MANGANESE MICROCHIMNEYS BURIED IN THE CENTRAL PACIFIC PELAGIC SEDIMENTS - EVIDENCE OF INTRAPLATE WATER CIRCULATION

Manganese oxide deposits of an irregular tube-like shape with unique c omposition and structure were found within Pliocene siliceous sediment s from an abyssal hill in the Central Pacific manganese nodule provinc e. 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 a nd sub-oxic diagenetic Mn deposits, low total REE abundance and strong negative Ce anomaly. The deposit consists of major todorokite with mi nor buserites, showing a hydrothermal signature. Bladelike crystals of the minerals typically grow within open tubes (several mm thick) and lack detrital minerals. All data indicate a low-temperature hydrotherm al or cold seep origin, or less likely a sub-oxic diagenetic origin. T his unusual manganese deposit from a pelagic environment is thought to have formed on the sea floor from Mn-bearing waters that passed throu gh sediments or basement rocks. We speculate that intraplate water cir culation took place during the Late Pliocene to Pleistocene due to the uplift and/or block faulting after the deposition of pelagic Pliocene sediments. (C) 1997 Elsevier Science B.V.