Chemical and isotopic investigations into the origin of clay minerals from the Galapagos hydrothermal mounds field

A dark green authigenic nontronite is the major component of the Galapagos hydrothermal mounds field sediments. Oxygen isotopic compositions of the chemically purified, <0.2-μm fraction of the nontronitic clays indicate formation temperatures of 25° to 47°C, in contrast with measured in situ mounds temperatures of up to 15°C. Assuming an authigenic origin, the Fe-rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27° to 39°C, compared with measured in situ temperatures of ca. 3.5° to 6.5°C. The higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the Holocene-Pleistocene. The apparent high isotopic temperature of the Fe-rich montmorillonite may reflect: (1) formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents, (2) a detrital origin of the mineral, or (3) a mixture of authigenic Fe-montmorillonite and detrital Al-montmorillonite in this region.