Monazite-xenotime thermobarometry: Experimental calibration of the miscibility gap in the binary system CePO4-YPO4
Monazite-xenotime thermobarometry: Experimental calibration of the miscibility gap in the binary system CePO4-YPO4
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DOI：
10.2138/am-1997-7-816
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发表时间：
1997-08
影响因子：
3.1
通讯作者：
Rene Gratz;W. Heinrich
中科院分区：
文献类型：
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作者：
Rene Gratz;W. Heinrich
摘要在300 ~ 1000 ℃，2、5、10和15 kbar压力下，水热合成了（Ce 5 Y）PO 4固溶体。用电子探针对实验产物进行了分析，并通过Rietveld分析XRD粉末衍射图，对实验产物的晶格参数进行了修正。在很宽的范围内的散装组合物，形成两个不混溶相：独居石与P21/n结构（REEO 9多面体）和磷钇矿与I41/amd结构（REEO 8多面体）。非对称可扩散性间隙的边界强烈地依赖于温度。在2kbar和300-1000 ℃条件下，独居石与磷钇矿共存时，钇的最大浓度从3- 16mol%增加。随着压力的增加，该分支向更高的Y含量移动，使得在15 kbar下，对于相同的温度范围，Y含量为6至约25mol%。相比之下，铈浓度在磷钇矿不超过3摩尔%，在1000 ℃在整个压力范围内。我们的实验确定的边界符合令人惊讶的好与（LREE-HREE，Y）PO 4-溶线边界的经验校准来自于一套低压的metalites（3至5千巴），在400-700 °C的峰值温度范围内平衡。自然（REE 5 Y）PO 4系统的行为可能是很好地描述了简单的（Ce 5 Y）PO 4二进制，至少对于变质成分。许多与磷钇矿共生的独居石天然样品在HREE + Y的掺入方面表现出明显的生长分带性。因此，我们的温度计沿着与高空间分辨率的U-Pb和Sm-Nd年龄测定相结合，可以提供有关变质PTt轨迹的分支的信息。
Abstract Solid solutions of (Ce5Y)PO4 were synthesized hydrothermally between 300 ℃ and 1000 ℃ at pressures of 2, 5, 10, and 15 kbar. Experiment products were analyzed by electron microprobe, and their lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, two immiscible phases formed: monazite with a P21/n structure (REEO9 polyhedron) and xenotime with an I41/amd structure (REEO8 polyhedron).I Variations of the unit-cell dimensions are directly correlated with variations in composition. The boundaries of the asymmetric miscibility gap are strongly dependent on temperature. At 2 kbar and from 300-1000 ℃, maximum Y concentrations in monazite coexisting with xenotime increase from 3-16 mol%. With increasing pressure, this limb is shifted to higher Y contents so that at 15 kbar it ranges from 6 to about 25 mol% for the same temperature range. In contrast, Ce concentrations in xenotime do not exceed 3 mol% at 1000 ℃ over the entire pressure range. Our experimentally determined boundaries fit surprisingly well with empirical calibrations of the (LREE-HREE,Y)PO4-solvus boundaries derived from a suite of low pressure metapelites (3 to 5 kbar) that equilibrated at peak temperatures ranging from 400-700 °C. The behavior of the natural (REE5Y)PO4 system is probably well described by the simple (Ce5Y)PO4 binary, at least for metapelitic compositions. Many natural samples of monazite coexisting with xenotime show prograde growth zonation with respect to the incorporation of HREE + Y The combination of our thermometer along with U-Pb and Sm-Nd age determinations of high spatial resolution may thus provide information about prograde branches of metamorphic PTt paths.