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We provide new experimental data on monazite, xenotime and U–Th-bearing cheralite solubility in slightly peralkaline to peraluminous granitic melts using dissolution and reverse (i.e. recrystallization after dissolution) experiments in water-saturated and flux-bearing (P + F + Li) granitic melts, at 800 °C and 200 MPa. Although a positive correlation between rare earth element (REE) solubility and melt peralkalinity is confirmed, monazite solubilities reported here are much lower than the values previously published. We suggest that the presence of elevated phosphorus concentrations in our melts depresses monazite solubility, principally because phosphorus complexes with Al and alkali, which normally depolymerize the melt through the formation of non-bridging oxygens. The new solubility data provide an explanation for the very low REE concentrations generally encountered in phosphorus-bearing peraluminous granites and pegmatites. This accounts for the compatibility of REE in peraluminous systems, as the early crystallization of REE-bearing minerals (mainly monazite and zircon) leads to progressive REE depletion during liquid differentiation. In addition, dissolution and reverse experiments on U–Th-bearing cheralite–monazite display liquid–liquid immiscibility processes in our slightly peralkaline glass. The immiscible liquid forms droplets up to 10 µm in diameter and hosts on average 35 wt% P2O5, 25–30 wt% F, 22 wt% Al2O3, 4 wt% CaO, 5 wt% Na2O, 2 wt% La2O3, and 12 wt% ThO2 + UO2. We believe that the droplets formed during the runs and may have coalesced to larger droplets during quenching. We suggest that liquid–liquid immiscibility is a possible mechanism of REE concentration in highly fluxed melts and should be considered in natural systems where REE are extremely concentrated (up to thousands of µg g–1) in magmatic rocks. [ABSTRACT FROM AUTHOR]

Tapiolite [FeTa2O6] and columbite-group minerals [(Fe,Mn)(Ta,Nb)2O6] are common Nb-Ta-bearing accessory minerals in rare-element granites and pegmatites. Their compositional gap has inspired several experimental studies, but none of them have succeeded in reproducing the parameters that influence the compositional gap. In this study, tapiolite and columbite-group minerals (CGM) were crystallized from water-saturated, flux-rich granitic melts at various conditions of pressure, temperature, oxygen fugacity, and Ti contents. Crystals with a size as small as 500 nm were analyzed with a field emission gun (FEG) electron microprobe. The results show that temperature, pressure, and Ti content only slightly affect the compositional gaps between tapiolite and CGM, whereas high fO2 leads to complete solid solution between a rutile-structured component Fe3+TaO4 and (Fe,Mn)Ta2O6. The experimental CGM-tapiolite compositional gaps are compared with natural CGM-tapiolite pairs from rare-element granites and pegmatites worldwide. This study reveals that the crystallographic structure of tapiolite and CGM could be the dominant parameter that influences the position of the compositional gap. Order-disorder in CGM and tapiolite is tightly linked to disequilibrium crystallization triggered by supersaturation. Significant isothermal Nb-Ta fractionation is observed inside CGM crystals that grow at high degrees of supersaturation. The effect of supersaturation prevails over the solubility effect that is known to increase the Ta/(Ta+Nb) ratio in CGM and coexisting melts. Thus, even if global equilibrium in terms of the solubility of Nb-Ta-bearing minerals is attained, the Ta/(Nb+Ta) ratio in the crystals may differ significantly from equilibrium. It implies that Nb-Ta fractionation in Nb-Ta oxides is controlled by crystallization kinetics rather than equilibrium chemical fractionation (or any other processes such as F-complexing of Ta or fluid exsolution) in dynamic systems that can rapidly reach supersaturated conditions. These results have important implications for the understanding of crystallization processes in highly evolved and pegmatite-forming magmas. [ABSTRACT FROM AUTHOR]

The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-, P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7–1.3 wt% Li2O, 2–5.5 wt% F, 2.8–4 wt% P2O5 and 0–2.8 wt% B2O3, and their aluminum saturation index ranges from peralkaline to peraluminous (ASILi = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments, pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics, but additional experiments using natural manganotantalite and zircon of relatively pure composition (i.e., close to end member composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model relating the enthalpy of dissolution of manganotantalite to the ASILi of the melt is proposed: ∆ Hdiss (kJ/mol) = 304 × ASILi − 176 in the peralkaline field, and ∆ Hdiss (kJ/mol) = −111 × ASILi + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the melt, i.e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions. A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts. [ABSTRACT FROM AUTHOR]