ENIGMATIC ANDESITE IN THE IZU-BONIN-MARIANA ARC

The Izu-Bonin-Mariana (IBM) arc system stretches over 2800 km and is an excellent example of an intraoceanic convergent margin and has been well studied. However, two occurrences of andesites, firstly in the middle crust and secondly their eruption from volcanoes above thin crust, remain enigmas.

Seismic velocity images of the IBM arc reveal that there is a significant volume of material with seismic velocities of 6.0-6.8 km/s present at mid-crustal depths throughout the entire arc (Kodaira et al., 2007). This is remarkable because these velocities are similar to the mean velocity of andesitic continental crust, which should contrast with the oceanic arc where basalt and basic andesite are the predominant eruptive products. Moreover, it is thickest below basaltic volcanoes (Kodaira et al., 2007; Tamura et al., 2009).

Nishinoshima (27°15’N, 140°52’E) in the Bonin arc began its latest eruptive phase in November 2013 after being dormant for 40 years. Eruptions are ongoing and greatly expanding its surface area, but because of their explosivity access to the island is prohibited, as a result the new lava flows have not yet been sampled. Previous eruptions of Nishinoshima produced only andesitic lavas containing ~60 wt % SiO₂, similar to average continental crust. Recent eruptions of South Sarigan (16°36’N, 145°46’E) in the Mariana arc, including one in May 2010, have also been dominated by andesite (Embley et al., 2014).

Intuitively thick crust is necessary to produce andesitic magmas if AFC (assimilation-fractional crystallization) plays an important role in the differentiation from mantle-derived (primary) basaltic magmas to andesites. However, arc crust beneath the volcanic front at both Nishinoshima and South Sarigan is immature; approximately 10-20 km thick beneath Nishinoshima (Kodaira et al., 2007) and approximately 16 km beneath South Sarigan (Calvert et al., 2008). Thicker crust (~30 km thick) exists beneath the Izu volcanic front at Miyakejima, Hachijyojima and Aogashima, which are all dominated by basaltic eruptions.

The origin of andesitic middle crust and the eruption of andesitic magmas in the IBM arc are being addressed through sampling with Remotely Operated Vehicles and by IODP drilling.