Two Primary Basalt Magmatypes from Northwest Rota-1 Volcano, Mariana Arc

Primitive basalts are rarely found in arcs. The active NW Rota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyperdolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51-52 wt % SiO2) having 7.5-9.5 wt % MgO and Mg# of 61-67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 which contain 0.4 wt % NiO; Cr/(Cr+Al) of spinels are mostly 0.5-0.8, further indicating equilibrium with depleted mantle. There are three petrographic groups, based on phenocryst populations: 1) cpx-olivine basalt (COB); 2) plagioclase-olivine basalt (POB); and 3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3.1-3.2 and 1.2-1.5, respectively) than in COB (Zr/Y = 2.8-3.0 and Nb/Yb = 0.7-0.9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COBs have Ba/Nb (70-80) and Th/Nb (0.4-0.5) that are higher than for POBs (Ba/Nb = 30-35 and Th/Nb = 0.1-0.2), and also have steeper light Rare Earth element (LREE) enriched patterns, suggesting that COBs have a greater subduction component than POBs. We calculate primary COB magmas and POB magmas for the NW Rota-1 volcano. By using these primary magmas, existing melting experiments of mantle peridotites, and supra-subduction zone peridotites, we gain new insights into the nature and origin of arc magmas.The calculated primary POB and COB magmas indicate segregation pressures of 1.5- 2 GPa (50-65 km deep). These magmas were formed by 15-25 % melting of mantle peridotite having Mg# ~89.5. These two basalt magmatypes are similar to those found for Sumisu and Torishima volcanoes in the Izu-Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents. This finding helps explain the variation of fractionated arc magmas and begs the question: “why do COB and POB coexist in arc volcanoes and how are they produced together in the mantle wedge?”