2.1. Experimental strategy

Four compositions were synthesized (Table 1) with a base composition of a phonolite–carbonatite mixture in a ratio of 50:50 for the major elements (including Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Ba, Sr, F and Cl). The selected mixture composition corresponds to the immiscible carbonate melt and silicate melt of phonolitic composition from experiment T19_01 of Nabyl et al. [2020]. This base composition (composition PhCbn1, see Table 1; named “non-doped” in the following text) contains small amounts of F, Cl and P (F ∼ 1 wt%; Cl ∼ 0.3 wt%; P₂O₅ ∼ 0.7 wt%; Table 1). In order to characterise and isolate a possible volatile effect on the REE partitioning, three compositions were doped with ca. 5 wt% of P₂O₅ (composition PhCbn2, Table 1), F (composition PhCbn3, Table 1) and Cl (composition PhCbn4, Table 1) respectively. All four starting materials also contain 0.1 wt% of 𝛴REE-Y-Sc and other trace elements (see Table 1).

Most experiments were performed at 850 °C and 0.8 GPa [similar to experiment T19_01 in Nabyl et al. 2020]. Additional experiments were also performed at a higher temperature (1050 °C). Moreover, different amounts of water (0, 3 or 6 wt%) were added to the starting material to test its effect on each chemical system.

2.2. Starting compositions

The four starting compositions (Table 1) were prepared by mixing synthetic powders (SiO₂, TiO₂, Al₂O₃, AlPO₄, AlF₃, AlCl₃, FeO, FeS, MnCO₃, CaCO₃, Na₂CO₃, K₂CO₃, BaCO₃, SrCO₃ and Cr₂O₃) and natural dolomite (CaMg(CO₃)₂). For the three ±F-, Cl- and P-doped compositions (PhCbn2, PhCbn3 and PhCbn4 respectively, Table 1), additional AlF₃, AlCl₃ and AlPO₄ powders were used to reach the respective F, Cl and P concentrations needed (Table 1). The starting compositions are CO₂-rich, with bulk concentrations of ca. 24 wt%. REE and other trace elements were also added to the four compositions in the form of oxides, fluorides or pure elements (La₂O₃, CeO₂, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃, GdF₃, Tb, Dy₂O₃, HoF₃, Er₂O₃, Yb₂O₃, Lu, Y₂O₃, Sc₂O₃, Nb₂O₅, Ta₂O₅, ZrO₂ and HfO₂).

The synthetic powders were maintained in a dry oven at 120 °C, except for the strong hydrophilic Na₂CO₃ and K₂CO₃ powders which were preserved in an oven at 200 °C. Moreover, the AlF₃, AlCl₃ and AlPO₄ powders were also strongly hydrophilic at atmospheric conditions and could not be preserved at temperatures higher than 50 °C. Those powders and both the PhCbn3 and PhCbn4 compositions (Table 1) were thus manipulated using a Captair Pyramid 2200 ANM-XLS at controlled atmosphere under Argon gas to avoid any atmospheric water contamination, and maintained in a vacuum bell jar. For each composition, the powders were first mixed by hand in an agate mortar (30 min) and then in an automatic grinder with an agate mortar and ball mill (at least two times for 15 min).

2.3. Piston-cylinder experiments

We synthesized 20 experimental charges at 0.8 GPa, 850 and 1050 °C using an end-loaded piston-cylinder apparatus (Table 2). The starting materials were packed into Au capsules (diameter of 2.5–2.9 mm) for low temperature experiments (850 °C) and Au₈₀Pd₂₀ capsules for higher temperature ones (1050 °C). For experiments performed at 850 °C and 0.8 GPa and for almost all the starting materials, four capsules were synthesized: one capsule containing only the starting material, two capsules containing 3 and 6 wt% of water respectively (Table 2), and one capsule with additional graphite (+1 wt%, see Table 2). Water and graphite were added in order to test their effect on carbonate and silicate melt compositions.

