Unexpected submarine volcanoes, petit-spot, were recently discovered on the subducting plate, which occur in a region of the plate-flexure prior to subduction into the trench off NE Japan and Chile. The magmas produced by these volcanoes originate from the asthenosphere immediately under the plate (Hirano
et al., 2006). Monogenetic petit-spot volcanoes located on the NW Pacific Plate are less than 2 km in diameter and yield ages of 1.8, 3.8, 4.2, 6.0, 6.2, 8.5, and 9.2 Ma by Ar-Ar datings, suggesting the episodic eruption of magma over a large eruption area (over 800 km of plate motion) but with low volumes of magma production. As for the Chilean petit-spots, the back calculation along present absolute motion of the Nazca Plate using Ar-Ar age results of two volcanoes shows the eruptive portions associated with lithospheric bending in response to both of plate subduction and to loading related to the presence of seamounts (Hirano
et al., submitted). The location of the tiny submarine volcanoes is therefore an important indicator of the stress field of the plate, in addition to providing information on the geochemical composition of the mantle below the plate (Hirano, 2011).
The most important feature of petit-spot lavas is their high vesicularity (up to 60 vol.%) in spite of the eruption under submarine hydrostatic pressure encountered at 4000 and 6000 mbsl. This observation is caused by CO2, as the solubility of CO2 is very low in alkaline magmas (Dixon, 1997) compared with the high solubility of H2O. It is anticipated that petit-spot magmas originate in the asthenosphere as incipient partial melts that form as a result of the presence of H2O and CO2 (Wyllie, 1995). More recently, carbonatite melt has been proposed as a key material in explaining the electrical conductivity of oceanic asthenosphere (Gaillard et al., 2008; Yoshino et al., 2010). High CO2 contents in petit-spot lavas raises the possibility that CO2 affects the source components and their melting.
