Primitive Submarine Basalts and Magmatic Variation of Pagan and Daon, Mariana Arc

Pagan is an active volcano located in the central island province of the Mariana arc (18°07'N) and is one of the largest volcanoes in the Mariana arc; its main edifice rises from a base ~3,000 m below sea level (b.s.l.) and has a volume of 2,160 km³ (Bloomer et al., 1989). Daon is a small reararc seamount 25 km SW of Pagan (17°58'N). We visited the submarine portions of the two volcanoes in 2010 (NT10-12), using ROV Hyper-Dolphin and RV Natsushima. Rocks were collected from the northeastern and southwestern flanks of the Pagan volcano at 1,500-2,000 m b.s.l. (dive HPD1147) and at 2,020-2,330 m b.s.l. (HPD1148), respectively, and from the southern flank of Daon at 2,360-2,580 m b.s.l. (HPD1149). Fresh pillow lavas dominate in all three dives, but the rocks recovered from HPD1147 seem to be the youngest based on very light sediment cover and no Mn coating. Sediment cover is considerably more extensive at HPD1148, and all rocks from Daon (HPD1149) had 0-10 mm thick Mn coating. Submarine Pagan lavas show major element compositions typical of subaerial Pagan basalts (Marske et al., 2011; Elliott et al., 1997; Woodhead, 1989), although the least fractionated compositions recovered from HPD1147 extend to much higher MgO (7-11 wt %) and Mg# (60-70), than the subaerial lavas. We recognize two types of primitive basalts from Pagan and Daon. Daon has plagioclase-olivine basalt (POB) and clinopyroxene-olivine basalt (COB), petrographic types that are similar to those reported from NW Rota-1 volcano (Tamura et al., 2011). Pagan has two types of COB, both having 10-11 wt % MgO; COB-1 has higher Ba/Zr and Sr/Zr and lower Zr/Y than COB-2 at the same MgO content, indicating that COB-1 has a greater subduction component and formed from higher degrees of mantle melting than COB-2. Similar distinct primitive magmas like those recognized from NW Rota-1 also coexist at Pagan and Daon.