Geochemistry of hydrothermal manganese deposits from the Pitcairn Island hotspot, southeastern Pacific

Abstract

Hydrothermal Mn crusts from a submarine volcano in the Pitcairn Island region have been studied mineralogically, radiometrically, and by bulk and partition chemical techniques. The crusts contain layers that comprise three morphological types. A mechanism of formation by downward growth from a Mn-cemented surface sand hardpan is proposed. Age dating of the crusts suggests relatively recent, and rapid, formation. Mineralogically, the crusts exhibit variable amounts of 10Å and 7Å manganite. Chemical data, however, indicate that the crusts do not show element associations characteristic of marine birnessite, suggesting that the 7Å manganite phase may have transformed from structurally unstable 10Å manganite as a result of air drying. Trace element concentrations in massive, finely laminated types 2 and 3 crust layers are significantly lower than those in similar crusts from island arc and back-arc settings and, in general, are comparable with, or lower than, those of the most trace element poor crusts from mid-oceanic ridge environments. REE data suggest that these crusts are almost pure hydrothermal deposits with little or no hydrogenous component. In the types 2 and 3 crusts, only Mo shows a significant enrichment that is likely to be hydrothermal in origin, although it is enriched to lower levels than in hydrothermal crusts from most other geological settings. Friable, Mn-cemented sand type 1 crusts contain elevated trace element concentrations, in part as a result of incorporation of biogenic and volcaniclastic phases, but also as a result of hydrogenous inputs consistent with their greater age and closer proximity to seawater. In contrast to types 2 and 3 crusts which show no Li enrichment, type 1 crusts are enriched in Li to levels above those that can be accounted for by hydrogenous input and a hydrothermal source for it is proposed. The observed variability in the composition of hydrothermal Mn crusts between midplate hotspot, mid-oceanic ridge, island arc, and back-arc environments may be influenced by variability in basement composition and sea water/rock ratio in the hydrothermal systems. As both parameters are directly related to tectonic setting, this may play an important role in controlling the chemistry of hydrothermal deposits.