[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

[SCG55-04] The geochemical structure of pre-subducting oceanic lithosphere explored from petit-spot volcano

*Kazuto Mikuni¹, Naoto Hirano¹, Norikatsu Akizawa², Shiki Machida³ (1.Tohoku University , 2.University of Tokyo, 3.Chiba Institute of Technology)

Keywords:Petit-spot, Lithosphere, Metasomatism, Mantle Xenolith, Pacific Plate

Chemical composition of the oceanic lithosphere has been represented previously by “abyssal peridotite” exposed along transform faults in the mid-ocean ridge. However, nobody has ever seen the lithospheric mantle of the oldest part of Pacific Plate. A subseafloor petit-spot (small conical knoll) cluster was recently discovered in the south of Minamitorishima (Marcus) Island, where the age of Pacific Plate is estimated as 160 Ma. Peridotite and pyroxenite xenoliths, and pyroxene xenocrysts transported to the ocean floor through the petit-spot volcanism could directly provide petrological and geochemical information of the oldest Pacific lithosphere. This study presents in-situ chemical compositions, determined using an electron micro-probe, of quenched glasses, xenoliths and xenocrysts in petit-spot basalts on the oldest part of Pacific Plate. The whole-rock chemical compositions were analyzed using an inductively coupled plasma-mass spectrometry. Eruption ages were estimated from thickness of palagonite, an altered quenched glass rind by the interaction with seawater.

The eruption ages of eight petit-spot volcanoes in this region were dated at 0.03–2.2 Ma, suggesting recent eruptions due to lithospheric flexure of the subducting Pacific Plate into the Mariana Trench. The petrographic and geochemical features of petit-spot basalts obtained in this study are similar to those of previously reported petit-spot basalts in the northwestern Pacific Plate.
Most of clinopyroxenes of peridotite and pyroxenite xenoliths are Ti-rich Al-augite, indicating compositional difference with the depleted abyssal peridotite. Ti-rich Al-augite is possibly originated from phenocryst crystallized at high pressure, cumulate from alkalic magma or metasomatic vein in lithospheric mantle. However, the petrographic characteristics of clinopyroxene in these xenoliths (e.g., lamellae) specify the presence of metasomatic veins in the lithospheric mantle. The estimated temperature and pressure of pyroxenite and peridotite xenoliths peculiarly fit on a geotherm younger than 160 Ma. These results imply that the oldest Pacific Plate has a high geothermal gradient due to multiple petit-spot magma injections as previously indicated in the northwestern Pacific Plate. If petit-spot volcanism is ubiquitous phenomenon, the nature of subducting oceanic plate must be revised drastically in previous recognition.