Abstract

Four submarine volcanoes in the southern and central Mariana arc were dredge-sampled. The recovered lavas were analyzed for major- and trace-element abundances, including rare-earth elements (REE), mineral compositions, and Sr and Nd isotopic ratios.

The samples exhibit a great range of chemical and mineral compositions relative to subaerial exposures in the central arc. Zealandia Bank produces high magnesium basalts (MgO = 8–9 wt %) enriched in Ni and with forsteritic olivine, as much as Fo88. This lava composition is probably parental to more fractionated basalts and basaltic andesites exposed on the adjacent islands of Guguan and Sarigan. The southernmost seamounts in the Mariana arc (south of lat. 15°N) produce dacitic pumice, a composition that is not common in the active arc. REE data preclude their derivation from a basaltic parent by fractional crystallization. They may be derived by direct partial mantle melting under water-saturated conditions at low pressure.

Nd and Sr isotopic data (143Nd/144Nd = 0.5130–0.5131; 87Sr/86Sr = 0.7032–0.7035) indicate a mantle source for the Mariana arc lavas similar in isotopic characteristics to sources for ocean islands. Significant contributions to these arc magmas from subducted oceanic crust or sediment are not evident.

A spatial distribution in magmatic characteristics coincides with a structural break in the arc near 16°N. South of this, submarine seamounts produce incompatible element–enriched lavas with relatively high 87Sr/86Sr (0.7034–0.7035). These lavas were produced by a relatively low degree of partial melting (5%–15%) of a shallow (spinel-bearing) mantle. To the north, Zealandia Bank and the central arc islands are characterized by lower incompatible element abundances and lower 87Sr/86Sr (0.7032–0.7033). These lavas were produced by higher degrees of partial melting (15%–25%) of a deeper (garnet-bearing) mantle.

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