock samples collected during International Ocean Drilling Program (IODP) Expedition 352. The drilling was conducted at four sites in the outer Bonin forearc, and rock samples including fore-arc basalt (FAB) and boninite were collected. Physical property measurements were performed on cubic samples, including porosity, density, and P-wave velocity under water-saturated conditions. Magnetic properties, including magnetic susceptibility were also measured to assess the role of magnetic minerals. The measured grain densities ranged from 2.1 to 3.1 g/cm³, porosities ranged from 5% to 40%, and P-wave velocities ranged from 3.0 to 5.5 km/s. A general negative correlation was observed between porosity and P-wave velocity, indicating that porosity has a primary effect on seismic velocity of FAB and boninite. Nevertheless, P-wave velocities varied by up to ~2 km/s even at similar porosity ranges. This likely reflects differences in pore geometry or in the elastic moduli of the solid phase. The elastic moduli of the solid phase are related to mineralogy; they tend to be lower when alteration products or glass are present and higher when magnetic minerals are abundant. Magnetic susceptibility measurements revealed a bimodal distribution, with FAB, basaltic boninite, and high-magnesium andesite samples exhibiting values two orders of magnitude higher than high-silica boninite samples. Samples in the low-susceptibility group generally showed higher P-wave velocities at similar porosity compared to those in the high-susceptibility group. This trend suggests that the observed velocity variations at constant porosity are not directly related to the abundance of magnetic minerals but may instead reflect the influence of alteration minerals, glass content, or cracks.