From diagenesis to hydrothermal recrystallization : Polygenic Sr-rich fluoropatite from the oolitic ironstone of Saint-Aubin-des-Châteaux (Armorican Massif, France)
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-Châteaux (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 (~ 4.8 wt.%), with mean formulae (Ca4.73Sr0.24(Mn, Fe)0.02)S=4.99(PO4)3.02[F0.68(OH)0.32] and (Ca4.75Sr0.24(Mn,Fe)0.01)S=5.00(PO4)3.00[F0.63(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 1 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.