The primary geochemical cycle of the solid Earth is directly associated with plate tectonics and mantle convection; it is illustrated schematically in Figure 12.1. This is a box model in which the principal geochemical reservoirs are included. These are the core, the mantle, the oceanic crust, the continental crust, the oceans, and the atmosphere. In terms of the formation of the Earth, the mantle was the primary reservoir. The core was formed by the differentiation of the dense iron-rich components. The oceanic and continental crusts were formed by the differentiation of the light early melting silicic components. The origin of the oceans and atmosphere remains controversial. The relative contributions of internal degassing and extraterrestrial inputs are also uncertain.
The creation of the oceanic crust at mid-ocean ridges leads to the strong concentration of incompatible elements from the upper mantle into the basaltic oceanic crust through the partial melting process. Gases and fluids generated by this volcanism transfer incompatible and volatile elements to the oceans and atmosphere. Hydrothermal processes also exchange material between the oceanic crust and the oceans. The oceanic crust is coated with sediments that are primarily derived from the continents.
Ridges Hot Spots
Island Arcs Hot Spots
Figure 12.1. Schematic diagram of the geochemical reservoirs and interactions involved in the chemical geodynamic behavior of the Earth.
At ocean trenches the altered oceanic crust is cycled back into the Earth's interior. Along with the descending lithospheric plate, some continental material, including altered oceanic crust and entrained sediments, is recycled into the mantle at subduction zones. At a depth of about 100 km, the upper part of the oceanic crust melts and partial melting occurs in the overlying mantle wedge; these processes further concentrate incompatible elements and result in island arc volcanism. Island arc volcanism, together with continental flood basalt volcanism and hot spot volcanism, form new continental crust. However, all these sources have compositions that are considerably more mafic (basaltic) than the present silicic composition of the continental crust. Further differentiation of the continental crust is attributed to remelting events and delamination of the mafic lower continental crust (see Section 2.7). The magmas from the mantle associated with subduction zone volcanism, flood basalt volcanism, and hot spot volcanism intrude the continental crust and in the presence of water produce silicic (granitic) magmas. These magmas rise into the upper crust, making the upper crust more silicic and the lower crust more mafic. Subsequently the mafic dense rocks of the lower crust are returned to the mantle by delamination. The net result is that continental crust becomes more silicic with time, transforming into a reservoir for the incompatible elements, including the radiogenic elements U, Th, and K and the light rare earth elements.
Although the continental crust is small in volume, its enrichment in incompatible elements is so large that it constitutes a significant reservoir for these elements. The atmosphere constitutes an important reservoir for the radiogenic gases helium and argon. While the oceanic crust plays a critical role in chemical geodynamics, its volume is so small that it can be neglected in isotopic mass balances.
The mantle reservoir in Figure 12.1 is divided into two parts, an upper mantle reservoir and a lower mantle reservoir. Since the style of mantle convection still has uncertain aspects, we consider the mantle as both a two-layer system with limited transport between the layers and a single convecting layer. Even with two-layer mantle convection, substantial transport of material between the layers is likely to occur. Subducted lithosphere may penetrate into the lower layer while plumes generated in the lower layer may penetrate through the upper layer. Active convective processes in either layer may entrain material from the other layer.
The core reservoir in Figure 12.1 is also divided into two parts in accordance with our knowledge of the structure of the core. Cooling of the Earth through geologic time has resulted in the growth of a solid inner core. In this process, light alloying elements such as Si are concentrated into the liquid outer core which becomes progressively richer in the light elements with time. Of particular importance here are possible modes of interaction of the lower mantle and outer core including chemical reactions at the core-mantle boundary, exsolution of light elements from the outer core into the lower mantle, and dissolution of heavy elements from the lower mantle into the outer core. While there is much speculation about core-mantle mass exchange, there is no strong evidence for significant material transport between these reservoirs and in the following we assume such transport is negligible and neglect the core reservoir.
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