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首页> 外文期刊>The American mineralogist >Two types of metamorphic monazite with contrasting La/Nd, Th, and Y signatures in an ultrahigh-pressure metapelite from the Pohorje Mountains, Slovenia: Indications for pressure-dependent REE exchange between apatite and monazite?
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Two types of metamorphic monazite with contrasting La/Nd, Th, and Y signatures in an ultrahigh-pressure metapelite from the Pohorje Mountains, Slovenia: Indications for pressure-dependent REE exchange between apatite and monazite?

机译：斯洛文尼亚Pohorje山的超高压变质岩中两种具有La / Nd，Th和Y特征相反的变质独居石：指示磷灰石与独居石之间压力相关的REE交换的迹象吗？

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Two monazite generations (M1; M2) were distinguished in a kyanite-garnet gneiss from the UHP terrain of the Pohorje Mountains, Slovenia. P-T estimates reveal a peak event at 760℃/2.6 GPa and isothermal decompression down to 700℃/0.6 GPa. M1 type provides a Th-U-Pb mean date of 100 ± 6 Ma, ThO2 contents between 3–7 wt%, Y2O3 values <0.3 wt%, and La/Nd ratios (1.2–1.4) that are clearly higher than for the whole-rock La/Nd (1.1). The absence of Y zoning in M1 and the lack of monazite inclusions in garnet indicate that M1 formed after the main stage of garnet growth (>1.2 MPa), probably close to the P-T peak. M2 type is slightly younger than M1 (74 ± 16 Ma), and has a lower La/Nd (0.3–0.9), lower ThO2 (0.1–5 wt%), and higher Y2O3 (up to 3.2 wt%). Most M2 monazites occur as tiny needles within apatite (subtype M2-a) or along apatite margins (M2-b). Parasitic growth of M2-a and -b from apatite is supported by its low ThO2 (<1 wt%) and La/Nd (<0.5). Isolated matrix grains (M2-c) and overgrowths around M1 (M2-d) have slightly higher La/Nd (0.5–0.9) and higher ThO2 (5 wt%) and were supplied from an apatite and M1 source. Elevated yttrium suggests that M2 formed during decompression, when garnet was consumed and Y was released. These observations imply that at UHP conditions MREE-rich apatite coexisted with low-MREE M1 monazite and reacted during decompression to Ca-F-apatite plus MREE-rich M2 monazite. This provides strong arguments that REE-partitioning between apatite and monazite is pressure-dependent.
机译：在斯洛文尼亚Pohorje山的UHP地形中，以蓝晶石-石榴石片麻岩区分了两个独居石世代（M1; M2）。 P-T估计值表明在760℃/ 2.6 GPa出现峰值事件，等温减压至700℃/ 0.6 GPa。 M1型提供的Th-U-Pb平均日期为100±6 Ma，ThO2含量在3-7 wt％之间，Y2O3值<0.3 wt％，La / Nd比（1.2-1.4）明显高于全岩La / Nd（1.1）。 M1中没有Y分区，石榴石中没有独居石夹杂物，表明M1是在石榴石生长的主要阶段（> 1.2 MPa）之后形成的，可能接近P-T峰。 M2类型比M1（74±16 Ma）稍年轻，并且具有较低的La / Nd（0.3–0.9），较低的ThO2（0.1–5 wt％）和较高的Y2O3（最高3.2 wt％）。大多数M2独居石以细针状出现在磷灰石内（亚型M2-a）或沿磷灰石边缘（M2-b）。磷灰石的低ThO2（<1 wt％）和La / Nd（<0.5）支持了磷灰石对M2-a和-b的寄生生长。孤立的基体晶粒（M2-c）和M1附近的过度生长（M2-d）具有较高的La / Nd（0.5-0.9）和较高的ThO2（5 wt％），它们是由磷灰石和M1来源提供的。钇升高提示减压期间，石榴石被消耗，Y被释放时，形成了M2。这些观察结果表明，在超高压条件下，富含MREE的磷灰石与低MREE的M1独居石共存，并在减压过程中反应为Ca-F-磷灰石加上富含MREE的M2独居石。这提供了强有力的论据，即磷灰石和独居石之间的稀土元素分配是压力依赖性的。

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