Experimental investigation of the alluaudite plus triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates (Articolo in rivista)

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Label
  • Experimental investigation of the alluaudite plus triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates (Articolo in rivista) (literal)
Anno
  • 2011-01-01T00:00:00+01:00 (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#doi
  • 10.1007/s00410-010-0547-6 (literal)
Alternative label
  • Hatert F. [1]; Ottolini L. [2]; Schmid-Beurmann P. [3] (2011)
    Experimental investigation of the alluaudite plus triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates
    in Contributions to mineralogy and petrology
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Hatert F. [1]; Ottolini L. [2]; Schmid-Beurmann P. [3] (literal)
Pagina inizio
  • 531 (literal)
Pagina fine
  • 546 (literal)
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  • ID GEO:cnr.igg/2011-A0-012 (literal)
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  • 161 (literal)
Rivista
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  • 4 (literal)
Note
  • ISI Web of Science (WOS) (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • [1] Univ Liege, Dept Geol, Lab Mineral, B-4000 Sart Tilman Par Liege, Belgium [2] CNR, IGG, Unità Pavia, I-27100 Pavia, Italy [3] Inst. Mineral, D-48149 Munster, Germany (literal)
Titolo
  • Experimental investigation of the alluaudite plus triphylite assemblage, and development of the Na-in-triphylite geothermometer: applications to natural pegmatite phosphates (literal)
Abstract
  • In order to assess the stability of the primary alluaudite + triphylite assemblage, we performed hydrothermal experiments between 400 and 800A degrees C, starting from the LiNa(2)Mn (x) Fe (3-x) (2+) Fe(3+)(PO(4))(4) compositions (x = 1.054, 1.502, 1.745) that represent the ideal compositions of the alluaudite + triphylite assemblages from the Kibingo (Rwanda), Hagendorf-Sud (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 500A degrees C, while the association alluaudite + triphylite + mariA double dagger ite appears at 600 and 700A degrees C. The limit between these two domains, at ca. 550A degrees C, corresponds to the maximum temperature that can be reached by the alluaudite + triphylite assemblages in granitic pegmatites, because mariA double dagger ite has never been observed in such geological environments. At 800A degrees C, the formation of the X-phase + triphylite assemblage indicates a strong reduction of the bulk composition, according to the reaction 0.5LiM(2+)PO(4) (triphylite) + 3Na(2)M(2) (2+)Fe(3+)(PO(4))(3) (alluaudite) + 1.5H(2)O = 4.5NaM(2+)PO(4) (mariA double dagger ite) + Li(0.5)Na(1.5)M(5) (2+)(PO(4))(4) (X-phase) + H(3)PO(4) + 0.75O(2) (M(2+) = Fe(2+), 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 Li(2)O data over a wide concentration range spanning from few ppm Li up to similar to 11 wt%. Li(2)O. Our SIMS analyses of the synthesized phosphates indicate that the Li contents of alluaudites, mariA double dagger ites, 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 (Na) (Tri) /x (Na) (All) ) = -7.0(7) 10(3)/T + 5.4(9). This equation can be used to estimate the crystallization temperature of triphylite-alluaudite assemblages independently of the oxygen fugacity. (literal)
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