Trace Element content in Primary Fe-Mn Phosphates from the Tryphilite-Lithiophilite, Graftonite-Beusite and Triplite-Zwieselite Series: Determination by La-ICP-MS Methods and Preliminary Interpretation

E. RODA-ROBLES; PESQUERA A.; GARCÍA DE MADINABEITÍA S.; GIL IBARGUCHI J; NIZAMOFF J.; SIMMONS W.; FALSTER A.; GALLISKI M. A.

Bartlett, New Hampshire

Simposio; PEG 2013-6th International Symposium on Granitic Pegmatites; 2013

TRACE ELEMENT CONTENT IN PRIMARY FE-MN PHOSPHATES FROM THE TRIPHYLITE-LITHIOPHILITE, GRAFTONITE-BEUSITE AND TRIPLITE-ZWIESELITE SERIES: DETERMINATION BY LA-ICP-MS METHODS AND PRELIMINARY INTERPRETATIONE. Roda1, A. Pesquera1, S. García de Madinabeitia2, J.I. Gil Ibarguchi1, J. Nizamoff3, W. Simmons3, A. Falster3, M.A. Galliski41 Dpto. Mineralogía & Petrología, UPV/EHU, BIlbao, Spain, encar.roda@ehu.es2 SGIker-Geochronology, UPV/EHU, Spain3 Dept. of Earth and Environmental Sciences, University of New Orleans, USA4 IANIGLA-CCT Mendoza, CONICET, Mendoza, ArgentinaPhosphates of Fe-Mn are common accessory mineral phases in some granites and in manypegmatites, and they become important constituents in the beryl-columbite-phosphate pegmatite subtype of Černý & Ercit (2005), and in some hydrothermal quartz-rich dykes (e. g. Garate-Olabe et al., 2012).The chemical composition of phosphates has been used to establish the degree of evolution of thepegmatites in which they occur, just as the composition of some silicates (micas, K-feldspar,tourmaline, garnet and beryl) and of oxides (columbite-group minerals) is used to indicateevolution. Indeed, a decrease of the Fe/(Fe+Mn) value in phosphates is associated with increasingevolution of pegmatites (Ginsburg 1960; Fransolet et al. 1986; Keller et al. 1994; Roda et al. 2005; Roda-Robles et al. 2010). However, this ratio may be influenced by the presence of other Fe-(Mg-Mn)-rich minerals, such as tourmaline, biotite and/or garnet (Roda-Robles et al. 2012), and, therefore, this criteria must be followed with caution. A more complete chemical characterization of the Fe-Mn phosphates, including trace elements, could help to better understand the petrogenetic role of phosphates during the evolution of pegmatites. With thispurpose, samples belonging to the triplite-zwieselite (Mn2+,Fe2+,Mg,Ca)2(PO4)(F,OH), triphylitelithiophilite Li(Fe2+,Mn2+)PO4 and graftonite-beusite (Ca,Fe2+,Mn2+)3(PO4)2 primary phosphate series were analyzed by microprobe and LA-ICP-MS methods at the Université Paul Sabatier (Toulouse, France) and at the SGIker-Geochronology Laboratory of the Universidad del País Vasco UPV/EHU (Bilbao, Spain) respectively. For each series the widest possible Fe-Mn compositional range was covered. Overall, more than 700 microprobe analyses and close to 400 LA-ICP-MS analyses were made on eight samples of the triplitezwesielite series, six samples of the triphylitelithiophilite series, and five samples of the graftonite-beusite series from pegmatites andhydrothermal quartz dykes in Europe (Hagendorf Sud, Cañada, Folgosinho and Nossa Senhora de la Assunçao), Argentina (Cema, El Criollo and El Gigante), Canada (Swanson), and the United States (Tourmaline Queen, Beryl, Storm Mountain, Palermo n.1, Keystone and Emmons).The results obtained show important differences for the studied series of primary Fe-Mn phosphates.Members of the triphylite-lithiophilite series contain extremely low amounts of all the analyzed elements, except for Zn (570-8990 ppm). Members of the triplite-zwieselite series, in contrast, are enriched in Nb (78-537 ppm) and Zn (437-4093 ppm), with a broader range for HREE (0-124 ppm), Ta (4-176ppm), Y (0-280 ppm), Zr (6-233 ppm), and U (7-50ppm). Samples of the graftonite-beusite series are the richest in REE, with HREE up to 300 ppm andof LREE up to 345 ppm. Other trace elements, such as Zn (1508-4238 ppm), Sr (3-91 ppm) and Y (0-509 ppm) occur also in significant amounts in the graftonite-beusite. The variations in the trace element contents observed among the different phosphate series are most probably controlled by the crystal structure more than by the bulk composition.For example, triphylite and graftonite coexisting in the Palermo n.1 pegmatite show clearly differenttrace element contents. Nevertheless, the broad range in the content of some trace elements for thesame series could be attributed to the different geological settings. The plot of the different traceelements contents versus the Fe/(Fe+Mn) ratio shows a complex correlation, which would indicateeither that this ratio does not reflect the differentiation degree of the hosting rock, or that thetrace element concentrations in phosphates do not account (or not solely) for the evolution degreeattained by these phosphates-bearing pegmatites. Further investigation must be done to determine (i) the factors controlling the variability in the content in trace elements for the different primary phosphate series, (ii) the distribution coefficients between phosphates and silicates such as biotite, tourmaline and garnet, and (iii) the influence of the bulk composition and geological setting on thegeochemistry of the primary Fe-Mn phosphates.ReferencesČerný, P. & Ercit, T. S. (2005): The classification of granitic pegmatites revisited. Canadian Mineralogist, vol. 43, pp 2005-2026.Fransolet, A. M., Keller, P. & Fontan, F. 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