Chemistry of deep (3500–5600 m) Pacific MORB—Why is the Pacific anomalous?

Abstract

In contrast with the Atlantic and Indian Oceans, the Pacific shows the global correlations of axial depth and basalt chemistry only weakly or not at all for unaveraged data. In the past, this observation has been partly explained by the limited depth window of available Pacific samples. We report on new, very young basalt samples collected by submersible from Pacific ridges at ∼3500–5600 m, nearly doubling the depth interval of Pacific samples. These samples, from the Pito Deep, Hess Deep, and the Garrett transform, do not conform to the global correlation of depth vs. chemistry and show the local trend of chemical variation. The Pacific as a whole is thus unusual compared to the Atlantic and Indian Oceans. We propose that the anomalous behavior of the Pacific is due mostly to its relatively rapid spreading rate and tectonic shifts leading to an absence of a dynamic equilibrium between mantle temperature and seafloor depth observed elsewhere.