The conditions for potential abiotic formation of organic compounds from inorganic precursors have important implications for understanding past and present global carbon budgets. With respect to the early Earth, much of the current discussion has focused on whether the Earth's inventory of organic compounds was introduced from space or was a natural consequence of reactions taking place in the atmosphere, hydrosphere, and geosphere. It is, of course, of great interest in geochemistry to determine plausible pathways for abiotic synthesis of organic compounds. It is, however, equally important to identify natural settings that would be conductive for such reactions, as well as for the accumulation of simple organic compounds and for their subsequent reaction to form more complex macromolecules. The geosphere with its submarine hydrothermal systems offers a type of environment that in principle has not changed during the four billion years that have passed since the Earth's crust was formed.

Formation of oceanic crust and hydrothermal systems

New oceanic crust is created mainly by basalt production at ocean ridge spreading centers (Figure 4.1). Some basalt is also produced at oceanic hotspots, like the one that has created the Hawaiian Island Chain and is now overlain by the Loihi Seamount. During the process of basaltic crust formation, convecting water acts as a cooling fluid. The convecting water carries thermal energy away toward the relatively cold rock surface, thus creating hydrothermal systems. Geologists differentiate hydrothermal systems on the basis of their tectonic settings. In Figure 4.1, the most common types are shown: sediment-free on-axis systems on plate tectonic spreading centers, and off-axis systems on the flanks of the spreading centers.