The capsules were introduced into an alumina tube with a powder composed of 50% AlSiMag (mixture of Al, Si and Mg)–50% haematite, and closed at the top and bottom with MgO-plugs. The fO₂ for the experiments was estimated at between FMQ and FMQ+2, due to the presence of the haematite powder which creates an oxidised environment (i.e. H₂-poor). Graphite-bearing experiments were more reduced, likely in the range FMQ−2 [Stagno and Frost 2010]. The whole assemblage was packed into a 3∕4-inch piston-cylinder assembly, composed of graphite, pyrex and talc. Two steel-plugs surrounded by pyrophyllite were placed at the top and bottom of the assembly to maintain the assemblage in place; the thermocouple (B-type, Pt₉₄Rh₆–Pt₇₀Rh₃₀) was introduced in the top of the assemblage. For the experiments conducted at 850 °C (Table 2), the samples were first taken up to 975 °C for two hours to ensure bulk composition melting and homogenisation, then the final temperature (850 °C) was reached within a few minutes (cooling rate of 10 °C per minute). Each experiment was quenched by switching off the heat source at isobaric conditions. Uncertainties for the temperature and the pressure are considered to be ±12 °C and ±0.1 GPa respectively [Dasgupta et al. 2004; Sifré et al. 2014].

2.4. Analytical methods

Polished sections were prepared from the experimental charges, and observed using a Merlin Compact Zeiss electron microscope (voltage 15 kV), equipped with a micro-analyser system EDS (Bruker-QUANTAX-XFlash6, resolution 129 eV) at the ISTO laboratory, in order to determine the phases and their textures. Carbonate and silicate melt sections were then analysed with a Cameca SXFive electron microprobe equipped with 5 WDS detectors (ISTO, France) at 15 kV and 6 nA. Analyses were performed with a large beam size (from 10 to 70 μm) to avoid any Na-loss and to average carbonate melt compositions. The standards used for the calibration were albite, apatite, orthose, andradite, topaz, vanadinite, MgO, Al₂O₃, Fe₂O₃, MnTiO₃, BaSO₄, and SrCO₃. The counting time was 10 s for all the elements, except for Ba (20 s), Sr and Nb (30 s) and S (60 s). Higher counting times were also needed for F, Cl and P to ensure representative analyses (30 s for Cl, 60 s for F, and 60 or 120 s for P).

Trace elements were analysed using two different LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) devices: the Agilent 7500 CS Quadrupole ICP-MS of the LMV laboratory (Clermont-Ferrand, France; He: 550 mL/min; Ar: 850 mL/min; N₂: 2 mL/min) and the Agilent 7900 Quadrupole ICP-MS of the ISTO laboratory (Orléans, France; He: 350 mL/min; Ar: 930 mL/min). Both mass spectrometers are coupled to a 193 nm excimer laser ablation system (Resonetics), with He flushing the ablation cell. Carbonate and silicate melt analyses were performed with a frequency of 2 Hz, an ablation energy of 1.5–3 mJ and a beam size of 14 to 60 μm in diameter, depending on both melt/crystal-free areas. Carbonate and silicate melts from several samples were analysed with both spectrometers to ensure similar trace element concentrations were obtained and that no bias was induced by using the two different machines. The following isotopes were analysed: ⁴³Ca, ⁴⁴Ca, ²⁹Si, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁷²Yb, ¹⁷⁵Lu, ⁸⁹Y, ⁴⁵Sc, ⁴⁹Ti, ⁵³Cr, ⁵⁵Mn, ⁸⁸Sr, ¹³⁷Ba, ⁹³Nb, ¹⁸¹Ta, ⁹⁰Zr, ¹⁷⁸Hf, ¹⁹⁷Au and ¹⁰⁵Pd to verify any potential contamination by the capsule during the experiment.

