Hydrothermal oxide and nontronite deposits on seamounts in the eastern Pacific

Abstract

Deposits of Fe oxide mud, nontronite, and Fe-Mn crusts were sampled from the summits of two seamounts in the eastern Pacific. Where low temperature (0–15°C) hydrothermal fluids are issuing, the deposits consist of X-ray amorphous Fe oxyhydroxide and are rich in Fe (43 wt.%), contain minor Si and P (4% and 3.5%, respectively), and have very low Mn (<0.01%) and other trace element contents. Other deposits, where no current hydrothermal activity was observed, consist of mud and crusts composed of amorphous material and poorly crystalline hematite, goethite, and smectite. These deposits have slightly higher Mn (up to 2%) and other trace element contents. The mineralogical and chemical differences between the active and inactive deposits and crusts are interpreted to be due to recrystallization and contamination with detrital, biogenic, hydrogenous, and possibly hydrothermal sulfide material.Nontronite deposits, capped by Fe-Mn oxides, were sampled from one of the seamounts. These deposits have compositions similar to other seafloor nontronite deposits, and formed at low temperatures (30°C) during mixing of low temperature hydrothermal fluids with seawater.The oxide deposits consist almost entirely of long, delicate filaments, remarkably similar in morphology to genera of Fe oxidizing bacteria. Such bacteria were probably important in catalyzing the oxidation and precipitation of iron from hydrothermal fluids on the seamounts. The occurrence of nearly identical filament-rich oxide deposits on two widely separated seamounts suggests that this type of deposit may be common on seamounts, but previously unrecognized.