Coupling of in-situ Sm–Nd systematics and U–Pb dating of monazite and allanite with applications to crustal evolution studies
Coupling of in-situ Sm–Nd systematics and U–Pb dating of monazite and allanite with applications to crustal evolution studies
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DOI：
10.1016/j.chemgeo.2007.07.020
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发表时间：
2007-10
期刊：
影响因子：
3.9
通讯作者：
C. McFarlane;M. McCulloch
中科院分区：
文献类型：
--
作者：
C. McFarlane;M. McCulloch
在这里，我们展示了LA-MC-ICPMS Sm-ND同位素测量应用于地壳中两种最常见的轻稀土富集型矿物：独居石和尿晶石的能力。我们发现，在激光光斑大小从16到50μm的范围内，可以实现与基于溶液的MC-ICPMS测量(<；0.5epsilon单位)相当的高精度测量。在这个尺度上，以前使用原位U-Th-Pb年代学(例如离子探针)测年的亚晶区可以通过原位Nd同位素和Sm/ND元素测量来表征，以确定可靠的初始143Nd/144Nd比值和研究颗粒尺度的同位素不均一性。此外，通过Ce、NdSm、Eu和Gd的现场测量，可以同时确定轻稀土元素的富集型式和Eu的⁎异常。这种方法在这里被应用于麻粒岩相变质岩，以前使用离子探针原位测年。结果表明，LA-MC-ICPMS在独居石和尿晶石颗粒中精确测量的初始143Nd/144Nd比值和LREE浓度，可以为同位素遗传提供约束，并在结构背景下理解组成域的起源(例如，岩心过度生长特征)。独居石U-Pb和石榴石全岩Sm-ND等时线年龄的差异也可以用来估算地壳冷却速率，从而阐明高等级地体的地壳驻留时间尺度。
Here we demonstrate the capabilities of LA-MC-ICPMS Sm–Nd-isotope measurements applied to two of the most common LREE-enriched minerals in the Earth's crust: monazite and allanite. We show that high precision measurements of143Nd/144Nd comparable to solution-based MC-ICPMS measurements (<0.5 epsilon units) can be achieved for laser spot sizes ranging from 16 to 50 μm. At this scale, sub-grain domains previously dated using in-situ U–Th–Pb geochronology (e.g., ion-microprobe) can be characterized with in-situ Nd-isotopic and Sm/Nd elemental measurement to determine robust initial143Nd/144Nd ratios and investigate grain-scale isotopic heterogeneities. In addition, the LREE-enriched pattern and Eu⁎ anomaly can be simultaneously determined from in-situ Ce, Nd Sm, Eu, and Gd concentration measurements. This approach is applied here to granulite-facies metamorphic rocks previously dated in-situ using an ion-microprobe. It is shown that precise initial143Nd/144Nd ratios and LREE concentrations measured in monazite and allanite grains by LA-MC-ICPMS on grain mounts and in-situ in polished thin sections can provide constraints on isotopic inheritance, and an understanding of the origin of compositional domains (e.g., core-overgrowth features) within a textural context. Differences between monazite U–Pb and garnet-whole-rock Sm–Nd isochron ages can also be used to estimate crustal cooling rates, thereby elucidating the timescales of crustal residence of high-grade terrains.