Ti substitution mechanisms in phlogopites from the Suwalki massif-type anorthosite, NE Poland

Namur, O. and Hatert, F. and Grandjean, F. and Long, G.J. and Krins, N. and Fransolet, A.-M. and Auwera, J.V. and Charlier, B. (2009) Ti substitution mechanisms in phlogopites from the Suwalki massif-type anorthosite, NE Poland. European Journal of Mineralogy, 21 (2). pp. 397-406. ISSN 0935-1221 DOI 10.1127/0935-1221/2009/0021-1894

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Abstract

Intercumulus titanian phlogopite occurs in leuco- and gabbro-noritic cumulates from the Suwalki anorthosite massif, NE Poland. The degree of Ti enrichment in the micas ranges from 2.59 to 9.41 wt.% TiO2. The chemical composition is highly variable for several other elements: Al 2O3 (13.07-16.75 wt.%), K2O (7.90-10.16 wt.%), FeOtot (6.92-16.69 wt.%), Fe2O3 (0.82-2.95 wt.%), and MgO (9.86-19.54 wt.%), with a Mg/(Fe + Mg) ratio ranging from 0.47 to 0.83. Substitution mechanisms for Ti are proposed, which suggest the presence of exchange vectors involving octahedral and tetrahedral cations. In samples characterized by low Ti contents (0.147-0.239 Ti a.p.f.u.), the Ti incorporation mechanism is: [6]Ti4+ + [6]□ = 2( [6]Mg2+, [6]Fe2+, [6]Mn2+), where [6]□ corresponds to a vacancy in octahedral coordination (Ti-vacancy). In the two groups with intermediate (0.164-0.326 Ti a.p.f.u.) and high Ti contents (0.477-0.532 Ti a.p.f.u.), the Ti substitution mechanism corresponds to the reaction: [6]Ti4+ + 2([4]A13+, [4]Fe3+) = ([6]Mg2+, [6]Fe2+, [6]Mn2+) + 2 [4]Si4+ (Ti-Tschermak). The Mössbauer spectral investigation shows that 0.046-0.167 a.p.f.u. Fe3+ occur on the octahedral sites of the structure. The substitution mechanism responsible for the incorporation of Fe3+ in phlogopites from Suwalki is 3( [6]Mg2+, [6]Fe2+) = 2( [6]Al3+, [6]Fe3+) + [6](M3+-vacancy). The use of the Ti content of phlogopite as geothermometer reveals crystallization temperatures from 729 ± 15 to 874 ± 15 °C for the phlogopites. © 2009 E. Schweizerbart'sche Verlagsbuchhandlung.

Item Type: Article
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: European Journal of Mineralogy
Volume: 21
Page Range: pp. 397-406
Identification Number: 10.1127/0935-1221/2009/0021-1894
Depositing User: Sarah Humbert
Date Deposited: 23 Mar 2012 12:05
Last Modified: 23 Jul 2013 10:03
URI: http://eprints.esc.cam.ac.uk/id/eprint/2415

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