The geochemical cycle, Stage 2 : sinking convection cells

It is now well established that the new crust and lithosphere. Formed and chemically modified at ocean ridges, spreads away from the ridges and is almost completely returned to the mantle near the great trench systems of the Karth, in what are termed subduction zones. In this process all the chemical substances fixed in the altered ocean crust are returned to the mantle, to depths perhaps as great as 700 km, as seismic studies have shown. Thus atmosphere-hydrosphere components are recycled deep into the mantle.

Before the older, cooler and heavier crust reaches the subduction zone it is covered by a sediment layer. In the subduction process these sediments may be scraped off to build continental crust, or sub-ducted. The exact balance of these two possible processes is not well known at the present time, but recent studies have shown that, where the angle of descent of the lithosphere is steep, sediments appear to be subducted. As typical deep ocean sediments are rich in Si0₂, Al₂O₃ , potassium-bearing clay minerals and even uranium, these species may be returned to the mantle. There is no doubt that much research in the next decade will be directed at quantifying exactly what materials are subducted. The process has profound implications for mantle chemistry and models of crustal evolution.

One feature of this process is clear. Given the present knowledge of what is subducted there must be a most efficient process of return flow. Consider just one species, water. At present water is returned to the mantle at a rate of about 1.5 x 10¹² kg/a (assuming 5 per cent water in the ocean crust). This rate might be doubled if pelagic sediments were subducted. As the ocean mass is 1.4 x 10²¹ kg, this entire mass would be removed in a thousand million years (or half this time, if sediments are subducted). Since there are still oceans now this water must be returned to the surface.

The return flux is not well quantified. Some fluids must be released by metamorphic dehydration processes as the crust descends and becomes hotter. But the most obvious return process involves the volcanism that is associated with the regions above subduction zones. As the descending crust becomes sufficiently heated dehydration processes may cause melting of the materials and melting in the hotter overlying mantle (Figure 4.18).

4.18: Left : the ocean ridge where hot mantle materials rise and create new oceanic crust. The surface B consists of basaltic lava flows and pillow lavas; layer C below is made up of magma-filled cracks or dykes, and layer D of gabbro-peridotite. A large, basaltic magma chamber is shown at the ridge. The crust cools by conductive heat flow and by driving seawater convection cells, which vent to the ocean near the ridge (E). In favourable cases sulphide ore deposits may form from the discharge of metals in solution (Cu, Zn, Fe, Ag).

The new crust moves awayfrom the ridge and becomes sediment-covered (G). Fluid convection becomes confined in the basaltic layer beneath the impermeable sediments. Below left: a subduction zone where sediment-covered ocean crust and lithosphere (about 100 km thick) descends back to the mantle near a trench environment (A). Sediments may be scraped off, adding to the continent, but some may be dragged back to the mantle. At depth (D) water released from the descending slab may trigger melting and production of andesite melts which rise to form volcanoes. The thermalflux may lead to crustal fusion and the creation-of large granite plutons (B), which may be cooled by convecting groundwaters (C), again leading to metal concentrations and ore deposits (Cu. Mo. W, Sn, etc).

These rising andesite melts have exactly the compositions that might be expected. They are generally similar to the basalts formed at the ridges, but have higher contents of elements that are involved in the modification of the primary ridge materials, with a possible contribution from sediments. Thus the andesite magmas are often rich in gases (H₂0, CO₂ ) and elements such as potassium, sodium, silicon and uranium. The exact balance between what goes down and what comes back to the surface is not quantified, but is a subject of intense research at the present time.

The magmas that rise above subduction zones carry heat to the overlying crust. In many regions this crust is continental, and it responds to the heating from below by partial melting. The process leads to the production of magmas of the granite family, and these rocks form a major part of the crust above subduction zones. Granitic magmas tend to rise in giant (often 500 km³) bubbles and, where they penetrate to near the surface, they also cool, crack and can be water-cooled by typical continental ground waters. Again the fluid convective processes associated with the rise of these hot materials lead to the formation of major ore deposits, particularly involving elements such as copper, tin, tungsten, molybdenum and gold.