In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)
In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)
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铝长石热液蚀变的现场研究：科西嘉岛（法国东南部）钙碱性和非造山花岗岩的实例
DOI：
10.1007/s00410-002-0353-x
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发表时间：
2002
影响因子：
3.5
通讯作者：
F. Poitrasson
中科院分区：
文献类型：
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作者：
F. Poitrasson
铜榴石与独居石一起是大陆地壳中轻稀土元素 (REE) 的主要储存库，可用于 U-Th-Pb 地质年代学。这种矿物已被证明易于发生蚀变。使用背散射扫描电子显微镜、电子微探针和激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）探索了铜榴石和热液之间发生的地球化学交换。发现的蚀变机制显示了变生化作用的限制，并且对应于铜榴石到绿帘石的转化或铜铜矿 A 晶体位点的浸出。因此，尽管富含稀土的重流体可能会在铜榴石蚀变带上留下其地球化学特征，但稀土元素大部分在流体-矿物相互作用过程中从这一相中被去除。看来，即使该 REE 持有者提供了蚀变证据，在研究的案例中，散装矿物和全岩 REE 以及 Sm-Nd 同位素特征也没有受到显着影响。蚀变主要导致普通铅的输入，因此在蚀变发生时限制了铝长石用于 U-Th-Pb 定年的有用性。最后，这里发现的改变机制证实了与磷酸盐相比，硅酸盐晶格的灵活性更加有限。
Allanite is, with monazite, the main repository for light rare earth elements (REE) in the continental crust and can be used in U–Th–Pb geochronology. This mineral has been shown to be prone to alteration. The geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy, electron microprobe, and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The alteration mechanisms found show the restricted role of metamictization and correspond to the allanite to epidote transformation or to the leaching of the allanite A-crystallographic sites. Hence, the REE are mostly removed from this phase during fluid-mineral interactions, although a heavy rare earth-rich fluid may imprint its geochemical signature on the allanite altered zones. It appears that even if this REE holder presents evidence of alteration, the bulk mineral and whole-rock REE and Sm-Nd isotope signatures were not significantly affected in the cases studied. Alteration results mostly in the input of common lead, thus restricting the usefulness of allanite for U–Th–Pb dating when alteration occurs. Finally, the alteration mechanisms found here confirm the much more limited flexibility of silicates crystalline lattice as compared to phosphates.