Data reduction and trace element concentrations were carried out using the GLITTER4.4 software [Van Achterbergh et al. 2001] for both the carbonate and silicate melts. The elements were quantified using the NIST 610 standard glass [Pearce et al. 1997] and other standards were used to check the analysis validity [NIST 612 and BCR-2G natural basaltic glass; Pearce et al. 1997; Rocholl 1998; Jochum et al. 2016]. The standards were analysed at the same beam size, frequency and energy ablation as the samples to verify the efficiency of the analysis. The Ca was used as the internal standard, its concentration having first been determined by electron microprobe analyses in both the carbonate and the silicate phases.

3.1. Carbonate and silicate melt textures

Nineteen samples were synthesized at 0.8 GPa and 850 and 1050 °C. For the sake of clarity, each chemical system is represented with a different colour in all the figures: black for the non-doped system, red for the P-rich system, green for the Cl-rich system and blue for the F-rich system.

For all starting materials (±F, Cl and P), all run products have an alkaline silicate liquid conjugated with a carbonate liquid, apart from those produced at 850 °C with graphite (samples noticed “…_04”, see Table 2) which have only a carbonate melt coexisting with a few silicate crystals. The silicate liquids quenched into homogeneous glass in all the samples (Figures 1 and 2). The coexisting carbonate melts show typical textures of quenched crystallised dendritic blebs in almost all the systems (non-doped system, P- and Cl-rich system; Figures 1 and 2), the exception being the F-rich system where they have a near-glassy texture (Figures 1c, 2e and f). In the Cl-rich system (Figures 1d and 2g), carbonate blebs display a porous texture. For almost all the samples, the clear contact between both liquids, the homogeneous chemical compositions and the size of the carbonate melts (up to 200 μm) indicate that equilibrium has been attained between both liquids at given P–T conditions, and this attests to the fact that immiscibility is not related to quench processes [Brooker and Kjarsgaard 2011]. In two samples from the non-doped system that were synthesized at 850 °C (PCPC1_01 and PCPC1_02, Figures 2a and b; Table 2), a second distinct carbonate liquid phase is observed (noted “CL2”, in Figure 2b), characterised by carbonate blebs of <15 to 50 μm. This phase will be treated separately in the following section of the results.

In some samples (Table 2), bubbles are observed (Figure 1d; Figures 2d and f) and attest to the presence of a fluid phase coexisting with both liquids. Moreover, at 0.8 GPa and 1050 °C carbonate melts contain a large amount of apatite crystals in the P-rich sample (Figure 1b), while no apatite are observed in the F- and Cl-rich systems (Figures 1c and d respectively). At the lower temperature (850 °C), all charges contain crystals (Figure 2 and Table 2). Clinopyroxene is mainly present in non-doped samples (Figures 2a and b) and in Cl-rich samples (in green, Figure 2g). Apatite is also present in significant proportions in the carbonate melt zones of P-rich samples (Figures 2c and d; Table 2), and a few apatite crystals are also observed in the Cl-rich samples (Figure 2g, Table 2). Other phases are also observed, such as nepheline, garnet, spinel, alkali feldspar, feldspathoid, calcite and fluorite (Table 2, see Figure 2). The chemical composition of all minerals is not discussed here as this study is mainly focused on the effect of F, Cl and P on REE partitioning between the carbonate and alkaline silicate melts.

3.2. Chemical composition of the melts

The major element concentrations of the conjugate silicate and carbonate melts are presented in Table 3.

