[S-CG52] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG52-07] Volatile contents beneath the NW Pacific plate visualized using petit-spot basalts

*Makoto Sano¹, Naoto Hirano¹, Satoshi Okumura¹, Norikatsu Akizawa², Akihiro Tamura³, Tomoaki Morishita³ (1.Tohoku University, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Kanazawa University)

Keywords:petits-spot, volatile, alkali basalt, upper mantle

Suboceanic asthenosphere plays an important role in tectonic plate motion, where the physical state is recognized as a ductile zone observed seismic low velocity and electrical high conductivity in contrast to the rigid lithosphere. Some workers have ever assumed that the asthenosphere plausibly occurs due to the presence of water, carbon, and/or basaltic melt. As the H2O, CO2, and other contents in the upper mantle have been previously estimated only using the world MORBs (mid-ocean ridge basalts), H2O and CO2 contents are still enigmatic, in particular, from suboceanic upper mantle outside the mid-ocean ridges. The petit-spot basalts ascent from asthenosphere erupt on an old Pacific lithosphere prior to subduction, which show some peculiar components on potassium, high field strength elements, and high CO2 content. Here, this study reports the volatile, major, and trace element compositions of fresh quenched glass rinds of petit-spot basalts.
The H₂O content in the upper mantle is estimated using the H₂O/Ce ratio of quenched glass rind erupted on deep submarine environment because of the similarity in their partition coefficient during the melting process of the mantle. Water contents of the upper mantle below some petit-spot volcanoes are estimated as 38-138 ppm. This amount of water is clearly insufficient to lower the solidus to induce the partial melting. Otherwise, some experimental studies recently suggest that CO2 could potentially lower the solidus of the asthenosphere and explain the presence of alkaline partial melt. Because of the degassing of CO₂ in our samples, unfortunately, it would be impossible to estimate some relation between CO₂ and trace elements in this study. In the future studies, the CO2 and trace element compositions of melt inclusions would be important to estimate their degassing process about CO2 using in order to investigate the CO2 on the asthenosphere.