Abstract
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.
Highlights
The upper oceanic crust is mainly composed of basaltic lava erupted and solidified at mid-ocean ridges, where the buoyance of heated fluid drives vigorous circulation of oxygenated seawater
U1365E-8R4 and U1365E-12R2 were associated with fractures/veins mainly filled with celadonite and iron oxyhydroxides, respectively (Supplementary Table S2)
A layer with fibrous material mainly composed of Si, Fe, and Mg without a diagnostic Raman spectrum was found between a celadonite-filled fracture and basaltic groundmass in U1365E-8R4 (Fig. 1f)
Summary
The upper oceanic crust is mainly composed of basaltic lava erupted and solidified at mid-ocean ridges, where the buoyance of heated fluid drives vigorous circulation of oxygenated seawater. As a result of thermally driven fluid circulation, basaltic lava is hydrothermally altered to form secondary minerals such as a mica mineral called celadonite [K(Mg, Fe2+)(Fe3+, Al)(Si4O10)(OH)2] and iron oxyhydroxides[5]. On the young ridge flank, the circulation of oxygenated seawater is restricted, because thermally driven fluid circulation is weakened by rapid crustal cooling[6]. At this stage, the crustal fluid is enriched with reducing compounds such as Fe(II) and HS−, to form pyrite [FeS2] and a smectite mineral called saponite [Ca0.17Mg3(Si, Al)4O10(OH)2·n(H2O)]5. We characterised unsedimented basaltic basement cores collected at Sites U1365, U1367, and U1368 with crustal ages of 104 Ma, 33.5 Ma, and 13.5 Ma, respectively[14,15,16] (Supplementary Fig. S1, Supplementary Table S1)
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