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.

中文翻译：

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花岗岩熔体中独居石-赤霞石和磷钇矿的溶解度，以及浓缩 REE 中液液不混溶性的实验证据

我们使用水饱和和含助焊剂 (P + F + Li) 中的溶解和反向（即溶解后重结晶）实验，提供了关于独居石、磷钇矿和含 U-Th 的白钨矿在微过碱性到过铝花岗岩熔体中溶解度的新实验数据花岗岩熔体，温度为 800 °C 和 200 MPa。尽管确认了稀土元素 (REE) 溶解度与熔体过碱度之间的正相关关系，但此处报道的独居石溶解度远低于先前公布的值。我们认为，我们的熔体中磷浓度升高会降低独居石的溶解度，主要是因为磷与铝和碱络合，这通常会通过形成非桥连氧来解聚熔体。新的溶解度数据解释了含磷过铝花岗岩和伟晶岩中通常遇到的非常低的 REE 浓度。这解释了 REE 在过铝质系统中的相容性，因为含 REE 矿物（主要是独居石和锆石）的早期结晶导致在液体分化过程中 REE 逐渐耗尽。此外，在我们的微过碱性玻璃中，对含 U-Th 的白辉石-独居石的溶解和逆向实验显示了液-液不混溶过程。不混溶的液体形成直径达 10 µm 的液滴，平均含有 35 wt% P2O5、25–30 wt% F、22 wt% Al2O3、4 wt% CaO、5 wt% Na2O、2 wt% La2O3 和 12 wt% % ThO2 + UO2。我们相信这些液滴是在运行过程中形成的，并且可能在淬火过程中结合成更大的液滴。