Experimentally metasomatised monazite was studied in terms of preservation of U-Pb and Th-Pb ages during alkali-bearing fluid-induced alteration over a broad range of temperature conditions 250–750 °C. Starting materials for experiments included Burnet monazite (Concordia age 1100.5 ± 11.6 Ma, 2σ), albite, K-feldspar, biotite, muscovite, SiO₂, CaF₂, Na₂Si₂O₅ and H₂O. Monazite from experiments at 250–550 °C is partially replaced by secondary REE-rich fluorapatite [(Ca,LREE,Si,Na)₅(PO₄)₃F], fluorcalciobritholite [(Ca,REE)₅(SiO₄,PO₄)₃F] and REE-rich steacyite [(K,?)(Na,Ca)₂(Th,U)Si₈O₂₀], and developed patchy zoning, whereas partial replacement by fluorcalciobritholite and cheralite [CaTh(PO₄)₂] occurred at 650 and 750 °C, with no signs of compositional alteration based on EPMA data and BSE imaging. Raman microspectroscopy results show narrowing of the ν₁(PO₄) stretching band in unaltered domains, which indicates advancing annealing of the monazite structure with increasing temperature, and narrow ν₁(PO₄) band with low FWHM values in altered domains. TEM investigations revealed that unaltered domains of monazite from experiments at 250–550 °C have mottled diffraction contrast, similar to the starting Burnet monazite, which indicates low to moderate degree of metamictization. On the contrary, the altered domains of monazite (patchy zones) show no mottled contrast, suggesting an ordered crystalline structure. TEM imaging demonstrated low degree of metamictization in monazite from the experiment at 650 °C; fluid-aided alteration along the cleavage planes resulted in the development of nanoporosity or partial replacement by fluorcalciobritholite and cheralite. Monazite from the experiment at 750 °C has crystalline structure with no signs of metamictization and shows significant development of nanoporosity and formation of secondary cheralite nanocrystals across the grain. For comparison, TEM and Raman evaluation of xenotime from similar experiments at 350 and 650 °C revealed that both starting xenotime and xenotime from experimental products are crystalline with no signs of radiation damage or fluid-induced alteration affecting internal domains on submicron scale, which could result in compositional alteration of the xenotime.

The unaltered domains of monazite from runs at 250–550 °C yielded U-Pb and Th-Pb dates similar to the age of Burnet monazite, whereas altered domains yielded discordant dates due to various degree of Pb-loss (up to 99.4%). Linear regressions on the Concordia diagrams show lower intercept ages from −266 ± 160 Ma (run 350 °C, 200 MPa) to −1 ± 48 Ma (450 °C, 800 MPa), which reflect the “true age” of experimental alteration. The monazite from runs at 650 and 750 °C yielded data indicating initial disturbance of the U-Th-Pb system, ranging from 8.4% Pb-gain to 18.6% Pb-loss. Linear regressions with lower intercepts of −53 ± 420 Ma and −55 ± 610 Ma roughly correspond to the timing of the experiments. Furthermore, LA-ICPMS results demonstrate discrepancy between Th-Pb and U-Pb dates suggesting higher mobility of ²⁰⁸Pb than that of ²⁰⁷Pb and ²⁰⁶Pb.

To summarize, TEM and Raman data indicate increasing annealing of the radiation damaged monazite with increasing temperature. Alteration processes induced by alkali-bearing fluid can result in recrystallization of monazite and various degrees of the age disturbance at temperatures 250–550 °C, whereas isotopic U-Th-Pb microanalysis provide an opportunity to constrain the age of the metasomatic processes as the lower intercept in the Concordia diagram. The particular importance of this study lies in submicron alteration of monazite at 650–750 °C induced by alkali-bearing fluid and/or melt, which remains unnoticed using common electron microscopy BSE imaging. Such alteration, however, induces substantial disturbance of U-Pb and Th-Pb ages, which can cause misinterpretations in reconstructions of geological processes.

