Tubular Nontronite-Bearing Hydrothermal Deposits from a Galápagos Seamount

Abstract:

An unusual tubular deposit of Fe-rich hydrothermal nontronite has been discovered by remotely operated vehicle (ROV) exploration on a young seamount of the Wolf-Darwin lineament in the Galápagos Islands. X-ray diffraction, ICP-MS, ICP-AES, and SEM-EDS analyses show that this deposit is chemically and mineralogically similar to other deep-sea hydrothermal nontronites indicating a likely formation temperature of about 30º to 50º C by diffuse hydrothermal activity. These deposits contain Fe-oxides and Fe-rich, Al-poor nontronite, with bulk composition of 38-51 wt.% SiO₂ and 40-50 wt.% Fe₂O₃. The presence of filamentous and lepispherical structures in the samples suggests that mineral deposition was facilitated by chemosynthetic bacteria. Although hydrothermal nontronite has been sampled at a number of seafloor sites by coring and dredging, this is the first in situ documentation of its unusual sinuous, tubular structure, on the seafloor. Quantitative image-analysis of ROV imagery indicates that hydrothermal fluid pathways that developed through an underlying pillow lava sequence likely control the distinctive pattern.