At 0.8 GPa and 1050 °C for all four chemical systems, the silicate melts present a composition of alkaline nephelinite type, with the SiO₂ content varying slightly from 42.9 ± 0.2 wt% (sample PCPC6_02 in the Cl-rich system, Table 3) to 46.3 ± 0.6 wt% (sample PCPC6_03 in the P-rich system, Table 3), and alkali contents (Na₂O + K₂O) from 15.1 ± 0.8 wt% (sample PCPC6_02 in the Cl-rich system, Table 3) to 17.8 ± 0.6 wt% (sample PCPC6_01 in the non-doped system, Table 3). Figure 3 presents ternary diagrams in the Hamilton projection—i.e. in the (Na₂O + K₂O)–(SiO₂ + TiO₂ + Al₂O₃)–(CaO + MgO + FeO) space—which is widely used to represent the miscibility gap between carbonate and silicate melts [Freestone and Hamilton 1980; Kjarsgaard and Hamilton 1988]. The silicate melts in the P- and F-rich systems are slightly richer in silica and alkali elements compared to those in the non-doped and Cl-rich systems (Figure 3a, see Table 3). The conjugate carbonate melt compositions vary in the four chemical systems from 10.1 ± 1.6 to 17.1 ± 0.7 wt% Na₂O (samples PCPC6_02 and PCPC6_03 respectively, Table 3) and from 29.4 ± 0.9 to 34.3 ± 1.2 wt% CaO (samples PCPC6_03 and PCPC2_01, respectively, Table 3). The carbonate melts of the Cl-rich system are poorer in Na₂O content compared to the others, and thus closer to the (CaO + MgO + FeO) pole (Figure 3a).

The F, Cl and P₂O₅ silicate melt concentrations at 1050 °C for all the systems are relatively high, reaching almost 4, 1.3 and 1 wt% in the F-, Cl- and P-rich systems respectively (Table 3). In the carbonate melts, F and P₂O₅ concentrations are much higher in the F- and P-rich systems: F reaches 11.8 ± 0.7 wt% in the F-rich system (sample PCPC6_01, Table 3), and the carbonate melt in the P-rich system contains 9.5 ± 0.9 wt% P₂O₅ (sample PCPC6_03, Table 3). However, in the Cl-rich system for the carbonate melt the Cl concentration remains low (0.9 ± 0.3 wt%, sample PCPC6_02, Table 3).

At the lower temperature (850 °C, Figure 3b and Table 2), partial crystallisation changes the silicate melt composition from evolved nephelinite to phonolite type, i.e. from 45.8 ± 0.9 to 53.7 ± 0.8 wt% SiO₂ (samples PCPC5_03 and PCPC4_01 respectively, Table 3) and 16.2 ± 0.6 to 18.0 ± 0.5 wt% Na₂O + K₂O (samples PCPC5_03 and PCPC3_03 respectively, Table 3). For all four chemical systems, the miscibility gap between the carbonate and silicate melts is enlarged as temperature decreases from 1050 to 850 °C (Figures 3a and b), with silicate melts getting closer to the (SiO₂ + TiO₂ + Al₂O₃) pole and carbonate melts getting closer to the (CaO + MgO + FeO) pole. Silicate melts present similar compositions for all the chemical systems (Figure 3b; Table 3) and for samples ± hydrated (dashed and dotted lines, 3 and 6 wt% of H₂O respectively, Figure 3b). In contrast, the conjugate carbonate melts vary significantly in the four chemical systems. Carbonate melts are broadly richer in CaO with concentrations varying from 31.0 ± 1.4 to 36.8 ± 5.1 wt% (samples PCPC1_03 and PCPC5_03 respectively, see Table 3), except for the P-rich system, which shows lower CaO concentrations, between 24.4 ± 1.8 and 25.4 ± 2.8 wt% (samples PCPC3_01 and PCPC3_02, Table 3). Carbonate melts are also slightly richer in Na₂O in the non-doped system and in the F- and P-rich systems, with concentrations varying from 14.7 ± 0.7 to 20.6 ± 0.7 wt% (samples PCPC1_01 and PCPC3_03 respectively, Table 3). On the contrary, the Na₂O contents are lower in the Cl-rich system (<11 wt%, see Table 3). In fact, carbonate melts become much richer in CaO (35.1 ± 5.6 and 36.8 ± 5.1 wt% of CaO, samples PCPC5_02 and PCPC5_03 in the Table 3) and poorer in Na₂O (8.2 ± 3.5 and 4.8 ± 2.3 wt% of Na₂O, Table 3) and K₂O (1.2 ± 0.7 and 0.8 ± 0.2, Table 3) with increasing bulk water content (from 3 to 6 wt%; see the green arrow in Figure 3b). The carbonate melts thus slightly deviate from the (Na₂O + K₂O) pole and evolve towards the (CaO + MgO + FeO) pole (Figure 3b). This water effect that was predominant in the Cl-rich system is not observed in the non-doped system or the P- and F-rich systems (Figure 3b).

