**7.3.3. Fission track and apatite fission-track analysis**

Recently, man has only a slightly stronger influence on the total amount of the Earth's phosphorus than his prehistoric ancestors. If man made a significant alteration in the cycles of phosphorus, it had an impact on the cycles of fresh surface waters. The detergent phos‐ phates have been blamed for degrading freshwater lakes and there is no doubt that several lakes have been overabundant with phosphates and sewage. Sewage treatment will alleviate

The overall natural and artificial cycles involving phosphorus are introduced in **Fig. 13**.

Weathering and leaching processes from millions of years ago led to the transfer of phos‐ phate to rivers and oceans where it was concentrated in shells, bones and marine organism that were deposited on the sea floor. Subsequent uplift and other geological movements led

Generally, weathering of apatite occurs synergistically through biotic and abiotic processes and leads to the release of mineral phosphate. Inorganic phosphate cannot be assimilated by plants, but it can be converted to the bioavailable form orthophosphate (HPO4 2−, H2PO4 <sup>−</sup>

some species of phosphate-solubilizing fungi and bacteria. The main mechanism underlying the microbial phosphate solubilization is the secretion of organic acids that, by changing the soil pH and acting as chelators, may induce the dissolution of phosphorus from minerals and its release into the pore water of soils [79],[80]. The dissolution of apatite is described in

Apatite represents an important source of inorganic P for natural ecosystems and may favor the establishment of microbial communities able to exploit it [79]. The microorganisms can cause the fixation or immobilization of phosphate, either by promoting the formation of inorganic precipitates or by the assimilation of phosphate into organic cell constituents on intracellular polyphosphate granules. Insoluble forms of inorganic phosphorus, e.g. calcium, aluminum and iron phosphates, may be solubilized through the microbial action. The

**i.** The first mechanism may be the production of inorganic or organic acids that attack

**ii.** The second mechanism may be the production of chelators such as gluconate and 2-

**iii.** The third mechanism of phosphate solubilization may be the reduction of iron in

**iv.** The fourth mechanism is the production of hydrogen sulfide (H2S), which can react

portion of insoluble phosphate salts and thus force their dissociation.

ketogluconate, citrate, oxalate and lactate. All these chelators can complex the cation

ferric phosphate, e.g. strengite (Fe3+PO4·2H2O [81]), to ferrous iron by enzymes and metabolic products of nitrate reducers such as *Pseudomonas fluorescens* and *Alcali‐*

with iron phosphate and precipitate it as iron sulfide, thereby mobilizing phos‐

mechanisms by which the microbes accomplish this solubilization vary [44]:

the insoluble phosphates.

*genes* spp. in sediments.

phate, as in the reaction [44]:

) by

most of the problems associated with point-source loading of lakes [78].

360 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

to these accumulations becoming dry land deposits [25],[78].

**7.3.2. Weathering of apatite**

**Section 3.4**.

The fission-track (FT) dating is a radiometric dating method26 based on the analysis ofradiation damage trails (fission tracks) in uranium-bearing, non-conductive minerals and glasses. It is routinely applied to the minerals apatite, zircon and titanite. Fission tracks are produced continuously through geological time as a result of spontaneous fission track of 238U atoms27 that undergo spontaneous fission.28 The atom splits into two parts that move rapidly in opposite directions, creating a long thin region of damage. The submicroscopic features with an initial width of approximately 10 nm and the length of up to 20 μm can be revealed by chemical etching.29 Crucially, fission tracks are semi-stable features that can self-repair (shorten and eventually disappear) by the process known as annealing at a rate that is a function of both time and temperature. The extent of any track shortening (exposure to elevated tempera‐ tures) in a sample can be quantified by examining the distribution of fission-track lengths [6], [85],[86],[87],[88].

This unique sensitivity of the apatite fission-track system is now of considerable economic importance due to the coincidence between the temperature range over which annealing occurs and that over which liquid hydrocarbons are generated. Other applications include the determination of timing of emplacement and the thermal history of ore deposits. There is

<sup>26</sup> Radiometric dating techniques are, in general, complementary to one another, and each method produces an age with the special meaning, such as the last outgassing, the last melting accompanied by mixing with isotopically separate material and the last heating to remove the track. Fission-track dating is conceptually the simplest of several dating techniques that provide absolute measures of time from slow but statistically steady decay of radioactive nuclides [86].

<sup>27</sup> Most radiometric dating processes are based on the statistical regularity of the decay of one parent radionuclide into a daughter nuclide, for example, 40K into 40Ar, 87Rb into 87 Sr or 238U ,235U and 232Th into 206 Pb, 207Pb and 208Pb, respectively. The age of sample can be then determined by measuring relative abundance of parent and daughter in any pair. The major isotope of uranium (238U) decays at a spontaneous rate of ~10−16 per year [86].

<sup>28</sup> Fission track can also be created artificially (induced track) by irradiating the mineral specimen with thermal neutrons in a nuclear [88].

<sup>29</sup> The mineral grain is ground and polished to expose a flat surface inside the crystal. It is then immersed in a chemical etchant that preferentially attacks the regions of damage, widening them and making them visible under optical microscope. The track appears in the apatite torch readily in 20 to 30 s when immersed in diluted nitric acid [88].

abundant literature on both fission-track dating and its use in evaluating the tectonic and thermal history of rocks [6],[89],[90],[91].

Apatite is the most frequently used material for fission-track dating [92]. Apatite fissiontrack (AFT) analysis serves as a thermochronological tool to investigate the low-temperature thermal history of rocks below ~120°C [93],[94]. The estimates of closure temperatures for fission-track retention in apatite are usually in the range from 75 to 120°C at cooling rates between 1 and 100°C/m.y. [6].

Thermochronology may be described as the quantitative study of the thermal histories ofrocks using temperature-sensitive radiometric dating methods such as 40Ar/39Ar and K-Ar, fission track and (U-Th)/He. Among these different methods, apatite fission track and apatite (U-Th-Sm)/He (AHe) are now, perhaps, the most widely used thermochronometers, as they are the most sensitive to low temperatures (typically between 40 and 125°C forthedurations of heating and cooling in the extent of 106 years). They are ideal forinvestigating the tectonic and climatedriven surficial interactions that take place within the top few (<5 km) kilometers of the Earth's crust. These processes govern the landscape evolution, influence the climate and generate the natural resources essential to the well-being of mankind [85],[95].
