火成碳酸岩Sm-Nd等时线定年技术

Sm-Nd Isochron Dating Technique for Carbonatites

选取庙垭铌-稀土矿床中5件新鲜的碳酸岩样品,依据内部等时线法原理,依次采用醋酸、王水、氢氟酸对其进行分步溶解,并分别对其醋酸溶解相、王水溶解相、氢氟酸溶解相和全岩进行了Sm-Nd 同位素组成分析。结果显示不同溶解相之间147Sm/144Nd同位素比值变化全岩之间由0.02 扩大为0.09,5件样品共计17个点拟合获得的等时线年龄为233±12 Ma (MSWD=2.7),其中单件样品的分相Sm-Nd等时线定年结果为235±17 Ma (MSWD=1.4)。这些结果与在该套碳酸岩中的独居石SHRIMP U-Th-Pb 年龄(233.6±1.7 Ma)(Geochimica et Cosmochimica Acta, 2014,143: 189 - 206.)、独居石LA-ICP-MS Th-Pb年龄(~240 Ma)以及铌铁矿LA-ICP-MS U-Pb年龄(232.8±4.5 Ma)(Lithos, 2017, 290 - 291:159 - 171.)在误差范围内基本一致,表明火成碳酸岩分相Sm-Nd定年结果可靠。

Five fresh carbonatite samples from Miaoya Nb-REE deposit were selected and dissolved step by step with acetic acid, aqua regia and hydrofluoric acid according to the principle of internal isochron method. Sm-Nd isotopic compositions of phases dissolved by acetic acid, Aqua regia, hydrofluoric acid and whole rock were analyzed respectively. The results show that 147Sm/144Nd isotope ratios between different dissolving phases change. The age of isochron obtained by fitting 17 points of 5 samples was 233±12 Ma (MSWD=2.7). Among them, the result of phase separation Sm-Nd isochron dating of single sample was 235±17 Ma (MSWD=1.4). These results are basically consistent with the SHRIMP U-Th-Pb age of monazite(233.6 +1.7 Ma)(Geochimica et Cosmochimica Acta, 2014,143: 189-206.), LA-ICP-MS Th-Pb age of monazite (~240 Ma) and LA-ICP-MS U-Pb age of niobite (232.8 +4.5 Ma)(Lithos, 2017, 290-291:159-171.) within analytical errors, indicating that the Sm-Nd internal isochron dating of carbonatite by phase separation is reliable.