A second carbonate liquid phase is observed in two samples from the non-doped system that were synthesized at 850 °C (PCPC1_01 and PCPC1_02, Table 2; see Figure 3b, empty black circles). In comparison to the main carbonate melts (Table 3), this second carbonate liquid is rare, presents higher CaO concentrations and higher totals during the EMPA analyses (see Supplementary Table 1). The occurrence of this second carbonate phase suggests that the equilibrium was not achieved in these two samples. When plotted in the chemical diagrams (see results and discussion sections, empty black circles in figures), the corresponding data points are outliers. For the sake of clarity, these two samples are included in the diagrams, but are not discussed further here.

The F, Cl and P₂O₅ silicate melt concentrations at 850 °C for all the systems are lower than those at 1050 °C. In the non-doped system (see Table 2), the silicate melts contain relatively small amounts of F (0.3–0.4 wt%), Cl (0.07 wt%), and P₂O₅ is below the detection limit (samples PCPC1, Table 3). The concentrations are slightly higher in F, Cl or P doped systems (Table 2), varying from 1.4 to 1.8 wt% for F in the F-rich system (samples PCPC4, Table 3), 0.6 to 0.8 wt% for the Cl in the Cl-rich system (samples PCPC5, Table 3) and only around 0.2 wt% for the P₂O₅ in the P-rich system (samples PCPC3, Table 3). Carbonate melts are also more enriched in those elements at the higher temperature, although P₂O₅ and Cl carbonate melt concentrations remain low in the P- and Cl-doped systems respectively (see samples PCPC3 and PCPC5_05, Table 3).

Trace element concentrations—including the REE—of both silicate and carbonate melts are presented in Table 4. At 1050 °C, the silicate melts contain 18.2 ± 0.5 to 35.9 ± 0.7 ppm of La as representative of other LREE (light REE), and 32.1 ± 0.9 to 50.3 ± 0.7 ppm of Lu, as representative of other HREE (high REE; Table 4), whereas REE concentrations in the carbonate melts are higher and vary from 87.6 ± 2.8 to 108.1 ± 5.6 for La and 50.0 ± 2.3 to 74.9 ± 2.7 ppm for Lu (Table 4). The silicate melt REE concentrations are lower at 850 °C, varying from 1.8 ± 0.0 to 8.4 ± 0.1 ppm of La, and from 3.9 ± 0.0 to 17.87 ± 3.1 ppm of Lu (Table 4). At this temperature, carbonate melt REE concentrations are generally much higher than at 1050 °C, varying from 77.7 ± 5.4 to 140.5 ± 9.8 ppm and 51.6 ± 3.3 to 116.2 ± 5.2 for La and Lu respectively (Table 4). In the Cl-rich system, the middle REE and high REE (MREE and HREE) concentrations decrease slightly in the carbonate melts with increasing water content (67.0 ± 1.9 to 34.8 ± 5.0 ppm of Gd and 34.0 ± 1.3 to 13.0 ± 1.9 ppm of Lu, samples PCPC5_02 to PCPC5_03, from 3 to 6 wt% of water; Table 4), whereas no significant variation in concentration is observed in the silicate melts (Table 4). This behaviour is not observed in other chemical systems.

3.3. REE partitioning between carbonate and silicate liquids

Trace element partitioning is defined using the Nernst partition coefficient D which corresponds to the mass concentration ratio in ppm of the element x in the carbonate liquid (CL) and the silicate liquid (SL; DxCL∕SL=xCL∕xSL). Trace element partition coefficients (including REE) are presented in the Table 5.

