**7.3.4. Extraterrestrial apatite**

On Earth, magmatic volatiles (i.e. H2O, F, Cl, C-species and S-species) play an important role in the physicochemical processes that control thermal stabilities of minerals and melts, in magma eruptive processes and in the transportation of economically important metals. On the Moon, magmatic volatiles in igneous systems are poorly understood, and the magmatic volatile inventory of lunar interior, aside from being very low, is not well constrained. Although the Moon is a volatile-depleted planetary body, there is evidence indicating that magmatic volatiles have played a role in igneous processes on the Moon. Specifically, magmatic volatiles were implicated as the propellants that drove fire-fountain eruptions, which produced the pyroclastic glass deposits encountered at the Apollo 15 and 17 sites [96]. That is supported by recent discoveries of water-rich apatite from lunar mare basalts [97],[98].

Apatite was found in a large number of samples of igneous lunar rocks, although it typically occurred in only trace amounts and is typically reported as coexisting with REE-merrillite [(Mg,Fe)2REE2Ca16P14O56], and those two minerals make up the primary mineralogical budget for P on the Moon [96]. Merrillite, also known as the mineral whitlockite (or more precious and dehydrogenated whitlockite) [99], is one of the main phosphate minerals, along with apatite, which occur in lunar rocks, in Martian meteorites and in many other groups of meteorites. Significant structural differences between terrestrial whitlockite and lunar (and meteoritic) varieties require the use of "merrillite" name forthe H-free extraterrestrial material, and the systematic enrichment of REE in lunar merrillite requires the use of "REE-merrillite". Lunar merrillite, ideally (Mg,Fe2+,Mn2+)2[Ca18−x(Y,REE)x] (Na2−x)(P,Si)14O56, contains high concentrations of Y + REE [100],[101].

**Fig. 14.** The structure of lunar merrillite (a) [102] and terrestrial whitlockite (b).

abundant literature on both fission-track dating and its use in evaluating the tectonic and

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

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

On Earth, magmatic volatiles (i.e. H2O, F, Cl, C-species and S-species) play an important role in the physicochemical processes that control thermal stabilities of minerals and melts, in magma eruptive processes and in the transportation of economically important metals. On the Moon, magmatic volatiles in igneous systems are poorly understood, and the magmatic volatile inventory of lunar interior, aside from being very low, is not well constrained. Although the Moon is a volatile-depleted planetary body, there is evidence indicating that magmatic volatiles have played a role in igneous processes on the Moon. Specifically, magmatic volatiles were implicated as the propellants that drove fire-fountain eruptions, which produced the pyroclastic glass deposits encountered at the Apollo 15 and 17 sites [96]. That is supported by recent discoveries of water-rich apatite from lunar mare basalts [97],[98].

Apatite was found in a large number of samples of igneous lunar rocks, although it typically occurred in only trace amounts and is typically reported as coexisting with REE-merrillite [(Mg,Fe)2REE2Ca16P14O56], and those two minerals make up the primary mineralogical budget for P on the Moon [96]. Merrillite, also known as the mineral whitlockite (or more precious and dehydrogenated whitlockite) [99], is one of the main phosphate minerals, along with apatite, which occur in lunar rocks, in Martian meteorites and in many other groups of meteorites. Significant structural differences between terrestrial whitlockite and lunar (and meteoritic) varieties require the use of "merrillite" name forthe H-free extraterrestrial material, and the systematic enrichment of REE in lunar merrillite requires the use of "REE-merrillite". Lunar merrillite, ideally (Mg,Fe2+,Mn2+)2[Ca18−x(Y,REE)x] (Na2−x)(P,Si)14O56, contains high

natural resources essential to the well-being of mankind [85],[95].

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

thermal history of rocks [6],[89],[90],[91].

between 1 and 100°C/m.y. [6].

**7.3.4. Extraterrestrial apatite**

concentrations of Y + REE [100],[101].

