Esmeralda Bank: Geochemistry of an active submarine volcano in the Mariana Island Arc

Esmeralda Bank is the southernmost active volcano in the Izu-Volcano-Mariana Arc. This submarine volcano is one of the most active vents in the western Pacific. It has a total volume of about 27 km³, rising to within 30 m of sea level. Two dredge hauls from Esmeralda recovered fresh, nearly aphyric, vesicular basalts and basaltic andesites and minor basaltic vitrophyre. These samples reflect uniform yet unusual major and trace element chemistries. Mean abundances of TiO₂ (1.3%) and FeO^* (12.6%) are higher and CaO (9.2%) and Al₂O₃ (15.1%) are lower than rocks of similar silica content from other active Mariana Arc volcanoes. Mean incompatible element ratios K/Rb (488) and K/Ba (29) of Esmeralda rocks are indistinguishable from those of other Mariana Arc volcanoes. On a Ti-Zr plot, Esmeralda samples plot in the field of oceanic basalts while other Mariana Arc volcanic rocks plot in the field for island arcs.Incompatible element ratios K/Rb and K/Ba and isotopic compositions of Sr (⁸⁷Sr/⁸⁶Sr=0.70342–0.70348), Nd (_ND=+7.6 to +8.1), and O(¹⁸O=+5.8 to +5.9) are incompatible with models calling for the Esmeralda source to include appreciable contributions from pelagic sediments or fresh or altered abyssal tholeiite from subduction zone melting. Instead, incompatible element and isotopic ratios of Esmeralda rocks are similar to those of intra-plate oceanic islands or hot-spot volcanoes in general and Kilauean tholeiites in particular. The conclusion that the source for Esmeralda lavas is an ocean-island type mantle reservoir is preferred.Esmeralda Bank rare earth element patterns are inconsistent with models calling for residual garnet in the source region, but are adequately modelled by 7–10% equilibrium partial melting of spinel lherzolite. This is supported by consideration of the results of melting experiments at 20 kbars, 1,150° C with CO₂ and H₂O as important volatile components. These experiments further indicate that low MgO (4.1%), MgO/FeO^*(0.25) and Ni(12 ppm) in Esmeralda Bank melts are characteristic of initial melts generated by moderate degrees of melting of hydrous and carbonated mantle. Consideration of experimental determinations and spinel-lherzolite to garnet-lherzolite stabilities indicates Esmeralda Bank melts were generated by partial melting within the upper 60–110 km of the mantle.