Iron-rich Smectite Formation in Subseafloor Basaltic Lava in Aged Oceanic Crust

Basalt weathering in oceanic crust controls long-term elemental cycling on Earth. It is unknown whether basalt weathering tends to continue in unsedimented oceanic crust with formation ages of >10–20 million years (Ma), when fluid circulation is restricted by the formation of secondary minerals in fractures/veins. We investigated basalt weathering in 13.5-, 33.5- and 104-Ma oceanic crust below the South Pacific Gyre by combining bulk and in-situ clay mineral characterisations. Here we show the formation of iron-rich smectite at the rims of fractures/veins in 33.5-Ma and 104-Ma core samples from depths as great as 121 metres below the seafloor. In contrast, iron-rich smectite formation was not observed in three 13.5-Ma core samples, which suggests that iron-rich smectite formation may be affected by the dilution of aqueous silica supplied from basalt dissolution by actively circulating fluid. As iron-rich smectite from the 33.5-Ma and 104-Ma core samples was more enriched in Mg and K than that typically found at hydrothermal mounds, iron-rich smectite formation appears to result from basalt weathering rather than hydrothermal alteration. Our results suggest that unsedimented basaltic basement is permeable and reactive to host microbial life in aged oceanic crust on Earth and possibly in the deep subsurface on Mars.