 |
 |
|
Moelo, Y., Rouer, O. & Bouhnik-Le Coz, M. (2008) From diagenesis to hydrothermal recrystallization: polygenic Sr-rich fiuorapatite from the oolitic ironstone of Saint-Aubin-des-Chateaux (Armorican Massif, France). Eur. J. Mineral. 20 205–216.
Added by: Laurent Cournède (2016-03-10 21:58:42) |
| Type de référence: Article DOI: 10.1127/0935-1221/2008/0020-1792 Numéro d'identification (ISBN etc.): 0935-1221 Clé BibTeX: Moelo2008 Voir tous les détails bibliographiques  |
Catégories: CESES Mots-clés: apatite-group minerals, Armorican Massif, crystal-structure, fluorapatite, France, iron, ironstone, mineralization, monazite, ordovician, rare earth elements, rare-earth-elements, strontium, substitution, xenotime Créateurs: Bouhnik-Le Coz, Moelo, Rouer Collection: Eur. J. Mineral. |
Consultations : 1/515
Indice de consultation : 3% Indice de popularité : 0.75% |
| Résumé |
|
Four generations (I to IV) of Sr-rich fluorapatite were observed in the hydrothermally altered oolitic ironstone interbedded within the lower Ordovician sandstone of Saint-Aubin-des-Chateaux (Armorican Massif, France). Main type I is diagenetic to weak metamorphic; its remobilization along fractures gave type II, which just preceded the hydrothermal pyritization of the ironstone. Types I and II have a quite similar SrO content (similar to 4.8 wt.\%), with mean formulae (Ca4.73Sr0.24(Mn, Fe)(0.02))(Sigma-4.99)(PO4)(3.02)[F-0.68(OH)(0.32)] and (Ca4.75Sr0.24(Mn,Fe)(0.01))(Sigma=5.00)(PO4)(3.00)[F-0.63(OH)(0.37)], respectively. Types III and IV postdate the main hydrothermal process. Type III results from the breakdown of a Sr phosphate, lulzacite. It presents a patchy texture, where each constitutive sub-grain has a relatively homogeneous Sr content (from I to 7 wt.\%), with an almost constant F ratio (0.83 apfu). The last type IV (geodic) shows a strong oscillatory growth zoning (Sr {<}-{>} Ca substitution) with SrO content ranging from 0 to 18 wt.\%, and F close to 0.75 apfu. On the basis of trace analysis of REE and Y by LA-ICPMS, chondrite-normalized spectra of apatite types I, II and IV present dissymmetric convex shapes, with a significant deficit of LREE and a slight one of HREE relatively to MREE (Gd to Dy). Bond valence calculations indicate that this shape is determined mainly by the heterovalent substitution 2 Ca2+ -{>}) Na+ (REE3+, Y), which controls the incorporation of REE and Y in the crystal structure of apatite. The best fit is for REE = Dy or Ho. This crystal chemical constraint favours at low temperature the fractionation of REE and Y between apatite and REE phosphates. This fractionation is stronger with monazite than with xenotime. The normalized spectrum of apatite type III has a symmetric convex shape, with a strong positive Eu anomaly inherited from its precursor lulzacite (close ionic radii of Eu2+ and Sr2+). Owing to its complex geochemistry and geological evolution, the Saint-Aubin ironstone appears as a basic example for the crystal-chemistry of apatite in low-temperature conditions, especially when phosphate-rich sediments and their metamorphic or hydrothermal derivatives are concerned.
Added by: Laurent Cournède |