Theoretical modeling of monazite growth in a low-Ca metapelite
Theoretical modeling of monazite growth in a low-Ca metapelite
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DOI：
10.1016/j.chemgeo.2010.02.016
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发表时间：
2010-04
期刊：
影响因子：
3.9
通讯作者：
F. Spear;J. Pyle
中科院分区：
文献类型：
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作者：
F. Spear;J. Pyle
独居石的生长和组成已在K2 O-Na 2 O-CaO-MnO-MgO-FeO-Al 2 O3-SiO2-H2 O-Y2 O3-Ce 2 O3-P2 O 5-F系统中进行了建模，并给出了一个假剖面，该剖面描绘了独居石模态量和YPO 4组成、石榴子石量和YAG组成以及磷钇矿量。独居石的量和组成强烈地受磷钇矿的存在的控制，并且当磷钇矿不存在时，受石榴石的生长或消耗的控制。独居石在最合理的变质P-T路径上在亚固相线区域连续生长，YPO 4含量通常随温度升高而增加，特别是在存在磷钇矿的地方。当熔体存在时，独居石随着熔融的增加而消耗，并且在没有磷钇矿的情况下，YPO 4含量降低。提出了两种正演模型来说明石榴石分离结晶对计算结果的影响。当存在磷钇矿并且石榴石的模态量低时（例如，在低压下），分级结晶对相量或组成影响不大。在磷钇矿不存在而石榴石更丰富的地方（例如，在中高压下），石榴石分馏控制独居石的生长历史和组成。特别是，YPO 4独居石的含量下降到接近零，在岩石中的石榴石隔离钇作为YAG成分的中间等级。这种Y可以在较高温度下通过扩散释放，并促进具有较高YPO 4的独居石生长。因此，独居石的生长历史推断从YPO 4分区可能是相当复杂的，并反映不同的反应和过程，即使在一个简单的散装组成。
Monazite growth and composition has been modeled in the system K2O–Na2O–CaO–MnO–MgO–FeO–Al2O3–SiO2–H2O–Y2O3–Ce2O3–P2O5–F and a pseudosection is presented that is contoured for monazite modal amount and YPO4composition, garnet amount and YAG composition, and xenotime amount. Monazite amount and composition is strongly controlled by the presence of xenotime and when xenotime is absent, by the growth or consumption of garnet. Monazite grows continuously in the subsolidus region over most reasonable metamorphic P–T paths with YPO4content generally increasing with increasing temperature, especially where xenotime is present. When melt is present, monazite is consumed with increasing melting and, in the absence of xenotime, decreases in YPO4content. Two prograde forward models are presented to illustrate the effect of garnet fractional crystallization on the results. When xenotime is present and the modal amount of garnet is low (e.g., at low pressure), fractional crystallization has little effect on phase amount or composition. Where xenotime is absent and garnet is more abundant (e.g., at intermediate to high pressure), garnet fractionation dominates both monazite growth history and composition. In particular, YPO4content of monazite drops to nearly zero at intermediate grades in a rock in which garnet has sequestered the Y as YAG component. This Y may be released by diffusion at higher temperature and promote monazite growth with a higher YPO4. Therefore, the growth history of monazite as inferred from YPO4zoning may be quite complex and reflect different reactions and processes, even in a simple bulk composition.