Earth's Mantle below the Oceans [Preview]

Samples collected from the ocean floor reveal how the mantle's convective forces shape Earth's surface, create its crust and perhaps even affect its rotation

The dike complex, about one kilometer thick, was topped by a layer of pillow basalt, the form taken by basaltic magma when it cools and solidifies rapidly on eruption to the seafloor. During the next several days, we explored a different section and confirmed our previous findings. We were quite excited because no one had ever before observed a complete and relatively undisturbed section of oceanic upper mantle and crust. We immediately documented our discovery in a short paper that we mailed to Nature as soon as we docked a few weeks later.

Encouraged by the results of the Nautile dives, we conducted two other expeditions and established that the Vema lithospheric section is exposed on the seafloor for more than 300 kilometers. After mapping the magnetic anomalies produced by the seafloor, we could estimate the velocity with which the lithosphere moves away from the ridge axis. We thus established that the Vema section exposes lithosphere created gradually at the axis of the Mid-Atlantic Ridge during a time interval of more than 20 million years--a unique opportunity to study how the creation of lithosphere varies through time!

During the dives, we had used the Nautile's mechanical arm to grab a number of samples of mantle peridotite. We later sampled by dredging mantle peridotites at close intervals along the base of the section in lithosphere of increasing age. From the mineral composition of these rocks we estimated the variations in the degree of melting they had undergone over time during their ascent below the Mid-Atlantic Ridge. At the same time, we could estimate how crustal thickness varied through time, thanks to gravimetric data obtained from both ship and satellite measurements of the gravity field produced by rocks below the seafloor. Crustal thickness depends on the quantity of melt generated by mantle ascending below the ridge.

The results were quite unexpected. The degree of melting of the mantle and the crustal thickness both appear to have increased steadily from 20 million years ago to today. Small oscillations are superimposed on this general trend. The simplest interpretation of these results: the Mid-Atlantic Ridge is becoming steadily "hotter" over time.

Surprisingly, the increase of the temperature of the upwelling mantle is accompanied by a decrease in the spreading rate of the lithospheric plate generated at the ridge axis. This result contrasts with the concept of "passive" upwelling of the mantle in response to the diverging motion of the lithospheric plates--a concept that would require proportionality between spreading rate and degree of melting of the ascending mantle.

We were also able to estimate the velocity of the solid mantle that rises below the ridge, crucial information for refining our models on the formation of the oceanic crust. The speed of the rising mantle depends on its temperature and composition (both affect density and viscosity) and on the diameter of the rising column and is related to the velocity of the lithospheric spreading that diverges from the ridge axis.

How can we estimate the speed of the rising solid mantle? The rising mantle generates melt within a depth interval that can be estimated from experiments and theoretical considerations. The melt fraction rises rapidly, cooling and solidifying as basalt in the crust, while its parent mantle continues to ascend slowly.

When the "parent" mantle peridotite reaches the lithosphere and starts moving horizontally with the plate away from the ridge, the basalt it generated has moved farther away from the ridge. The horizontal distance between the parcel of basaltic crust and its parent mantle, translated as time, would allow us to estimate the velocity of the rising solid mantle. After correlating the temporal variations of the degree of mantle melting with the variations of crustal thickness along the Vema lithospheric section, we estimated the solid mantle rose at an average velocity of about 25 millimeters per year.

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