3.1.1 Surficial and ground waters

Seawater, representing only 0.02% of Earth's mass, is a major reservoir of Cl (∼24% by mass of the Earth's budget) but is a negligible host for F, containing less than 1% of the bulk Earth budget. When considering only the hydrosphere, seawater overwhelmingly dominates the mass balance. The preferred mean seawater value of F concentration is 1.3 μg g⁻¹ [11]. In rainwater, rivers, and other surface runoffs, where the F ion abundances are usually not measured, it is considered to be lower than that of seawater, as the mean halide ion abundances are very low and systematically lower than seawater. Fluorine in the surface runoffs is controlled dominantly by its content in the parental material, such as rock types in contact with water [12]. Volcanic plume emissions are reported to significantly increase the F⁻ contents of local surface waters [13].

Formation waters, underground waters often with high salinity trapped deeply in sedimentary basins, can be rich in dissolved chemical components. As it is the case for the surface water, F abundance in formation waters does not correlate with other halogen ions, and rather the abundance seems to reflect its content in the local material, varying from 0.5 to 50 μg g⁻¹ [2b]. Fluorine abundance of underground waters derived from crystalline basements (such as granite) shows the similar range of variations (Fig. 3). In summary, F⁻ is not an abundant anion in the Earth's hydrosphere; however, with an increase of dissolved halide in fluid, F⁻ potentially increases up to the level of 50 μg g⁻¹.

3.2.1 Continental crust

Fluorine in the continental crust (CC) is distributed differently in various rock types; however, an average CC value is modeled to be 553 μg g⁻¹ [14]. A CC is subdivided into two parts: the upper CC mostly constituted of sedimentary and low-grade metamorphic rocks, and the lower CC mostly constituted of high-temperature metamorphic rocks. The upper CC is enriched in F by 1.4 times relative to the lower CC, with the values of 611 and 429 μg g⁻¹, respectively [15]. With the exception of some phosphate shales (with very high F contents) and evaporites [16], there is no systematic variation in the F abundance related to rock types. In fact, the abundance can vary by several orders of magnitude in sedimentary rocks of both clastic and chemical types (Table 3; Fig. 4). Difficulty arises in accurately evaluating the relative contributions of rock types in the CC, because F abundances are not tied exclusively to the rock types or their mineral constituents. Further modification of the crust during hydrothermal alteration, mineralization, and weathering, and the overall high mobility of fluorine have led to an order of magnitude variations in the abundance, locally. Despite all this complications, F abundances of the crustal rocks are predominantly on the order of 100 s μg g⁻¹.

Fluorine content of different igneous (i.e., magma-derived) rock types in the CC (Table 3; Fig. 4) is somewhat variable. This reflects the range in corresponding abundances in silicate glasses and F-bearing silicate and phosphate minerals (predominantly) derived from magmas. This variation may also reflect the influence of the original F budget of igneous liquids. In addition, fluorine abundance in magma can be diminished by degassing [17] or increased by crystallization concentrating it in residual liquid [18]. Fluorine contents typically are in the range of hundreds to thousands of micrograms per gram, with highly fractionated peraluminous and peralkaline granitoids and some basaltic rocks showing the highest values. During the weathering of volcanic products, glass devitrification results in mobilization of F [19]. Data for F in metamorphic rocks are rarely reported, except for granulites, amphibolites, gneisses, and metapelitic rocks (Table 3). The relationship between metamorphic grade and F abundance is unclear.

3.2.2 Oceanic crust

The oceanic crust, which is the crust beneath ocean, comprises igneous rocks overlain by variable amounts of sediment. The Penrose ophiolite model [20] describes that an oceanic crust consists of a stratigraphic sequence of sediments, extrusive basaltic lavas, sheeted dikes, and plutonic gabbros, underlain by peridotite (i.e., mantle rock), in which igneous rocks originate from mantle-derived basaltic magmas. It should be noted that some oceanic crusts show variations from this standard model, having poor lava coverage as well as a lack of sheeted dikes, and instead, are dominantly composed of serpentinized peridotites (a hydrothermally modified mantle rocks) with some sediment cover [21]. Because variable magmatic activity occurs along mid-ocean ridges, the representative concentrations were assessed for each lithology in an attempt to provide an estimate of F abundance in the oceanic crust. These included fresh magmatic crust, hydrothermally altered magmatic crust, sediments, and serpentinite (Table 4). A combination of these lithologies in varying proportions, depending on the magmatic activity at mid-ocean ridges, should represent the F content of an oceanic crust.

