TECTONICS/Hydrothermal Activity 369 At temperatures exceeding 350–400 C, rocks begin to exhibit ductile behaviour Although this behaviour depends upon the rate which stresses are applied, ductile behaviour will tend to seal cracks Thus permeable pathways that may initially be opened by stresses resulting in brittle failure may gradually close This process may limit the depth to which cracks remain open in the crust and the extent to which hydrothermal circulation may approach magma bodies Water–Rock Chemical Reactions As aqueous fluids pass through hot subsurface rocks, chemical reactions occur Some chemical constituents may be removed from the fluid, others may be extracted from the rock The reactions may also involve isotopic exchange between the fluid and rock These reactions are complex functions of temperature, pressure, lithology, permeability structure, duration of activity, and other factors A detailed discussion of this topic is beyond the scope of this article; however, the use of geochemical thermometers and the formation of hydrothermal ore deposits are discussed briefly Geochemical thermometers The strong temperature dependence of solubility of certain chemical constituents in hydrothermal fluids, the temperature dependence of elemental partitioning between rock and solution, and the temperature dependence of isotopic partitioning between mineral and fluid phases have led to the development of a variety of geochemical thermometers to deduce subsurface conditions from surface samples The quartz geothermometer utilizes the strong temperature dependence of quartz solubility and the slow kinetics of quartz precipitation at low temperature As hydrothermal solutions in equilibrium with quartz at high temperature rise to the surface and cool, the high degree of disequilibrium in the measured quartz concentration permits a calculation of the equilibrium temperature at depth Other common geothermometers include: Na/K, which makes use of the temperature dependence of partitioning of these elements between aluminosilicate rocks and hydrothermal fluid; Na–K–Ca, which includes the effect of Ca in the partitioning; and ratios of stable isotopes such as d13C, d18O, dD, and d34S Various factors affect the resolution and reliability of each of these geothermometers, so often many independent ones are used Ore deposits–fossil hydrothermal systems As a result of water–rock chemical reactions at intermediate to high temperature, trace metallic ore-forming metals such as Fe, Cu, Zn, Sb, Au, Ag, and Pb are transferred from the rock to the hydrothermal solution Because most of these metals form metallic sulphides that are highly insoluble in water, solubility is achieved by the formation of bisulphide or chloride ion complexes Various mechanisms may cause local precipitation of these metal–ion complexes, resulting in a concentrated accumulation of metallic ore A rapid drop in the solution temperature because of thermal conduction or mixing with cooler fluids, a change in solution pH, and boiling can all lead to ore deposition Many types of ore deposits in the geological record, such as porphyry ore deposits associated with silicic volcanism, Mississippi Valley-type lead-zinc deposits, and volcanically hosted massive sulphide deposits are linked to hydrothermal activity Such ore deposits thus present an integrated fossil record of hydrothermal activity, and provide a window into subsurface heat transfer and fluid flow processes By coupling this integrated fossil record with studies of active oreforming processes on the seafloor and in other active hydrothermal environments, one can obtain a more complete picture of hydrothermal activity (see Mining Geology: Hydrothermal Ores) Temporal Variability in Hydrothermal Activity Temporal variability on a range of time-scales is a fundamental characteristic of hydrothermal activity (Table 2) Some of this variability is linked to episodicity in magmatic and tectonic activity, or climate changes The occurrence of these processes ranges from scales of plate reorganization of $106–107 years to magma replacement times of $101–104 years at fast and slow spreading ridges, respectively Temporal variability related to the fluid circulation system, mainly resulting from changes in crustal permeability, occurs on time-scales $1–102 years Seafloor hydrothermal activity is known to change on time-scales of hours to months following earthquakes, igneous intrusions (e.g., dykes), or volcanic eruptions Climate changes may alter precipitation patterns, and hence fluid recharge, on time-scales of 10–103 years; ice ages and glaciation may affect high-altitude systems on similar time-scale Two-Phase Flow Boiling and phase separation commonly occur in high-temperature hydrothermal systems For pure water, boiling is defined by the boiling point curve as a function of pressure Liquid phase occurs below