Fluoride distribution in volcanic rocks of the Aluto volcano complex, Main Ethiopian Rift.

Abstract

Full-text study online at
https://www.sciencedirect.com/science/article/pii/S1464343X25004133?via%3Dihub

Highlights

Fluoride (up to 0.32 wt%) varies with lithology and volcanic age.
Young glass-rich rocks have highest fluoride content.
Fluoride mainly occurs in volcanic glass, apatite, and riebeckite.

High fluoride in groundwaters of the Main Ethiopian Rift is of geogenic origin. Although the involvement of volcanic rocks in fluoride enrichment is well documented, the specific mineralogical sources and dissolution mechanisms remain incompletely understood. This study characterized the mineralogical sources of fluoride in volcanic rocks of the Aluto volcano complex, identifying the specific mineral phases and their fluoride concentration that control fluoride availability in volcanic aquifer systems. Pyrohydrolysis was employed for precise fluoride determination, while X-ray fluorescence and electron microprobe analyses enabled phase-specific fluoride distribution and quantification of fluoride-bearing minerals. Water-rock leaching experiments were performed to quantify fluoride release and characterize associated ion dissolution patterns for mineral phase identification. The study shows that: (1) The younger volcanic deposits like pumice and obsidian generally contain high fluoride concentrations of up to 0.32 wt%, while the older ignimbrites show lower concentrations between 0.11 and 0.15 wt%. (2) Fluoride behaves as an incompatible element in the post-caldera rocks showing strong linear correlations with other incompatible elements such as Zr, Nb, and Y (R² > 0.98). (3) Microprobe analyses of thin sections indicate that fluoride is incorporated into the glassy rock matrix, into apatite, and into the sodium amphibole riebeckite (the latter two occur only as accessory minerals). Mass calculations reveal that the largest proportion of fluoride is bound in the fine-grained matrix, as this phase represents 40 to 100 wt% of each rock sample. Thus, volcanic glass – particularly pumice – and glass-rich rocks have the highest potential to release fluoride, indicating that this phase controls fluoride availability in the rift groundwater system.

EXCERPTS:

1. Introduction

High fluoride (F^?) concentrations have been frequently reported in surface and groundwater throughout the East African Rift, particularly in the groundwaters of Ethiopia (Ayenew, 2008; Edmunds and Smedley, 2013; Haji et al., 2018; Olsson, 1979; Rango et al., 2009; Regenspurg et al., 2022; Tekle-Haimanot et al., 2006; Tumato et al., 2025). In rural areas, where access to public water treatment and supply is limited, the local population often relies directly on groundwater for drinking water. The World Health Organization (WHO) recommends a maximum fluoride concentration of 1.5 mg/L in drinking water (Fawell et al., 2006), a threshold exceeded by approximately half of the wells in the Ethiopian Rift Valley (Tekle-Haimanot et al., 2006). Prolonged consumption of high-fluoride water can have a significant impact on health, ranging from mild dental fluorosis to severe skeletal fluorosis (Ayoob and Gupta, 2006; Dissanayake, 1991; Fewtrell et al., 2006).