Lunar merrillite (**Fig. 14**(**a**), trigonal, the space group R3C with the cell parameters *a* = 10.2909 Å, *c* = 36.8746 Å, *c*:*a* = 3.5832 and *V* = 3381.93 Å3 ) and terrestrial whitlockite (**Fig. 14**(**b**), trigonal, the space group R3C with the cell parameters *a* = 10.3300 Å, *c* = 37.1030 Å, *c*:*a* = 3.5918 and *V* = 3428.79 Å3 ) have largely similar atomic arrangements, but the phases differ due to the presence or absence of hydrogen. In whitlockite, H is an essential element and allows the charge balance. Hydrogen is incorporated into the whitlockite atomic arrangement by disordering one of the phosphate tetrahedra and forming the PO3(OH) group. Lunar merrillite is devoid of hydro‐ gen; thus, no disordered tetrahedral groups exist. The charge balance for the substituents Y and REE (for Ca) is maintained by Si4+ ↔ P5+ tetrahedral substitution and □ ↔ Na<sup>+</sup> substitu‐ tion at Na site [99],[102].

A number of sources potentially contributed to the overall inventory of lunar water, includ‐ ing primary indigenous water acquired during lunar accretion, late addition of water through asteroidal and cometary impacts and solar wind implanting H into lunar soils. The average D/H ratios of apatite in norite (Apollo sample 78235) and in the granite clast (14303) are consistent with the estimates for the H isotopic composition of recent bulk-Earth and terres‐ trial mantle. By contrast, the average H isotopic composition of apatites in norite 77215 is lower. The content of water in norite parental melts provides strong evidence that the magmas involved in secondary crust production on the Moon were hydrated, in agreement with recent findings of water in lunar ferroan anorthosites. Water they contain, locked in the crystalline structure of apatite, is characterized by an H isotopic composition similar to that on Earth and in some carbonaceous chondrites [103].

Apatite preserves a record of halogen and water fugacities that existed during the waning stages of crystallization of planetary magmas, when they became saturated in phosphates. The thermodynamic formalism based on apatite-merrillite equilibria that makes it possible to compare the relative values of halogen and water fugacities in Martian, lunar and terrestrial basalts, accounting for possible differences in pressure, temperature and oxygen fugacities among the planets, was described by DOUCE and RODEN [104].

They showed that planetary bodies have distinctive ratios among volatile fugacities at apatite saturation and that these fugacities are, in some cases, related in a consistent way to volatile fugacities in the mantle magma sources. Their analysis shows that the Martian mantle parental to basaltic SNC meteorites was dry and poor in both fluorine and chlorine compared to the terrestrial mantle. Limited data available from Mars show no secular variations in mantle halogen and water fugacities from ~4 Ga to ~180 Ma. Water and halogens found in recent Martian surface rocks have thus resided in the planet's surficial systems since at least 4 Ga and may have been degassed from the planet's interior during the primordial crust-forming event. In comparison to the Earth and Mars, the Moon, and possibly the eucrite parent body too, appear to be strongly depleted not only in H2O but also in Cl2 relative to H2O. The chlorine depletion is the strongest in mare basalts, perhaps reflecting an eruptive process characteris‐ tic with large-scale lunar magmatism [104].

Mars does not recycle crustal materials via the plate tectonics. For this reason, the magmatic water reservoir of the Martian mantle has not been affected by the surface processes, and the deuterium/hydrogen (D/H) ratio of this water should represent the original primordial Martian value. Following this logic, hydrous primary igneous minerals on the Martian surface should also carry this primordial D/H ratio, assuming no assimilation of Martian atmospher‐ ic water during the crystallization and no major hydrogen fractionation during the melt degassing. Hydrous primary igneous minerals, such as apatite and amphibole, are present in Martian meteorites here on Earth. Provided these minerals have not been affected by terres‐ trial weathering, Martian atmospheric water or shock processes after the crystallization, they should contain a good approximation of the primordial Martian D/H ratio. As Nakhla was seen to fall on the Egyptian desert in 1911, the terrestrial contamination is minimized in this meteorite. The nakhlites are also among the least shocked Martian meteorites. Therefore, apatite within Nakhla could contain primordial Martian hydrogen isotope ratios. The similar D/H ratios indicate that the Earth and Mars, and possibly the otherterrestrial planets, accreted water from the same source [105].

Vesta, as the second most massive asteroid, has long been perceived as anhydrous. Recent studies suggesting the presence of hydrated minerals and past subsurface water have challenged this long-standing perception. The volatile components indicate the presence of apatite in eucrites. Eucritic apatite is fluorine rich with minor chlorine and hydroxyl (calcu‐ lated by difference) [106],[107],[108].
