Geochemistry of basalt from the Ayu Trough, equatorial western Pacific

Abstract

The Ayu Trough is an approximately 600-km-long divergent margin between the Philippine Sea and Caroline plates located in the western Pacific near the equator. Characterized by rugged seafloor topography and deep axial depths that average greater than 4000 m, the Ayu Trough has been considered as a slow spreading center. Here we report geochemical analyses of Ayu Trough basalts (ATB) dredged from seven sites along the axis from 0°30′ to 4°30′N. The ATB vary geochemically from N-MORB (La/SmN < 1) to relatively enriched (E)-MORB (La/SmN > 1.4). N-MORB were recovered from five sites of axis wall, while E-MORB were sampled from one site, an axial volcanic ridge inside the median valley at 2°45′N. At one site, transitional (T)-MORB were recovered in addition to N-MORB. Variations in incompatible element distribution patterns and Sr-Nd-Pb isotopic ratios between the N-MORB and E-MORB samples are too large to be explained solely by melting of a uniformly depleted mantle source. It appears that at least two distinct mantle components, depleted and enriched, were involved in the formation of the ATB. N-MORB isotope ratios are similar to those associated with Indian-Ocean-type mantle, whereas those of E-MORB lie on a trend towards those of EM2. Based on incompatible element and isotope ratios, we suggest that the enriched component, source of E-MORB, beneath the Ayu Trough was generated in a subduction zone by metasomatism of low-degree melt and addition of a small amount of sediment. Although N-MORB generally conform to global correlations between the axial depth and basalt chemistry, E-MORB retrieved from the axial volcanic ridge include two distinct lava suites with (Sm/Yb)N ratios of ∼ 1.8 and 1.3, suggesting that they were generated at different mean depths of melting, and possibly different mean degrees of partial melting. If the low-degree melt samples are younger than the higher degree of melt samples, then this finding is consistent with inferences based on geophysical observations such as at least 50-70-m-thick sediment on the axis, that spreading in the Ayu Trough may have recently slowed dramatically or even ceased, leading to a diminished melt production and construction of a localized axial volcano.