Fluorine (F) is the 13th most abundant element in the Earth’s crust and occurs ubiquitously in rocks and waters (Fawell et al., 2006; Koritnig, 1972). While anthropogenic sources such as brick manufacture, coal combustion, and aluminum smelting, or fertilizer production can locally increase environmental fluoride levels, most fluoride found in groundwater worldwide originates from geogenic processes (Ayoob and Gupta, 2006; Edmunds and Smedley, 2013; Finkelman et al., 1999; Fuge, 2019; Fuge and Andrews, 1988). This happens particularly through interactions between fluoride-bearing rocks, volcanic exhalations, and the hydrosphere (Borchert, 1952; Boyle, 1976). The average fluoride content of magmatic rocks is approximately 715 ppm (Hem, 1985) but the presence of fluoride-rich mineral phases determines the fluoride content of the rocks. Minerals in which fluoride is an essential structural component generally occur as trace phases (Fuge, 2019). These accessory minerals include halides such as villiaumite (NaF) and cryolite (Na₃[AlF₆]), as well as more common minerals like fluorite (CaF₂) and topaz (Al₂[(F,OH)₂|SiO₄]). Fluoride can also substitute hydroxide in the crystal lattice of various minerals due to their similar ionic radii (F^?: 1.33 Å vs. OH^?: 1.40 Å) and charge (Borchert, 1952; Boyle, 1976). Phosphates such as apatite (Ca₅[(F,Cl,OH)|(PO₄)₃]) and silicates such as biotite (K(Mg,Fe^2+,Mn²⁺)₃[(OH,F)₂|(Al,Fe³⁺,Ti³⁺)Si₃O₁₀]), and amphibole such as arfvedsonite Na₃(Fe²⁺,Mg)₄Fe³⁺Si₈O₂₂(OH)₂ are typical hosts for fluoride, as are clay minerals and sediments (Fuge and Andrews, 1988). Identifying which mineral phases control the bulk fluoride content in volcanic rocks is crucial for understanding how fluoride is released into groundwater.

Rift formation and volcanism play a key role in shaping the hydrogeological and geochemical structure of the East African Rift, where geothermal activity, active faults, horsts, and trenches affect the heterogeneity of the groundwater flow and chemistry (Kebede et al., 2010). Recent studies confirm that elevated fluoride concentrations in groundwater are primarily of geogenic origin and are linked to the dissolution of volcanic rocks (Bianchini et al., 2020; Fentaw et al., 2025; Haji et al., 2018; Regenspurg et al., 2022). Comparable findings from other African regions highlight the continental scale of the problem (Kazapoe et al., 2024; Ligate et al., 2021; Usman et al., 2024).

However, there are differing interpretations about the origin of fluoride in groundwaters of the Ethiopian Rift System. Some authors suggest that fluoride originates directly from volcanic exhalations dissolving into groundwater (Gizaw, 1996; Kebede, 2013), while others emphasize the dissolution of fluoride from volcanic rocks into groundwater (Ayenew, 2008; Bianchini et al., 2020; Fentaw et al., 2025; Malago, 2017; Regenspurg et al., 2022). Despite extensive hydrogeochemical work in the Ethiopian Rift, previous studies have largely focused on fluoride concentrations in water and bulk rock compositions (Rango et al., 2009b, Rango et al., 2010, Yirgu et al., 1999).

Various analytical approaches have been employed to quantify fluoride in volcanic rocks. Previous studies focused on bulk-rock analysis via XRF (Yirgu et al., 1999), phase-specific microprobe analysis of individual minerals (Rango et al., 2009a), or mineralogical identification through XRD (Nielsen, 1999). While some recent studies combined multiple methods (XRD, XRF, and leaching experiments (Onipe et al., 2021), a systematic integration of quantitative bulk and phase-specific fluoride determination with experimental validation of release mechanisms remains lacking. However, our approach combines pyrohydrolysis for the precise determination of total fluoride with electron microprobe analysis for identification of fluoride-bearing mineral phase and the fluoride-associated ion release pattern of volcanic rocks from the Aluto volcano complex, providing more comprehensive insights than previous single-method studies.

This study makes three key contributions to the current understanding: First, it provides the first detailed, phase-specific quantification of fluoride distribution between the glass, apatite and amphibole phases in rocks from the Aluto volcano. Second, it explores the possible origin of the fluoride in the rocks. Third, quantitative mass balance calculations reveal the distribution of fluoride concentrations in the rocks.

2. Geological and hydrogeological setting

The study area is situated about 130 km south of the capital Addis Ababa in Ethiopia. As a part of the East African Rift Valley, the Aluto volcano complex is located in the Central Main Ethiopian Rift (Fig. 1). The Ethiopian Rift structure extends over approximately 1000 km from northern Kenya to the northeast, reaching the Afar Depression, and covers a width of about 50 km (Chorowicz, 2005). Extension between the Nubian and Somali plates formed the eastwest-oriented rift axis (Corti, 2009). The extension is characterized by magmatic intrusions and tectonic faults (Corti, 2009). The initial rift formation phase of the Main Ethiopian Rift (MER) took place more than 30 million years ago (Hofmann et al., 1997). Further development of the rift was driven by diffuse volcanism with episodic eruptions, uplift, subsidence, and widespread fault patterns (Corti et al., 2019; Hutchison et al., 2015; Paola, 1972).

