Coupling of in-situ Sm–Nd systematics and U–Pb dating of monazite and allanite with applications to crustal evolution studies

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 of 143Nd/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 initial 143Nd/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 initial 143Nd/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.