Experimental investigation of the alluaudite聽+聽triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates

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• 作者：Frederic Hatert (1) fhatert@ulg.ac.be
  Luisa Ottolini (2)
  Peter Schmid-Beurmann (3)
• 关键词：Alluaudite &#8211 ; Triphylite &#8211 ; Na&#8211 ; Li&#8211 ; Mn&#8211 ; Fe2+&#8211 ; Fe3+ phosphates &#8211 ; Pegmatites &#8211 ; Phase relations &#8211 ; Geothermometry &#8211 ; SIMS
• 刊名：Contributions to Mineralogy and Petrology
• 出版时间：April 2011
• 出版年：2011
• 期刊代码：29_0010-7999
• 类别：ch
• 卷：161
• 期：4
• 页码：531-546
• 数据来源：sp

摘要

In order to assess the stability of the primary alluaudite + triphylite assemblage, we performed hydrothermal experiments between 400 and 800掳C, starting from the LiNa₂Mn _x Fe_3−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-S眉d (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掳C, while the association alluaudite + triphylite + marićite appears at 600 and 700掳C. The limit between these two domains, at ca. 550掳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掳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) + Li_0.5Na_1.5M₅ 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.