proportion to the total, the salinity, which is about 35 parts per thousand by weight on average. A final component, Ultimately, the concentrations of all components in seawater are controlled both by the rates at which they are added from sources and the rates at which they are removed by sinks. Rivers are the major source of most elements in seawater, but the atmosphere is the major source for dissolved gases as well as a few metals such as Al, Pb, and Th, which reach the ocean in wind-blown dust. Ridge crust hydrothermal activity is an important source of some elements, but it is also an important sink for others. Sediments are the major sink for most elements, and half the ocean floor is covered by the carbonate and siliceous shells of planktonic organisms. Evaporites are the major sink for Na+, K+, Cl–, and
Biological processes exert an extremely important influence on ocean chemistry. Unlike the other major components, the concentration of
Unlike the major elements, concentrations of most minor and trace elements are quite variable in the oceans and much of this variation is due to biologic activity that imposes vertical concentration gradients, as these elements are taken up by phytoplankton in the surface water and released by respiration in the deep water. This includes not only nutrients such as P, Si, and Fe, but also nonutilized elements such as Ge because organisms take them up incidentally. A few elements, such as Al and Pb, show the opposite pattern: enrichment in the surface water and depletion in deep water because wind-deposited dust is the primary source of these elements and they are quickly scavenged onto particle surfaces after deposition.
In the final chapter we see how geochemistry can be used to address the needs of society, specifically, its need for mineral resources and environmental protection. The story of civilization is in some respects the story of increasingly sophisticated tools. The Stone Age ended when people learned to produce copper metal from copper sulfide ores around 7000 years ago. Copper tools were subsequently replaced by bronze ones and then by iron ones beginning around 3000 years ago. In a sense, we still live in the Copper and Iron Ages, however, as 21 million tons of copper ore and 2.5 billion tons of iron ore were mined globally in 2018. In the United States, about half the demand for metals is met by recycling, but modern society still need enormous amounts. Furthermore, modern technology requires a great variety of metals, many of which were unknown as recently as two centuries ago. At least 80 different elements are incorporated in smartphones or used in their production, including exotic ones like neodymium, europium, and tantalum. Two other exotic elements, cadmium and tellurium, are used to produce CdTe solar panels, which have the highest efficiency and can be produced in thinner films than other solar cells.
We'll discuss the process of geochemical exploration and consider examples of the formation of a variety of ore deposit types. The first of these is the Bushveld complex of South Africa, which is an example of orthomagmatic ores, in which the ore had precipitated directly from magma. The Bushveld, which outcrops over an area the size of Ireland, is a layered mafic intrusion that formed 2 billion years ago and hosts the world's largest reserves of platinum group elements, Cr, and V. Decades of geochemical detective work have shown that these ores formed as fractional crystallization combined with repeated intrusions of magma and assimilation of surrounding crust periodically saturated the magma in ore-forming minerals, including chromite, magnetite, and sulfides that settled out of the magma chamber to formed distinct bands. In contrast, hydromagmatic ores such as porphyry copper deposits, which are the primary source of copper ore, form when a saline aqueous fluid exsolves from a magma and intrudes, often with violent force, into surrounding rock. Laboratory experiments together with analysis of fluid inclusions in these ores have revealed that many metals, including Cu, Zn, Pb, Co, Sn, and Au, form highly soluble chloride and sulfide complexes in these fluids at elevated temperatures and partition into the fluid phase from the magma, then precipitate when the solution cools. These form mainly from subduction-related magmas because they are rich in water and oxidizing; the latter prevents premature precipitation from the magma of the ore metals as sulfides. Many tin deposits form in a similar way but the magmas are produced by melting of Sn-rich sediments within the crust and reducing conditions allow Sn concentrations to build up through fractional crystallization and Sn is often complexed by F rather than Cl.
Hydrothermal ores also precipitate from aqueous solution and chloride complexes are also important in transporting metals in these deposits. The fluid, however, is derived from seawater or formation brines within the crust. These types of deposits include volcanogenic massive sulfides (VMS); mid-ocean ridge hydrothermal systems are actively forming examples of this type of deposit. The ore-forming fluids can be directly sampled and their chemistry determined; study of these systems has provided much insight into how VMS deposits form. Seawater is warmed as it penetrates the hot, young ocean crust and a series of reactions result in the solution becoming acidic and reducing. Under these conditions, metals, most notably Cu, Zn, and Pb, are leached from the rock. When temperatures reach 350–400°C, the fluid rises, eventually mixing with seawater whereupon the metals precipitate as sulfides.
We'll examine two examples of sedimentary ore deposits. The first is banded iron formations, which are the principal source of iron ore. Most of these formed around the time the atmosphere first became oxidizing about 2.3−2.4 billion years ago as ferrous iron-rich deep ocean water upwelled to the surface and the iron was oxidized to the insoluble ferric form. Directly or indirectly, the evolution of photosynthetic life appears responsible for them. Brines associated with saline lakes and salars, or salt flats, and their associated brines, particularly from the high plateaus of the Andes and Tibet, are becoming the most important source of lithium, which is needed for high performance batteries in everything from cell phones to electric cars. But not all such brines are Li-rich; we learn the conditions under which Li-rich brines form. Weathering-related ore deposits include