Figure 4 presents REE partition coefficients for the four chemical systems investigated. For all samples and for all experimental conditions, REE partition coefficients vary greatly, by almost two orders of magnitude. The large DREECL∕SL variation observed in the literature [Hamilton et al. 1989; Martin et al. 2013; Nabyl et al. 2020; Veksler et al. 2012, 1998] also occurs in F-, Cl- and P-rich systems. The DREECL∕SL are higher for LREE (light REE) than for HREE (heavy REE). For most charges in the four chemical systems, carbonate melts are richer in REE than silicate melts (DREECL∕SL>1, Figure 4). At 850 °C, the DREECL∕SL in the non-doped system vary from 10.7 ± 2.2 to 29.6 ± 3.3 for La and from 1.7 ± 0.3 to 5.8 ± 1.1 for Lu (Figure 4). The carbonate melt REE enrichment is greater in the P- and F-rich systems (Figure 4), being almost two times higher than in the non-doped system, with the DLaCL∕SL varying from 37.1 ± 4.1 to 46.2 ± 6.2 and from 26.5 ± 1.8 to 43.0 ± 5.7 in the P- and the F-rich systems respectively, and the DLuCL∕SL varying from 5.6 ± 0.4 to 6.8 ± 0.5 and from 7.3 ± 1.8 to 8.7 ± 2.0 (Figure 4, Table 5).

No clear effect of water on REE partitioning is identified in the non-doped system and the P- and F-rich systems (Figure 4 and Table 5). In the Cl-rich system, the REE partition coefficients decrease with the addition of water (3 and 6 wt% of water, samples PCPC5_02 and PCPC5_03, Table 2), from 21.3 ± 1.6 to 14.8 ± 2.0 for DLaCL∕SL and 4.6 ± 0.5 to 3.3 ± 0.5 for DLuCL∕SL (samples PCPC5_02 and PCPC5_03, green dashed and dotted lines in Figure 4).    

For some samples, the Ce partition coefficients show a negative anomaly. The DCeCL∕SL is lower compared to that of other LREE. This is particularly marked in Cl-rich samples (PCPC5_02 and PCPC5_03, green circles in Figure 4; Table 5), in the non-doped system (samples PCPC1_01 to PCPC1_03, black circles in Figure 4; Table 5) and in the F-rich system (PCPC4_02, blue circles in Figure 4; Table 5). This DCeCL∕SL decrease reflects the higher Ce concentrations in the silicate liquids for all these samples compared to other LREE, whereas the carbonate melt Ce concentrations do not present any particular variation (see Tables 4 and 5).

At the higher temperature (1050 °C, triangles in Figure 4), the carbonate melt enrichments in REE are lower in all four chemical systems, especially in the non-doped system with a DLaCL∕SL and a DLuCL∕SL of 2.2 ± 0.1 and 0.7 ± 0.1 respectively (Table 5). As for the experiments at 850 °C, REE partitioning is more in favour of carbonate liquids for samples doped in P and F (red and blue triangles, Figure 4), with a DLaCL∕SL of 4.7 ± 0.4 and 5.7 ± 0.2 respectively in the P- and F-rich systems, and a DLuCL∕SL of 1.6 ± 0.1 and 1.9 ± 0.1 (Table 5).

To summarise the REE partitioning, the experimental DREECL∕SL vary by two orders of magnitude; their values increase as temperature decreases and in addition are higher in the F- and P-rich systems. Nabyl et al. [2020] proposed an equation relating the REE partitioning between carbonate and silicate melts to the degree of alkaline silicate melt differentiation (i.e. composition, alkalinity and polymerisation of the silicate melt). The question arises as to whether this silicate melt composition effect still applies in systems rich in F, Cl or P, or whether these elements play a key role in REE concentrations in F-, Cl- or P-rich systems. This is tested below.