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首页> 外文期刊>Contributions to Mineralogy and Petrology >Experimental investigation of the alluaudite + triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates
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Experimental investigation of the alluaudite + triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates

机译：钙铝榴石+三叶虫组合的实验研究，以及三钠铝合钠地热仪的开发：应用于天然伟晶石磷酸盐

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In order to assess the stability of the primary alluaudite + triphylite assemblage, we performed hydrothermal experiments between 400 and 800°C, starting from the LiNa2Mn x Fe3−x 2+Fe3+(PO4)4 compositions (x = 1.054, 1.502, 1.745) that represent the ideal compositions of the alluaudite + triphylite assemblages from the Kibingo (Rwanda), Hagendorf-Süd (Germany), and Buranga (Rwanda) pegmatites, respectively. The pressure was maintained at 1 kbar, and the oxygen fugacity was controlled by the Ni–NiO buffer. The results of these experiments show that the alluaudite + triphylite assemblage crystallizes at 400 and 500°C, while the association alluaudite + triphylite + marićite appears at 600 and 700°C. The limit between these two domains, at ca. 550°C, corresponds to the maximum temperature that can be reached by the alluaudite + triphylite assemblages in granitic pegmatites, because marićite has never been observed in such geological environments. At 800°C, the formation of the X-phase + triphylite assemblage indicates a strong reduction of the bulk composition, according to the reaction 0.5LiM2+PO4 (triphylite) + 3Na2M2 2+Fe3+(PO4)3 (alluaudite) + 1.5H2O = 4.5NaM2+PO4 (marićite) + Li0.5Na1.5M5 2+(PO4)4 (X-phase) + H3PO4 + 0.75O2 (M2+ = Fe2+, Mn). Secondary ion mass spectrometry (SIMS) was used at our knowledge for the first time to measure Li in all the Li-bearing phosphates. A specific methodological procedure was developed with the ion microprobe to get accurate Li2O data over a wide concentration range spanning from few ppm Li up to ~11 wt%. Li2O. Our SIMS analyses of the synthesized phosphates indicate that the Li contents of alluaudites, marićites, and X-phase increase progressively with temperature, while the Li content of triphylite-type phosphates decreases due to the Li → Na substitution. The Na-exchange equilibrium between triphylite-type phosphates and alluaudite is correlated with the temperature according to the equation: ln(x NaTri/x NaAll) = −7.0(7) 103/T + 5.4(9). This equation can be used to estimate the crystallization temperature of triphylite–alluaudite assemblages independently of the oxygen fugacity.
机译：为了评估主要的钙铝榴石+三叶虫组合的稳定性，我们从LiNa 2 Mn x Fe 开始进行了400至800°C的水热实验。 3−x 2 + Fe 3 + （PO 4 ） 4 的组成（x = 1.054 ，1.502、1.745），分别代表了来自Kibingo（卢旺达），Hagendorf-Süd（德国）和Buranga（卢旺达）伟晶岩的变石+三叶虫组合的理想组成。压力保持在1 kbar，氧气逸度由Ni-NiO缓冲液控制。这些实验的结果表明，在400和500℃下，钙铝石+三叶石组合体结晶，而在600和700℃下，钙铝石+三钛石+水铝石组合出现。这两个域之间的限制约为。 550°C对应于花岗伟晶岩中的变石+三叶虫组合所能达到的最高温度，因为从未在这样的地质环境中观察到水云母。根据反应0.5LiM 2 + PO 4 （三叶石），在800°C时，X相+三叶石组合的形成表明本体组成的强烈减少。 ）+ 3Na 2 M 2 2 + Fe 3 + （PO 4 ） 3 （铝矾土）+ 1.5H 2 O = 4.5NaM 2 + PO 4 （水硬石）+ Li < sub> 0.5 Na 1.5 M 5 2 + （PO 4 ） 4 < / sub>（X相）+ H 3 PO 4 + 0.75O 2 （M 2 + = Fe 2 + ，Mn）。据我们所知，二次离子质谱仪（SIMS）首次用于测量所有含锂磷酸盐中的Li。用离子微探针开发了一种特定的方法学程序，以在从几ppm Li到约11 wt％的宽浓度范围内获得准确的Li 2 O数据。 Li 2 O。我们对合成磷酸盐的SIMS分析表明，脲醛金属，马氏体和X相的Li含量随温度逐渐增加，而三叶沸石型磷酸盐的Li含量由于Li→Na取代而降低。根据以下等式，三叶铁矿型磷酸盐与铝辉石之间的Na交换平衡与温度相关。ln（x Na Tri / x Na 全部）= −7.0（7）10 3 / T + 5.4（9）。该方程式可用于估算三叶石-铝矾土组件的结晶温度，而与氧逸度无关。

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