Manganese microchimneys buried in the Central Pacific pelagic sediments: evidence of intraplate water circulation?

Abstract

Manganese oxide deposits of an irregular tube-like shape with unique composition and structure were found within Pliocene siliceous sediments from an abyssal hill in the Central Pacific manganese nodule province. Effective separation of Mn from Fe and probably a rapid rate of Mn deposition are suggested from the chemical composition: a very high Mn/Fe ratio (up to 400) and low concentrations of transition metals. The REE pattern is similar to other marine low-temperature hydrothermal and sub-oxic diagenetic Mn deposits, low total REE abundance and strong negative Ce anomaly. The deposit consists of major todorokite with minor buserites, showing a hydrothermal signature. Blade-like crystals of the minerals typically grow within open tubes (several mm thick) and lack detrital minerals. All data indicate a low-temperature hydrothermal or cold seep origin, or less likely a sub-oxic diagenetic origin. This unusual manganese deposit from a pelagic environment is thought to have formed on the sea floor from Mn-bearing waters that passed through sediments or basement rocks. We speculate that intraplate water circulation took place during the Late Pliocene to Pleistocene due to the uplift and/or block faulting after the deposition of pelagic Pliocene sediments.