The Aluto volcano complex is part of the recent volcano chain along the MER. The oldest deposits encountered by deep wells in the study area are acidic ignimbrites, deposited during the voluminous caldera-forming eruption of the Munesa Volcano west of the Aluto volcano, about 3.5 million years ago (Hutchison et al., 2016; Woldegabriel et al., 1990). Starting in the early Pleistocene, around 2 million years ago, tectonic deformations formed the NNW-SSW oriented Wonji fault system, while lava flows and scoria cones were deposited at the Aluto Volcano. The Ziway-Shala basin began to develop in the Middle Pleistocene (before 500,000 years ago), when lake water flooded the entire area (Hutchison et al., 2016). Lacustrine sediments were deposited until the late Pleistocene (Le Turdu et al., 1999). Contemporaneously, the first trachytic lava of the Aluto volcano was deposited (Hutchison et al., 2016). The caldera-forming main eruption phase of the Aluto volcano followed around 310,000 years ago, during which the peralkaline welded grey and green ignimbrites were deposited. During an eruption pause, the water-filled basin regressed and partially drained (Hutchison et al., 2016; Le Turdu et al., 1999)Klicken oder tippen Sie hier, um Text einzugeben. Around 60,000 years ago, the first phase of post-caldera activity of the Aluto volcano began with rhyolitic deposits, followed by three further eruption phases (Hutchison et al., 2016). The post-caldera eruption phases are characterized by explosive episodes during which pyroclasts and pumice were deposited, followed by effusive phases with obsidian deposits (Hutchison et al., 2016). Accordingly, the outcropping rocks in the study area are Tertiary and Quaternary volcanic rocks. Active faults in the MER enhance the circulation of hot fluids, and hot springs and fumaroles further indicate ongoing geothermal activity in the study area.

The subsurface of the Central Main Ethiopian Rift is strongly influenced by faulting and volcanic processes, which control groundwater flow and aquifer distribution (Kebede et al., 2010). Fig. 1 show that the study area is characterized by NNE-SSW rift faults (Wonji Fault Belt) intersecting with older E-W and NNW-SSE volcano-tectonic structures. These structural features create complex groundwater flow patterns: major faults act as preferential flow pathways for fresh Highland waters, while intersecting volcano-tectonic structures locally impede flow and enhance water-rock interaction times(Ayenew, 2008; Darling et al., 1996; Kebede et al., 2010). Groundwater recharge originates primarily from the high-rainfall highlands, infiltrating through fracture systems, flowing along paths from highland recharge areas to rift discharge areas, with isotopic evidence indicating rapid infiltration (Ayenew, 2008; Darling et al., 1996; Kebede et al., 2010). The prolonged water-rock interaction time in these deep-circulating volcanic aquifer systems provides favourable conditions for geogenic fluoride enrichment (Kebede et al., 2010). Aquifer materials vary considerably over short depth intervals; local wells intersect aquifers composed of pumice, lacustrine sand, and ignimbrite within a few tens of meters. Tuff, basalt, and rhyolite also serve as aquifers when weathered or fractured (Ayenew, 2008). Overall, approximately 70 % of aquifer materials in the study area are made up of volcanic rock (JICA, 2012). The average depth of wells used as a source of potable water is about 50 m below sea level (mbsl), and between 80 and 200 mbsl for most volcanic aquifers (JICA, 2012; Kebede et al., 2010). High-production water sources can be found in areas where the slope meets the rift ridge, or in flat lowlands (JICA, 2012). The main aquifers have a range of transmissivity between 9 and 242 m² day^?1, whereby the most productive aquifer has transmissivities between 42 and 242 m² day^?1 (JICA, 2012).