在 250-750 °C 的广泛温度条件下，在含碱流体引起的蚀变过程中，研究了实验交代的独居石对 U-Pb 和 Th-Pb 年龄的保存。实验的起始材料包括 Burnet 独居石（Concordia 年龄 1100.5 ± 11.6 Ma，2σ）、钠长石、钾长石、黑云母、白云母、SiO ₂、CaF ₂、Na ₂ Si ₂ O ₅和 H ₂ O。来自 250 实验的独居石–550 °C 被次生富含 REE 的氟磷灰石 [(Ca,LREE,Si,Na) ₅ (PO ₄ ) ₃ F]、氟钙钛矿 [(Ca,REE) ₅ (SiO ₄ ,PO ₄) ₃ F] 和富含 REE 的硬脂石 [(K,?)(Na,Ca) ₂ (Th,U)Si ₈ O ₂₀ ]，并形成斑片状的分带，而部分被氟钙沸石和绿泥石取代 [CaTh(PO ₄ )_{图 2} ] 发生在 650 和 750 °C，根据 EPMA 数据和 BSE 成像没有成分改变的迹象。拉曼显微光谱结果显示未改变域中ν ₁ (PO ₄ ) 伸缩带变窄，这表明随着温度的升高独居石结构的提前退火，以及变窄的 ν ₁ (PO ₄) 在改变的域中具有低 FWHM 值的频带。TEM 研究表明，在 250-550 °C 的实验中未改变的独居石域具有斑驳的衍射对比度，类似于起始的 Burnet 独居石，这表明低至中等程度的变质化。相反，独居石的蚀变区域（斑块区域）没有显示出斑驳的对比，表明具有有序的晶体结构。TEM 成像表明，在 650 °C 的实验中，独居石的变质化程度较低；沿解理面的流体辅助蚀变导致纳米孔隙的发展或部分被氟钙硅沸石和cheralite取代。来自 750 °C 实验的独居石具有晶体结构，没有变质化的迹象，并显示出纳米孔隙率的显着发展，并在整个晶粒上形成了次生金缕石纳米晶体。为了进行比较，在 350 和 650 °C 的类似实验中对磷钇矿的 TEM 和拉曼评估表明，来自实验产品的起始磷钇矿和磷钇矿都是结晶的，没有辐射损伤或流体引起的改变影响亚微米尺度内部域的迹象，这可以导致磷钇矿的成分改变。

在 250–550 °C 下运行的独居石未改变的域产生的 U-Pb 和 Th-Pb 日期类似于 Burnet 独居石的年龄，而改变的域由于不同程度的 Pb 损失（高达 99.4%）而产生不一致的日期. Concordia 图上的线性回归显示截距年龄从 -266 ± 160 Ma（运行 350 °C，200 MPa）到 -1 ± 48 Ma（450 °C，800 MPa），这反映了实验变化的“真实年龄” . 在 650 和 750 °C 下运行的独居石产生的数据表明 U-Th-Pb 系统的初始扰动，范围从 8.4% 的铅增益到 18.6% 的铅损失。具有较低截距 -53 ± 420 Ma 和 -55 ± 610 Ma 的线性回归大致对应于实验的时间。此外，LA-ICPMS 结果表明 Th-Pb 和 U-Pb 日期之间存在差异，表明²⁰⁸Pb 高于²⁰⁷ Pb 和²⁰⁶ Pb。

总而言之，TEM 和拉曼数据表明辐射损坏的独居石随着温度的升高退火增加。含碱流体引起的变质过程可导致独居石在 250–550 °C 温度下重结晶和不同程度的年龄扰动，而同位素 U-Th-Pb 微量分析提供了限制交代过程年龄的机会，因为康科迪亚图中的较低截距。这项研究的特别重要性在于由含碱流体和/或熔体在 650-750°C 下引起独居石的亚微米变化，使用普通电子显微镜 BSE 成像仍然没有注意到。然而，这种改变会引起 U-Pb 和 Th-Pb 年龄的大量干扰，这可能会导致对地质过程重建的误解。