Two primary basalt magma types from northwest Rota-1 volcano, Mariana arc and its mantle diapir or mantle wedge plume

Abstract

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 Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51-52 wt % SiO2) with 7·5-9·5 wt % MgO and Mg-number of 61-67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 and contain 0·4 wt % NiO; the Cr/(Cr + Al) values of spinels are mostly 0·5-0·8, indicating equilibrium with depleted mantle. There are three petrographic groups, based on phenocryst populations: (1) cpx-olivine basalt (COB); (2) plagioclase-olivine basalt (POB); (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, COB have Ba/Nb (70-80) and Th/Nb (0·4-0·5) that are higher than for POB (Ba/Nb = 30-35 and Th/Nb = 0·1-0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched 87Sr/86Sr and 143Nd/144Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. 176Hf/177Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr, Y and HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB. The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2-1·5 GPa (equivalent to 65-50 km depth). These magmas formed by 24-18% melting of mantle peridotite having Mg-number ~89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at 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. © The Author 2011. Published by Oxford University Press. All rights reserved.