…5. Discussion

5.1. Enrichment of fluoride in the melt

The chemistry of the rocks is characteristic of the volcanism of the Ethiopian Rift Valley and has already been documented in other studies (Hutchison et al., 2015, 2016, 2018; Peccerillo et al., 2007; Yirgu et al., 1999). The trends toward SiO₂ enrichment described above can be attributed to fractional crystallization of a parental basaltic magma during magma evolution (Hutchison et al., 2016; Okrusch and Matthes, 2014; Rogers, 2015). This includes gravitational separation of early-formed minerals such as olivine, clinopyroxene, amphibole, and mica, which results in depletion of the major components iron, magnesium, and calcium, with simultaneous enrichment of sodium and potassium in the most evolved silicic melt. The trace elements also show fractionation with simultaneous enrichment of the lithophilic elements. Zirconium, an incompatible element in a peralkaline melt (Bailey and Macdonald, 1975; Markl, 2015), also increases its concentration with the increase in SiO₂. This trend is consistent with the results of other rock analyses from the Aluto Volcano (Hutchison et al., 2016) and Gedemsa Volcano (Peccerillo, 2003; Peccerillo et al., 2007; Yirgu et al., 1999) in the Ethiopian Rift Valley. Apart from the incompatible element strontium, which can be incorporated in the place of calcium in plagioclases due to its similar ionic radius and thus experiences depletion in the most evolved silicic melt, others such as the LREE La, Ce, and Y, but also the lithophile elements niobium and rubidium, accumulate in the melt. The fluoride concentration shows a very similar pattern in the measured post-caldera deposits and correlates strongly with the other incompatible elements (Table 2). This strong positive correlation between fluoride and trace incompatible elements (Zr, Nb and Y) indicates that magmatic differentiation through fractional crystallization primarily controls fluoride enrichment in the Aluto rocks, with fluoride partitioning into the residual melt alongside other incompatible trace elements. Consistent fluoride-to-incompatible-element ratios across different eruption types suggest that the observed fluoride distribution reflects primary magmatic processes rather than secondary post-emplacement alteration.

The solubility of fluoride in a silicate melt strongly depends on its chemistry (Wallace et al., 2015), but it predominantly accumulates with a distribution coefficient of D_melt/fluid > 1 (Markl, 2015) and is considered to be highly soluble in SiO₂-rich magmas (Agangi et al., 2010). The sampled peralkaline rocks show different forms (pumice, tuff, obsidian) and represent explosive and effusive post-caldera eruption phases. Assuming that the ratio of fluoride to the above-mentioned incompatible elements would have changed in the melt during F outgassing, the consistent ratio of these elements indicates that no significant fluoride loss occurred during the post-caldera rock deposition. This does not exclude the exhalation of fluoride gases at fumaroles and vents between eruptive cycles. Thus, the sampled rocks would largely reflect the fluoride concentration of the melt. The observed correlation seems to indicate that the processes that led to the accumulation of incompatible elements in the melt are the processes that determine the distribution of fluoride in the rocks. A similar magmaphilic behavior of fluoride and incompatible elements was also recognized by Barclay et al. (1996) in peralkaline volcanic rocks in New Zealand, Stecher (1998) in tholeiites from Iceland, and in peralkaline rhyolites from the Gedemsa volcano (Yirgu et al., 1999). Nevertheless, fluoride is also known to be a component in the gas phase of volcanic exhalation (Stecher, 1998) and can be adsorbed onto the surface of tephra as a nanometer-thin salt crust during a Plinian eruption (Óskarsson, 1980). However, the consistent behavior of fluoride relative to lithophilic elements, independent of the type of eruption, indicates that volcanic exhalation has no significant effect on the fluoride content of the analyzed rocks. Rock samples attributable to pre-caldera and caldera-forming volcanic activity (>300,000 years) have lower fluoride concentrations compared to younger volcanic rocks (<60,000 years) supporting the fractionation of fluoride into more highly evolved, late-stage erupting magmas. However, decreased fluoride variations with age could also be due to the devitrification and hydration processes described by Noble et al. (1967) and Scaillet and Macdonald (2004), whereby fluoride is released from rocks to the hydrosphere or atmosphere.

