Geospeedometers

Component exchange between phases is controlled by mass transfer. Between solid phases, mass transfer is through diffusion where the exchange of components may be used as a geospeedometer (Lasaga, 1983). Convection rather than diffusion may play a dominant role if fluid phases are involved. In reactions between solid and fluid phases, diffusion in the solid phase is usually the slowest step. However, dissolution and reprecipitation may occur and may accomplish the exchange more rapidly than diffusion through the solid phase. 

Only two high-temperature homogeneous reactions have been investigated in detail for their kinetics by geochemists. One is the Fe-Mg order-disorder reaction in orthopyroxene, and the other is the hydrous species interconversion reaction in rhyolitic melt. The two reactions have been applied as geospeedometers in various geochemical and meteoritic problems. Because they are often encountered in geochemical kinetics literature, the two reactions are discussed in depth below. 

The values of from the two expressions differ by about 20%. Most likely, this sudden jump of Xd atXps s 0.18 is an artifact. When applying the orthopyroxene geospeedometer, it is necessary to choose the appropriate relation of iC. ... 

Another geochemical reaction that has been investigated extensively as a geospeedometer by high-temperature geochemists is the hydrous species reaction in rhyolitic melt. As the H2O component dissolves in silicate melt, it partially reacts with oxygen in the melt to form OH groups (Reaction 1-10) ... 

In the literature, various methods to infer cooling rate from the measured species concentrations of a homogeneous reaction have been developed. These methods are summarized and assessed below. Then two specific geospeedometers based on two homogeneous reactions are presented. 

Four methods are available in the literature for inferring cooling rate or cooling timescale from measured species concentrations in homogeneous reaction geospeedometers temperature-time transformation, Ganguly s method, Zhang s equation, and the empirical method. They are outlined below. 

Unlike the theoretical method, which can be extrapolated to some degree (as long as the temperature dependences of K and k apply to lower temperatures), it is best not to extrapolate a geospeedometer calibrated using the empirical... 

With the above general background, we are now ready to apply some widely used geospeedometers based on homogeneous reactions. The equilibrium and kinetics of these reactions have already been discussed earlier. The geospeedo-metry application is the focus in the following two subsections. 

This section focuses on how the Fe-Mg order-disorder reaction (Section 2.1.4) is applied as a geospeedometer. The equilibrium and kinetics of the reaction are discussed in Section 2.1.4 and only a brief review is provided here. Although there is some complexity in the kinetics of this reaction (e.g.. Figure 2-5), it is minor, and is hence usually ignored so that the forward and backward reactions are treated as elementary reactions. The rate coefficient for the forward reaction of this reaction (Reaction 2-55)... 

The following factors should be considered in using this geospeedometer ... 

The experimental timescale is no more than a few years, but the cooling timescale of interest is often of the order of millions of years. That is, experimental kinetic data often must be extrapolated by 6 orders of magnitude in timescale (about 300 K in temperature). Because the formulation of this geospeedometer has a theoretical basis, some extrapolation is OK. However, the reliability of huge extrapolations by six orders of magnitude cannot be evaluated. 

In summary, the orthopyroxene geospeedometer is best applied to rocks that cooled rapidly so that Fe(Ml) can be measured to enough precision and extrapolation of experimental results is small. 

Oxygen isotope fractionation has been applied as a thermometer and geospeedometer. Oxygen isotopic ratios vary slightly from one phase to another. The small variations are conventionally expressed by 8-notation defined as... 

Based on the above discussion on various geospeedometers, a rock contains many clues from which its thermal history may be read. Some of these processes, such as homogeneous reactions and diffusion, are simpler and better understood, and hence can be more easily quantified as geospeedometers. Other processes are more complicated, and information stored by those remains to be deciphered. Often the more complicated processes may store more information on the thermal history. 

The geospeedometer based on the kinetics of interconversion of hydrous species in rhyolitic melt is also well developed although the reaction mechanism and rate law are not known. The empirical calibration covers a cooling rate range of 50 Klyx to 100 K/s, about eight orders of magnitude. Theoretically, this... 

Complicated heterogeneous reactions may contain the most information on the thermal history of a rock. Currently, few such reactions have been quantified as geospeedometers because rates of such reactions are more difficult to evaluate. Nonetheless, future development of kinetic theory may demonstrate the rich resources in the kinetics of such reactions and the possibility to infer complex thermal history from such reactions. 

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