Prof Chris MacLeod in April 2013 Nature Geoscience

15 Apr 2013

Writing in the April 2013 issue of Nature Geoscience, D. Sauter et alia, including Cardiff EARTH's Prof Chris MacLeod, present research that takes apart the long-established model for seafloor spreading at mid-ocean ridges.

One of the fundamental tenets of plate tectonics is that, as rigid plates are pulled apart at mid-ocean ridges, the mantle wells up from depth beneath the ridge axis to fill the gap. As it decompresses it starts to melt, generating magma that rises to create a 6-7km thick oceanic crustal layer and feed lava flows on the seafloor. Ocean lithosphere thus generated covers two-thirds of the Earth’s surface, and is entirely replaced every 200 Myr or so.

Recent advances, however, are revealing more in the mechanism of seafloor spreading than the classic model suggests. Several research groups, including Prof MacLeod's at Cardiff, have shown that huge extensional faults are present at the axis of slow-spreading ridges like the Mid-Atlantic Ridge. Greased by hydrous minerals such as talc, these ‘detachment’ faults are weak and slippery, and play a key role in helping the newly-formed oceanic plates separate. In places, the faults may be large enough and sufficiently long-lived to bring deep-seated mantle rocks up onto the seafloor from beneath the volcanic ocean crustal layer.

In this paper (also discussed in an accompanying News & Views article by D. Smith), Sauter and colleagues have shown that this ‘tectonic spreading’ mechanism is a far more extensive and extreme process than previously supposed. From a sonar survey and dredge sampling of the Southwest Indian Ridge – which at 14mm/yr is one of the slowest spreading ridges on Earth – they have shown that ocean crust is almost completely absent over huge swathes of the Indian Ocean floor. Instead, a succession of detachment faults, forming sequentially and operating in turn, has accommodated plate separation for at least the last 11 million years. No magmatic crustal layer ever formed; instead, only a few thin, isolated lava flows were erupted over this entire time interval.

This radical finding has implications not only for the underlying mechanisms of plate tectonics, but also for global biogeochemical cycles. The mantle rocks brought to the surface by the detachment faults are composed primarily of olivine, a mineral that is highly reactive in the presence of seawater. Olivine is transformed to the hydrated, low density mineral serpentine via a strongly exothermic reaction that generates high-pH fluids, hydrogen and abiotic hydrocarbons, causing widespread ancillary carbonate precipitation and creating a fertile environment for the development of sub-seafloor microbial communities. If, as Sauter et al.’s  study suggests, serpentinite seafloor forms a globally significant proportion of the ocean basement, the associated mineral carbonation reactions may constitute a highly important but hitherto unappreciated component of the global carbon cycle.

You can find the article here.