5.2. Fluoride distribution in the mineral phases

The release of fluoride from rocks to the environment is determined in particular by its form of binding within the mineral phases. Based on the analyses of the thin sections, the percentage of the total fluoride in the respective mineral phases was calculated (Table 4). For apatite, the P₂O₅ concentration was treated as a decisive factor for calculating the maximum fluoride concentration, as it was assumed that all phosphorus is bound in apatite because, e.g., no monazite was found. The analyses of the main constituents in the apatite were P₂O₅ (38.4–41.0 wt%), CaO (49.8–53.1 wt%), and F (3.26–4.22 wt%). A maximum of 10 % of the fluoride bound in the apatite was calculated in the sampled rocks. Due to their small size and abundance, however, the apatite crystals appear to play only a minor role in the total fluoride content of the rock.

Table 4. Calculation of each fluoride phase and its proportion in the percentage of the total fluoride of the rock.

		Pyrohydrolysis & XRF			EMPA analysis			calculated		EMPA analysis	estimated after Terry and Chilingar (1955)	calculated	EMPA analysis	estimated after Terry and Chilingar (1955)	calculated
N.	Rock type	Empty Cell			apatite					riebeckite			matrix
		Empty Cell			Ca₅[(F,Cl,OH)\|(PO₄)₃]					Na₂Fe₃²⁺Fe₂³⁺[(F₂,(OH)₂)Si₈O₂₂]			Empty Cell	Empty Cell	Empty Cell
		F	P₂O₅	CaO	P₂O₅	CaO	F	Proportion of mineral phase in the rock	Percentage of total fluoride in mineral phase^a	F in riebeckite	Proportion of mineral phase in the rock	Percentage of total fluoride in mineral phase^a	F in matrix	Proportion of mineral phase in the rock	Percentage of total fluoride in mineral phase^a
		[%]	[%]	[%]	[%]	[%]	[%]	[%]	[%]	[%]	[%]	Empty Cell	[%]	[vol%]	[%]
4	Obsidian	0.2104	0.02	<0.30	–	–	–	–	–	–	–	–	0.0540	98	25
7	Ignimbrite	0.1465	0.03	0.37	38.44	51.45	3.26	0.08	1.7	–	–	–	0	60	–
9	Pumice	0.3248	0.02	<0.30	–	–	–	–	–	–	–	–	0.2142	100	66
11	Obsidian	0.1841	0.02	<0.30	39.52	51.45	3.95	0.05	1.1	–	–	–	0.0043	99	2
13	Ignimbrite	0.1523	0.03	<0.30	–	–	–	–	–	3.49	0.5	11	0	40	–
14	Tuff	0.1525	0.17	2.22	41.03	53.77	3.72	0.41	10.1	–	–	–	0	60	–
15	Tuff	0.1895	0.02	<0.30	38.4	50.1	4.11	0.05	1.1	–	–	–	0.0806	70	30
16	Tuff	0.1138	0.02	0.43	40.22	53.05	4.22	0.05	1.8	–	–	–	0	60	–
17	Scoria	0.1781	0.02	<0.30	37.04	49.75	3.46	0.05	1.1	–	–	–	0.0121	60	4
20	Obsidian	0.1965	0.03	0.33	–	–	–	–	–	–	–	–	0.0130	90	6
21	Pumice	0.2105	0.03	0.27	–	–	–	–	–	–	–	–	0.0041	100	2

[%] unless otherwise indicated in wt.%.

a: calculated fluoride content in rock = F_{in mineral phase} $*$

proportion of mineral phase_rock/F_total-mineral phase not present in thin section.

The second identified fluoride-bearing mineral phase, riebeckite, was only found in the grey ignimbrite. Since the sodium-iron amphibole does not contain any element that occurs exclusively in this mineral phase, a geochemical calculation of the fluoride bound in riebeckite is not possible. The proportion of this mineral phase was estimated based on Terry and Chilingar (1955) at less than 0.5 vol%. The fluoride bound in the Na-Fe amphibole would thus represent a maximum of 11 % of the fluoride present in the rock.

