Effects of hydrothermal fluids on the heavy mineral assemblage of a late Pleistocene succession deposited in an oceanic ridge valley (Escanaba Trough, Juan De Fuca Plate)

This short contribution deals with the behaviour and modification of heavy mineral assemblages in a 500 m-thick turbidite succession deposited in the rift valley of the 3300m deep Escanaba Trough of the Juan de Fuca Plate. To our knowledge, there have been no previous studies on the effects of hydrothermal fluid circulation on heavy minerals in such an active geochemical environment. The sediments were transported mainly from glacial lakes of the Columbia drainage basin by cataclysmic floods during the last 60 ka. Climatic and sediment transport conditions imply minor weathering of the source rocks, with little sorting and mechanical abrasion of heavy minerals during transit. Our results indicate that the heavy minerals experienced severe chemical etching and dissolution by circulating hydrothermal pore fluids that caused, at various depths, the complete loss of particular species, especially the chemically highly unstable ortho- and clinopyroxenes. These specific geochemical conditions also generated new minerals, mainly titanite, iron-rich magnesite, barite, and pyrite. However, at present there is insufficient data to fully constrain the kinetics of dissolution of the various minerals as a function of pH, temperature, composition of pore fluids, and sediment-permeability. The length of time during which the sediments were affected by pore fluid movements is constrained by the very young (Late Pleistocene, 60 ka) age of the sediments.