[S-CG56] ICDP オマーン掘削プロジェクト

[SCG56-P04] Crack aspect ratio in the serpentinized peridotites inferred from onboard ultrasonic data by the Oman Drilling Projects

★Invited Papers

*片山 郁夫¹、阿部 なつ江²、岡崎 啓史³、畠山 航平¹、赤松 祐哉¹、道林 克禎⁴、Godard Marguerite⁵、Kelemen Peter⁶、The Oman Drilling Project Phase 2 Science Party (1.Department of Earth and Planetary Systems Science, Hiroshima University、2.Mantle Drilling Promotion Office, MarE3, JAMSTEC、3.Kochi Institute for Core Sample Research, JAMSTEC、4.Department of Earth and Planetary Sciences, Nagoya University、5.Géosciences Montpellier, CNRS, Université de Montpellier、6.Lamont Doherty Earth Observatory, Columbia University)

キーワード：空隙形状、蛇紋岩、オマーンオフィオライト

To assess pore geometry in the highly altered peridotites collected by the Oman Drilling Projects, we analyzed ultrasonic velocity of the serpentinized dunites and harzburgites from mantle sections at Holes BA1B, 3A, and 4A. First, we estimated serpentine fraction based on grain density to obtain the porosity-free reference velocity, suggesting a nearly complete serpentinization at shallow depth but decrease of hydration at deeper depth. We assumed that the difference between reference and measured velocity attributes to pores with spheroidal shape that are embedded in the samples. Application of effective medium theory to onboard P-wave and porosity data indicates that pore aspect ratio mostly lies between 0.1 and 0.01 and crack density is ranging from 0.58 to 0.02. We found a positive relationship between aspect ratio and serpentine fraction, suggesting a change in pore shape controlled mainly by the dissolution–precipitation processes during hydration, consistent with microstructural observations. The relatively high aspect ratio and hence high fluid flux at shallow depths also agree with the present-day hydration processes that are inferred from the borehole fluid chemistry. The inversion of ultrasonic data provides a series of elastic moduli, and we can estimate Poisson’s ratio, which is a key physical property to interpret geophysical observations in the oceanic lithosphere.