Monazite U–Pb and Th–Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock
Monazite U–Pb and Th–Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock
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DOI：
10.1016/s0009-2541(00)00239-4
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发表时间：
2001-02
期刊：
影响因子：
3.9
通讯作者：
R. Stern;R. Berman
中科院分区：
文献类型：
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作者：
R. Stern;R. Berman
利用9种前寒武纪岩石样品中同位素稀释U-Pb和Th-Pb年龄一致的独一石颗粒，采用高质量分辨率（R=5500, 1%）的离子探针，开发并评估了U-Pb、Th-Pb和Pb-Pb同位素定年技术。在~ 203.960 amu处有一条未识别的等压线，使高Th单氮石中204pb的测定变得复杂，但当使用轻微的能量偏移时，它似乎被消除了。单个斑点的207pb */206Pb*比值采用20 μm宽的O2 -主光束在10 min内测定，其不确定度为±0.3-0.5% (1σ)，而单代单氮石上几个斑点的平均值不确定度为±0.1-0.4% （2σ）。利用206pb +/UO+vs.之间的线性关系监测个体分析中Pb+/UO+和Pb+/ThO+值测量的仪器偏差。UO2 + / UO + and208Pb + / ThO + vs。UO2 + / UO +。偏倚似乎随着独居石的含量而有所不同，需要使用成分匹配的标准。在低至中等Th的未知独居石中，206pb /238U和208pb /232Th的比值可测定为±2% （1σ）。将测年技术应用于太古宙角闪岩级变长岩中小独居石包裹体的原位分析。研究发现，环境环境和独居石颗粒的形态是解释寄主变质矿物（如石榴石、橄榄石、斜长石）生长时间的重要因素。在低cao石榴石和基质斜长石中包裹的自面体独居石颗粒的年龄为2548±17 Ma，被解释为低压变质的早期阶段。高cao石榴石边缘部分吸收的独居石产生的年龄约为2500 Ma，但被解释为继承自第一次事件，而不是该石榴石生长的同成因。单脱石的年龄为1755±30 Ma，与晚期绿泥石充填的裂缝有关，明显反映了后构造退行。
Monazite grains from nine different Precambrian rock samples and having concordant isotope dilution U–Pb and Th–Pb ages were used to develop and evaluate techniques for U–Pb, Th–Pb, and Pb–Pb isotopic dating using an ion microprobe operating at high mass resolution (R=5500, 1%). An unidentified isobar at ∼203.960 amu complicated the determination of204Pb in the monazites with higher Th, but it appeared to have been eliminated when using a slight energy offset. The207Pb*/206Pb* ratios of individual spots were determined in 10 min analyses to ±0.3–0.5% (1σ) of the true values using a 20-μm-wide O2−primary beam, whereas the uncertainty in the mean of several spots on a single generation of monazite was typically ±0.1–0.4% (2σ). Instrumental bias in the measured Pb+/UO+and Pb+/ThO+values of individual analyses was monitored using linear relationships between206Pb+/UO+vs. UO2+/UO+and208Pb+/ThO+vs. UO2+/UO+. The bias appears to vary somewhat with monazite Th content, requiring the use of compositionally matched standards. Individual spot measurements of the206Pb/238U and208Pb/232Th ratios in unknown monazites of low to moderate Th can be determined to ±2% (1σ). The dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite-grade metapelite of Archean origin. Both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral (e.g., garnet, staurolite, plagioclase). Euhedral monazite grains armoured in low-CaO garnet and matrix plagioclase yielded an age of 2548±17 Ma, interpreted as dating an early episode of low-pressure metamorphism. Partially resorbed monazite within a high-CaO garnet rim yielded ages of ca. 2500 Ma, but are interpreted as inherited from the first event rather than syn-genetic with respect to the growth of this garnet. An age of 1755±30 Ma was obtained for anhedral monazite associated with late, chlorite-filled fractures, apparently reflecting post-tectonic retrogression.