**7.3. Geological role of apatite**

As was described above, the number of different elements can substitute into the structure of apatite,25 and this mineral can contain a number of trace elements by the substitution in both anion and cation sites. This means that apatite can be used as an indicator of planetary halogen compositions. The quantitative ion microprobe measurements of apatite from lunar basalts showed that portions of the lunar mantle and/or crust are richer in volatile species than previously thought [4].

<sup>25</sup> The tools based on the isotope composition of apatite were already described in **Section 6.5**.

Apatite was also used to determine the characteristics of metamorphic fluids within the mantle. For example, O'REILLY and GRIFFIN [69] and DOUCE et al [70] classified apatite within Phanero‐ zoic mantle material into two geochemically distinct types:


This classification is based on halogen content, presence or absence of structural CO2, Sr and trace elements (especially U, Th and light rare-earth) and association with either metasomat‐ ized mantle wall-rock peridotites (Apatite A) or high-pressure magmatic crystallization products (Apatite B) [4],[69],[70].

In addition, apatite can be used as a probe to determine the petrogenetic evolution of gran‐ ites, and significant amounts of research were devoted to the use of apatite in granitic rocks to distinguish between S- and I-type granites [4],[71].

The chemical composition of apatite is also a useful guideline for the petrogenetic and metallogenic history of magmas for the following reasons [4]:


In recent sedimentary systems, the major phosphorus deposition occurs within upwelling zones at continental margins. Upwelling of deep ocean waters rich in phosphorus triggers high biological production in the photic zone and eventually high concentration of phosphorus in organic-rich sediments, as in recent Namibian and Peruan shelves [72],[73], [74],[75].
