KINETICS OF EARTH PROCESSES - Chemical Fundamentals of Geology and Environmental Geoscience

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and reaction has effectively ground to a halt. This is

called the blocking or closure temperature .

The closure temperature is an important concept in

geochronology. In the potassium-argon dating method

(Chapter 10) the age of a rock is established by measur-

ing the minute amount of 40 Ar (argon is a gas) that has

accumulated in a potassium-bearing mineral from the

decay of 40 K. The method therefore depends critically

on the ability of the relevant mineral grains to imprison

this intra-crystalline argon component that, being a

gas, tends to escape. At temperatures above the closure

temperature for argon diffusion, the 40 Ar atoms diffuse

to grain boundaries and escape. A K-Ar age determ-

ination on an igneous rock therefore records not the

age of intrusion or eruption, but the date when the

rock had cooled to temperatures low enough for the

rate of diffusion of 40 Ar out of the grains of the potas-

sium mineral to be insignificant. A later phase of meta-

morphism in which the rock is again heated above the

closure temperature would discharge the 40 Ar accumu-

lated up to that point, and the isotopic clock would

thereafter record a 'metamorphic age', indicating the

time when the rock body had again cooled below the

closure temperature.

minerals are characterized by large activation energies,

and therefore only at high temperatures is there a sig-

nificant population of reactant 'molecules' possessing

the kinetic energy required to surmount the activation

energy hurdle (Figure 3.4). Disequilibrium textures in

many rocks (Box 3.1) testify to the rapid slowing-down

of chemical reactions as temperature falls, and at sur-

face temperatures disequilibrium is the rule rather than

the exception: the persistence of Fe 2+ minerals on the

Earth's surface, where atmospheric oxygen makes Fe 3+

the stable form of iron, is one obvious example. Both

the flow of silicate melts (Figure 3.8) and diffusion are

also strongly temperature-dependent (Figures 3.6 and

3.7), suggesting they too involve an activation step.

The marked slowing-down of diffusion with falling

temperature, and its effective cessation at the closure

temperature, are essential requirements for radiomet-

ric dating.

Many reactions in the atmosphere, however, rely on

another energy source, the Sun. Solar photons, particu-

larly those of UV wavelengths, are energetic enough to

tear apart chemical bonds in molecules like O 2 , O 3 , NO 2 ,

H 2 O 2 , CFCl 3 and HCHO (formaldehyde). The result of

such photodissociation reactions is often the formation

of free radicals such as O • , HO • , H • , HO 2 • , NO 3 • Cl • ,

ClO • and HCO • . Free radicals, possessing unpaired

electrons (represented by the symbol ' • '), are highly

reactive: through photodissociation they have already

reached the energy 'pass' and thus they are capable of

initiating gas reactions in a manner unrestrained by

considerations of activation energy. For example:

Review

A chemical reaction can be visualized in free energy

terms as a journey leading from one valley, the domain

of the reactants, to another where the product species

form the dominant population (Figure 3.3). The route

from one valley to the other leads across a high pass in

free energy space, the 'transition state', and reactant

molecules that cannot summon sufficient energy to

traverse this high ground will not reach their 'destina-

tion'. The key factor is the availability of energy: if

energy is plentiful, the traffic over the pass will be

heavy (the reaction rate will be high); if energy is hard

to come by, many reactant molecules will be forced to

turn back before attaining the pass.

What sources of energy drive geochemical reac-

tions? For reactions taking place in the Earth's interior,

reactants must rely on the kinetic energy possessed by

their constituent atoms and molecules. Accordingly

the temperature plays a crucial role in determining

the rates of geochemical reactions, as enshrined in

the Arrhenius equation. Reactions involving silicate

CH HO HC HO

+ →+

(3.18)

•

This reaction incidentally illustrates the key role of the

hydroxyl free radical HO· as an atmospheric cleansing

agent, removing many forms of pollution (including

the potent greenhouse gas methane, CH 4 ) from the air

we breathe.