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首页> 外文期刊>European journal of mineralogy >From diagenesis to hydrothermal recrystallization: polygenic Sr-rich fiuorapatite from the oolitic ironstone of Saint-Aubin-des-Chateaux (Armorican Massif, France)
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From diagenesis to hydrothermal recrystallization: polygenic Sr-rich fiuorapatite from the oolitic ironstone of Saint-Aubin-des-Chateaux (Armorican Massif, France)

机译：从成岩到热液重结晶：来自圣奥宾德沙托（法国阿莫里肯断层块）的橄榄岩铁矿的多基因富锶富氟磷灰石

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摘要

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.
机译：在Saint-Aubin-des-Chateaux（法国Armorican Massif）的下部奥陶纪砂岩中的热液蚀变的橄榄铁岩中观察到了四代（I至IV）富锶氟磷灰石。 I型主要成岩至弱变质;其沿裂缝的移动形成了II型，该类型恰好在铁矿的水热黄化之前。 I型和II型具有非常相似的SrO含量（约4.8 wt。％），平均分子式为（Ca4.73Sr0.24（Mn，Fe）（0.02））（Sigma-4.99）（PO4）（3.02）[F -0.68（OH）（0.32）]和（Ca4.75Sr0.24（Mn，Fe）（0.01））（Sigma = 5.00）（PO4）（3.00）[F-0.63（OH）（0.37）]。 III型和IV型推迟了主要的水热过程。 III型是由于磷酸Sr绿锌矿的分解而产生的。它呈现出斑驳的质地，其中每个本构亚晶粒具有相对均一的Sr含量（从1到7 wt。％），几乎具有恒定的F比（ap3为0.83）。最后一个IV型（大地）显示出强烈的振荡生长带（Sr-Ca替代），SrO含量为0至18 wt。％，F接近0.75 apfu。根据LA-ICPMS对REE和Y的痕量分析，I，II和IV型磷灰石的球粒陨石归一化光谱呈现不对称的凸形，相对于MREE（Ld Dy）。结合价计算表明，该形状主要由杂价取代2 Ca2 +-> Na +（REE3 +，Y）决定，它控制磷灰石晶体结构中REE和Y的掺入。最适合的是REE = Dy或Ho。这种晶体化学约束在低温下有利于磷灰石和REE磷酸盐之间的REE和Y的分馏。独居石比异种时更强。磷灰石类型III的归一化光谱具有对称的凸形形状，其前体蓝锌矿继承了强烈的正Eu异常（Eu2​​ +和Sr2 +的接近离子半径）。由于其复杂的地球化学和地质演化，Saint-Aubin铁矿石似乎是低温条件下磷灰石晶体化学的基本实例，尤其是当涉及到富含磷酸盐的沉积物及其变质或热液衍生物时。

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