Based on the estimated volume fractions after Terry and Chilingar (1955) and the analyzed fluoride concentration of the vitreous matrix, a maximum proportion of 66 % of the total fluoride bound in the glass was calculated in the pumice. The other rocks showed only smaller proportions of the fluoride bound in the matrix (<30 %). By comparing the total with the calculated fluoride concentration in the individual mineral phases (Table 4), it is evident that only a small proportion of the fluoride was assigned to mineral phases. Since the pyrohydrolysis measurements are considered most reliable due to the small deviations in repeated measurements, the lower fluoride concentrations calculated from microprobe analysis could result from analytical uncertainty associated with matrix effects and the limited representativeness of the few analyzed points per sample. The discrepancies between pyrohydrolysis and microprobe analysis underscore a fundamental limitation of point-analysis methods: in chemically heterogeneous groundmasses, matrix effects and the limited representativeness of few analyzed points per sample can lead to systematic underestimation of the true fluoride content (Devine et al., 1995; Mesto et al., 2006; Ottolini et al., 2000). However, the discrepancy can also be due to rare high fluoride-bearing mineral phases, like fluorite or villiaumite, which were not observed in the thin sections.

If fluoride was present in the volcanic exhalation, it would have the potential to adsorb as a nanometer-thin salt crust onto the surface of the explosive tephra during a Plinian eruption (Óskarsson, 1980). As a highly soluble phase, the salt crust typically causes greatly increased fluoride concentrations on a local level and for a short period of time, as observed in rivers and soils (Óskarsson, 1980). The coating might have been dissolved during thin section preparation and thus lead to non-detection via the microprobe analysis. However, the results of the batch experiments exclude the presence of F-halides in the pumice and tuff rock samples: i) The good correlation with Si and the typical pattern of H⁺ indicate a dominant glass dissolution (White and Claassen, 1980), whereby fluoride and Si are both transferred into the aqueous phase. When bound in volcanic glass, fluoride can replace the bridge-forming oxygen atom in the glass at the network former Si or Al, or be present as a complex at a network former like Na or K, in the undercooled melt (Baasner et al., 2014). The observed dissolution processes might be due to near surface ion exchange between H⁺ and Na⁺, as well as silicate hydrolysis and the associated H⁺ consumption (Dahlgren et al., 1999; White and Claassen, 1980). ii) The lack of and the incongruent behavior of chlorine during the leaching experiments, which adsorbs in the same way as fluoride to the surfaces of the tephra and dissolves even in higher concentrations into the aqueous phase (Armienta et al., 2002; Ayris and Delmelle, 2012; Witham et al., 2005), lead to the conclusion, that the missing fluoride is bound into the glassy matrix, which makes up the overwhelming part of the analyzed samples. Consequently, some of the differences between total fluoride and the fluoride calculated from microprobe analyses are likely due to analytical underestimation of fluoride in the glassy matrix. The fluoride-bearing volcanic glass is metastable and highly susceptible to weathering (White and Claassen, 1980). The amorphous structure and high surface area of pumice facilitate fluoride release through ion exchange and hydrolysis reactions which means that it can be considered as a controlling factor for groundwater chemistry in an aquifer. The pumice leaching experiments, for example, revealed that only 1.74 % of total fluoride was released after 24 h at 25 °C (11.33 mg/l maximum), a key limitation when transferring these findings to field conditions: experimental conditions do not reflect the variable temperature, pH, and residence time conditions of real aquifer systems and may underestimate long-term release rates under in situ conditions. This finding indicates that fluoride incorporated within the silicate network requires sustained dissolution over extended timescales to achieve significant aqueous transfer. Although the glass matrix is fundamentally reactive and functions as a rate-controlling factor in long-term groundwater fluoride chemistry in volcanic aquifers, its release rates are substantially lower than those of discrete soluble minerals, such as rapidly-soluble fluoride-bearing minerals like villiaumite or surface-adsorbed halides. This establishes the fluoride reservoir in Aluto volcanic glass as a persistent, long-term geochemical source. Taken together, these findings point to volcanic glass as the primary fluoride reservoir in the Aluto volcano system and, by extension, in similar peralkaline volcanic terrains in the Ethiopian Rift.

