Tectonic influence on chemical composition of ocean island basalts in the West and South Pacific: Implication for a deep mantle origin

Abstract

Pb, Nd, and Sr isotopic compositions and the 40Ar-39Ar ages of lavas from seamounts in the Joban seamount chain, which is the northernmost Cretaceous seamount chain in the northwest Pacific, suggest that these seamounts were produced by HIMU-type (high-μ or high-238U/204Pb) magmatism around 120 Ma. In addition, seamount chains in the western Pacific, which are coeval with the Joban seamount chain, also exhibit a HIMU isotopic signature. Since the seamount chains in the western Pacific are considered to have formed in the present-day South Pacific region, these isotopic features suggest that HIMU-type magmatism must have been active in the South Pacific since the Cretaceous. A notable geochemical characteristic of this magmatism is a correlation between the elemental ratios (La/Yb, Sm/Yb, Sr/Y, Nb/Zr, Ta/Zr, and Th/Ta) of the seamounts/islands and a “relative age” that is an index of the thickness of the lithosphere beneath hot spots. These correlations suggest the importance of the tectonic environment in determining the chemical composition of the magmas. They also suggest a genetic relationship between oceanic plateaus and island/seamount chains because the elemental ratios of oceanic plateaus follow the same trends. Another important geochemical feature is that the elemental ratios of mid-ocean ridge basalts (MORBs) from the East Pacific Rise (EPR) do not follow these trends. Since the source material for EPR MORBs is probably representative of the composition of the upper mantle beneath the South Pacific, this difference could imply an upward flow of material from the deep mantle.