**6.5. Trace elements and their isotopes**

Since apatite is an important accessory mineral in most common rock types, it is often used in trace element and isotope investigations of igneous and metamorphic rocks [123]. Stable isotope compositions of biologically precipitated apatite in bone, teeth and scales are widely used to obtain the information on the diet, behavior and physiology of extinct organisms and to reconstruct past climate in terrestrial and marine conditions [124].

Broad spectrum of substitutions in the apatite lattice allows the incorporation of various isotopes, which offer a number of instruments for the interpretation of paleoenvironment and diagenesis. The relative stability of francolite compared to other sedimentary minerals led to an enormous number of studies and applications. Various isotopes occupy the Ca2+ and PO4 3− sites in the lattice of apatite (**Fig. 16**).

**Fig. 16.** Possible isotopic substitutions in the structure of francolite [125].

Since the earliest application in deep-time study of Late Cretaceous paleotemperatures in 1950, the oxygen isotope paleothermometry is based on the temperature dependence of oxygen isotope fractionation between authigenic minerals13 [126] and ambient waters. Under the equilibrium conditions, the 18O/16O ratio of sedimentary carbonate and phosphate minerals depends only on the temperature of precipitation and on the 18O/16O ratio of ambient water. Thermodynamic relationships and bond vibrational frequencies can be used to determine the mineral-water isotopic fractionation relations but not with the precision and accuracy necessary for the paleothermometry. Such an application requires the calibrations based on mineral-water oxygen exchange experiments at high temperatures, mineral precipitation experiments at low temperatures and/or natural experiments using minerals grown under known conditions [127],[128].

Carbon, oxygen and sulfur isotopes are used to reconstruct the oxygenation stages of the sediments during organic matter degradation and precipitation of apatite. The application of this method gives good results for modern and Neogene deposits. In older occurrences, the signature of carbon and oxygen composition is commonly overprinted by diagenetic and burial diagenesis [125],[129]. The carbon isotope ratios of apatite can be used to interpret the source of carbon in magmas and metamorphic fluids using the assumption that the carbon isotope fractionations between phases are small in igneous and metamorphic systems [123].

The carbon isotope analysis of bioapatites was first applied to terrestrial mammals in early 1980s [130],[131],[132]. While it is now known that some bones do undergo the C-isotope exchange extremely readily, collagen, bone and enamelrecord different periods of time during

<sup>13</sup> The minerals of sedimentary rocks are subdivided into two main groups [126]: authigenic (formed on their present position) and allogenic (transported to its current position from elsewhere). Both groups will be further described in **Chapter 7**.

the life of a single individual, and the diet may change. That means, there is a fundamental ambiguity (preservation vs. normal intraindividual differences) in interpreting the isotopic differences among different tissues. Unfortunately, early results were taken to imply that all bioapatites are unreliable, and it was not until the 1990s that it became accepted that the tooth enamel, at least, is a robust recorder of diet. Thus, the early work of LEE-THORP and VAN DER MERWE [133],[134] was a struggle against the tide of misplaced opinions [129].

Fossil biogenic apatites display the trace element compositions that can record environmen‐ tal and biological signals, give insights into past water compositions or be used for dating paleontological and archeological bones and teeth. The mechanisms of the process of trace element and their isotopes incorporation into apatites of skeletal phosphatic tissues are described by REYNARD and BALTER [135]:


Age, land/sea distribution

known conditions [127],[128].

**Chapter 7**.

Land/sea distribution

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

**REE** *ε***Nd**

<sup>87</sup>**Sr/**86**Sr** <sup>234</sup>**U/**238**U**

**Sr U**

**Fig. 16.** Possible isotopic substitutions in the structure of francolite [125].

isotope fractionation between authigenic minerals13

Growth rates, age

Since the earliest application in deep-time study of Late Cretaceous paleotemperatures in 1950, the oxygen isotope paleothermometry is based on the temperature dependence of oxygen

equilibrium conditions, the 18O/16O ratio of sedimentary carbonate and phosphate minerals depends only on the temperature of precipitation and on the 18O/16O ratio of ambient water. Thermodynamic relationships and bond vibrational frequencies can be used to determine the mineral-water isotopic fractionation relations but not with the precision and accuracy necessary for the paleothermometry. Such an application requires the calibrations based on mineral-water oxygen exchange experiments at high temperatures, mineral precipitation experiments at low temperatures and/or natural experiments using minerals grown under

Carbon, oxygen and sulfur isotopes are used to reconstruct the oxygenation stages of the sediments during organic matter degradation and precipitation of apatite. The application of this method gives good results for modern and Neogene deposits. In older occurrences, the signature of carbon and oxygen composition is commonly overprinted by diagenetic and burial diagenesis [125],[129]. The carbon isotope ratios of apatite can be used to interpret the source of carbon in magmas and metamorphic fluids using the assumption that the carbon isotope fractionations between phases are small in igneous and metamorphic systems [123].

The carbon isotope analysis of bioapatites was first applied to terrestrial mammals in early 1980s [130],[131],[132]. While it is now known that some bones do undergo the C-isotope exchange extremely readily, collagen, bone and enamelrecord different periods of time during

13 The minerals of sedimentary rocks are subdivided into two main groups [126]: authigenic (formed on their present position) and allogenic (transported to its current position from elsewhere). Both groups will be further described in

<sup>230</sup>**Th/**234**U**

*<sup>δ</sup>*18**O**<sup>p</sup> *<sup>δ</sup>*18**O**<sup>c</sup>

**Ca10-a-bNaaMgb(PO4)6-x(CO3)x-y-z(CO3·F)y(SO4)zF2**

<sup>231</sup>**Pa/**235**U**

<sup>14</sup>**C/**12**C**

*δ*13**C**

Temperature, age, diagenetic environment

> Diagenetic environment

> > *δ*34**S**

[126] and ambient waters. Under the