5.3. Regional context and comparative analysis

Comparative analysis of fluoride systematics across the East African Rift reveals both systematic regional patterns and substantial site-specific variations controlled by local geology and eruption styles. At Oldoinyo Lengai in northern Tanzania, natrocarbonatitic lavas contain high total fluorine (3.2 wt%) but release limited water-extractable fluoride (319 mg/l) due to the low solubility of fluorite (CaF₂), whereas explosive tephras from the same volcano release substantially more fluoride (573–895 mg/l) as surface coatings of highly soluble villiaumite (NaF) with solubility of 42,200 mg/l (Bosshard-Stadlin et al., 2017). In contrast, the Mount Meru volcanic complex in southern Tanzania shows andesitic rocks as the primary fluorine source with mean concentrations of 7133 mg/kg, where sphene (titanite), hornblende, apatite and biotite are identified as discrete fluorine-bearing mineral phases, with fluorine concentrations in rocks directly proportional to dissolved fluoride in proximal groundwaters (Makoba, 2021). At the results presented here, fluoride distribution differs markedly, with volcanic glass identified as the dominant fluoride reservoir (up to 66 % in pumice samples), while discrete mineral phases like apatite and riebeckite contribute only minor amounts (?11 %), indicating that metastable, readily-weatherable glass controls fluoride release rather than resistant mineral phases. These contrasting fluoride distributions – from glass-dominated at Aluto to discrete minerals at Mount Meru to soluble surface coatings at Oldoinyo Lengai – demonstrate that local magma composition, eruption style, and cooling history fundamentally control which phases serve as fluoride sources. This regional and local variability makes it necessary to characterise the fluorine-bearing phases at individual sites using a mineralogical approach in order to assess water quality effectively and plan remediation across the rift system.

6. Conclusion

The study shows that fluoride enrichment in the Aluto volcano complex is primarily controlled by magmatic differentiation and mineralogical composition. The peralkaline post-caldera rocks contain the highest fluoride concentrations ranging from 0.19 to 0.32 wt%. Older rocks, like ignimbrites and other pre-caldera deposits, are depleted (0.11–0.15 wt%). Fluoride behaves as an incompatible element in post-caldera deposits and correlates strongly with Zr, Nb, and Y, indicating similar enrichment processes during late-stage melt evolution rather than degassing. Electron-microprobe data indicate that fluoride is mainly incorporated in volcanic glass, with minor amounts in apatite and riebeckite, and no discrete F-minerals were observed. Together with the leaching experiments, this identifies volcanic glass as the dominant and reactive reservoir controlling fluoride availability in rift groundwater.

CRediT authorship contribution statement

Lioba Virchow: Writing – review & editing, Writing – original draft, Methodology, Investigation, Data curation, Conceptualization. Franziska Wilke: Writing – review & editing, Methodology. Gezahegn Yirgu: Writing – review & editing, Methodology. Thomas Neumann: Writing – review & editing, Supervision, Resources. Simona Regenspurg: Writing – review & editing, Supervision, Resources, Project administration, Conceptualization.

Declaration of competing interest

The authors declare that they have no competing interests.

Acknowledgements

The authors would like to thank especially Dejene Driba for the great help and organization of the field work. Bonso Sedeto Bunke should also be thanked here as a local guide in the field. A big thanks goes to the following staff members of the TU Berlin and GFZ Potsdam: Cordelia Lange helped with the preparation of the thin sections, Petra Marsiske performed the XRF measurements, Iris Pieper analyzed the cations and anions and Oona Appelt helped at the microprobe. The field trip was funded by a PROMOS scholarship of the DAAD. This work was partially funded by European Union in the project CRM-geothermal (grant no. 101058163) under the Horizon Europe programme. We sincerely thank the reviewers for the quality of their comments, which helped to improve our manuscript.

Data availability

Data will be made available on request.

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Fluoride distribution in volcanic rocks of the Aluto volcano complex, Main Ethiopian Rift.

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