Table 4 reports the average F content of primitive basalts as a representation of the fresh igneous portion of the oceanic crust. The bulk F content of altered oceanic crust is difficult to assess, because it has been frequently demonstrated that F increases to various degrees with alteration of basaltic igneous rocks [22]. Furthermore, the degree of alteration of oceanic crust is highly variable [23]. A single value representing the composition of the altered ocean crust is probably only accurate to an order of magnitude at best. Several bulk measurements of marine sediments are reported as ranges in the F content [24]. Altered mantle (i.e., serpentinite) also experiences a significant increase in the F content through serpentinization (mineralization of serpentine), as the F content in serpentine (reaction product) is significantly higher than olivine and pyroxenes (reactant) by a factor of 20–1000 [22c]. Accounting for these, there are sufficient data to model the bulk F abundance of the altered ocean crust as a function of the degree of alteration and the proportions of sedimentary, mafic, and ultramafic lithologies. A recent estimate is 400 μg g⁻¹ [25]. As it was the case for the CC, F abundance of the oceanic crust is on the order of 100 s μg g⁻¹.

3.3.1 The mantle

The mantle is the most dominant medium in the Earth representing 67% of its mass. Furthermore, majority of magma is derived from the mantle. Therefore, the Earth's mantle is the most important reservoir providing the ingredients constituting the surface today. The mantle abundance of elements present in trace quantity, including F, is commonly assessed through compositional correlations and systematic chemical variations among primitive basalt, peridotite xenoliths, and chondritic meteorites [14,26]. The modeled F content of the bulk silicate earth (BSE, Table 5), which is a portion of the Earth excluding the core, hydrosphere, and atmosphere ranges from 18 to 25 μg g⁻¹, agreeing within uncertainty [27]. Mid-ocean ridge basalts are derived from a depleted mantle (DM) relative to the BSE due to the formation of CC. There are two studies that assessed the composition of DM [27d,28], while the methods and data slightly differ. Salters and Stracke [27d] report a F abundance (11 ± 5 μg g⁻¹), which is derived from an estimate of another better constrained element P, and an F/P = 0.27 ± 0.11, a ratio found to be nearly constant among mantle-derived magmas. The application of the F/P ratio to the P abundance by Workman and Hart [28] gives F = 22 ± 9 μg g⁻¹, a value higher by factor of 2. However, Workman et al. [29] advocate use of another ratio, F/Nd = 20.1 ± 5.8, finding a F abundance of 12 ± 3 μg g⁻¹. Recently, the variability of F content in DM is assessed through new studies. On the basis of the new Nd isotope study, F abundance in DM is possibly slightly lower, ranging from 6 to 12 μg g⁻¹ [30]. Also, Shimizu et al. [31] report enriched and varieties of DM compositions from an extensive submarine glass data set, F = 31 and 8 μg g⁻¹, respectively. Possibly, F abundance in mantle may vary over a factor of 5. As an approximation by taking median values, the F abundances in BSE and DM are 20 and 10 μg g⁻¹, respectively.

Data for the bulk F content in mantle xenoliths, rocks of mantle brought to the surface, are quite limited. For the few data available, variability appears to be significant, ranging from 7 [32] to 400 μg g⁻¹ [33]. In addition to the paucity of data, the composition of the subcontinental xenoliths has commonly been modified by metasomatic fluid injections. Therefore, an independent assessment of F abundances in the mantle, using only peridotite xenoliths, is not possible. A significant increase in the size of the whole rock xenolith database in the future could lead to independent constraints on the F abundance in the mantle.

Rare types of mantle-derived magmas, carbonatites from Ol Doinyo Lengai, Tanzania, contain between 1.2 and 3.5 wt% of F [34], which takes the form of an immiscible halide melt [35]. Halide melt inclusions are also found in magmatic minerals from an intrusive carbonatite from Saint-Honoré, Quebec, Canada [36]. Similarly, F-rich micas and amphiboles are reported from various mantle nodules. For example, up to 8 wt % F is reported for phlogopite found in peridotite xenoliths from ultrapotassic lavas [37]. Fluorine contents of 0.05–0.4 wt % have been reported in amphibole and mica in xenoliths from Kerguelen Island [38]. Furthermore, fluoride melt has been reported in peridotite xenoliths [39]. From these examples, it is clear that significant enrichment of F occurs in the mantle, suggesting significant mobility of F. However, high F content magmas are volumetrically insignificant and their contribution to the Earth's F budget is negligible.

3.3.2 The core

There is little direct information regarding the presence of F in the Earth's core. Even if the F content of the core is lower than that of mantle by 1/30th, given that the weight fraction of the core is 32% of the Earth, the core could contribute 1% of F budget of the Earth. Further research efforts will hopefully provide better constraints on this uncertainty. Poor F incorporation in the phases that may constitute the core (metals, metal alloys, and sulfides) can be inferred from experimental results with other halogens [40]. The interaction of F with Fe metal under atmospheric conditions commonly results in corrosion reactions, which form new fluorides. In general, F is not expected to form extensive solid solutions with metal alloys or Fe sulfides. A consequence of such observations leads to a common assumption that the most of the Earth's F budget is found outside of the core. In addition, we are not aware of any significant detection of F in iron meteorites, which are considered as fragments of planetary cores.