Journal of The School of Marine Science and Technology,Vol.3 No.3
Rocks and Minerals from The Hahajima Seamount in
Ogasawara Sea Area
─Aragonite, Precipitated Serpentine and
Quartzine in Serpentine Mud─
Toyoto AZUMA, Takayuki KATOH, Izumi SAKAMOTO and Yong Ui KIM
Abstract
The Hahajima Seamount is situated at about 20km west of the junction between the Izu-Ogasawara and the
Mariana Trench. Tokai University’s research vessel dredged amount samples from the 18 points on the Hahajima
Seamount since 1984 have revealed the topography and geological feature of the seamount. The estimated area of
distributions of serpentinites and serpentine mud accounts for only 2-5％ of all surface of the Hahajima Seamount.
Therefore, it is difficult to interpret the Hahajima seamount as the serpentinite seamount. The tectogenesis of the
Hahajima Seamount is considered that the fragments of oceanic crust undergone island-arc magmatism are crushed
and uplifted and of upper mantle intruded or injected to that. At the point 2003-St. 3 near northern summit of the
Hahajima Seamount,serpentinites,serpentine muds with acicular aragonites and cherts were dredged.Serpentinites
and serpentine muds composed of mostly lizardite and chrysotile with minor amount of antigorite.Some fructure of
serpentinites and cherts include precipitated serpentine and quartzine.Roundness and grain size of these serpentinite
and serpentine mud are not sorted, and serpentine mud is unconsolidated sediment. The modes of occurrence of
serpentinite and serpentine mud are similar to the serpentinite landslide on the land. It is favor that a serpentinite
submarine landslide occurred on the surface after emplacement of serpentinites of the Hahajima Seamount. The
authigenic minerals such as acicular aragonite containing serpentine fragments,precipitated serpentine and quartzine
are precipitated from the pore water in serpentinite and serpentine mud.It shows that the pore water is rich in Ca2+,
Mg2+, Fe2+and Fe3+,because these ions are supplying from the seawater flowing surface and internal shallow level
of the Hahajima Seamount with serpentinite.