An Experimental Study on the genesis of petit-spot Lavas

Asthenosphere as the seismic low-velocity zone plays an important role in stabilizing plate tectonics. Although the geochemical and petrological properties of the asthenosphere are essential to understand the surface environment of terrestrial planets, nature of asthenosphere is still enigmatic because we have never gained samples from the oceanic asthenosphere directly. Several studies have even proposed the presence of partial melt in the asthenosphere. However, rheological properties of the mantle minerals change due to the presence of water without partial melt.

Petit-spot, which is a new-type monogenic volcano discovered at the beginning of the 21^st century, mainly occurs as submarine knoll. Petrographical and geochemical data suggest that previously reported petit-spot samples from the Northwest Pacific Ocean have undergone some degrees of fractional crystallization of olivine and/or clinopyroxene etc. and have assimilated with ambient lithospheric mantle. Thus, their chemical composition could be considerably modified before the eruption. On the other hand, the samples newly collected from three isolated knolls show strongly primitive geochemical characters such as low-silica and high alkali and volatile element contents. The rapid ascending ratios are estimated for three primitive lavas by Fe-Mg diffusion of olivine xenocrysts, so called the "direct ascent" melts.

Here, we show some preliminary results of high-pressure experiments to seek out multiple saturation points of the "direct ascent" petit-spot lavas supported by the JSPS Japanese-German Graduate Externship. Multiple saturation points reveals P-T conditions and lithologies where primary melt lastly equilibrated with. The primary melt compositions of the "direct ascent" samples were estimated by backward calculation taking account of equilibrium degassing and olivine fractionation during the magma ascending. As the origin of the primitive samples are expected to be closer to asthenospheric condition than those of previously sampled petit-spot lavas, the experiments are a powerful tool to elucidate the geochemical and petrological properties of the asthenosphere. Measurements of Fe³⁺ partitioning between minerals and melt phases by Mössbauer spectroscopy are also planned on the same experiments.