Neodymium isotopic compositions of the standard monazites used in U-Th-Pb geochronology

Abstract

As an important carrier of thorium, uraniumand trace elements, monazite iswidely used in U-Th-Pb geochronology and Sm\Nd isotopic studies. The combination of U-Th-Pb age and Sm\Nd isotopic composition makes monazite one of the most versatile minerals in the study of crustal evolution. Nevertheless, most monazites are small and have complex internal textures that require isotopic measurements with high spatial resolution. The widely utilized in situ analyses, such as secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS), have emphasized the importance for uniform and homogeneous monazite standards for external calibration. Here we performed in situ simultaneous measurement of U-Th-Pb and Sm\Nd isotopes, as well as trace element determinations, of several monazite standards using a 193 nm excimer laser ablation system coupled with an Agilent 7500a quadruple (Q)-ICP-MS and a Neptune multicollector (MC)-ICP-MS.For U-Th-Pb isotopic analysis, instrumental mass bias and elemental fractionation were corrected by calibrating against a homogeneous monazite standard (44069), which makes it possible to precisely and accurately date other monazite grains. In terms of the Sm-Nd isotopes, the interference of 144Sm on 144Nd was corrected by using the interference-free 147Sm/149Sm for the Sm mass fractionation, and the fractionation between 144Sm and 144Nd was corrected by calibrating against an external monazite standard Manangoutry, which is homogeneous in Sm-Nd isotopic composition. The obtained Sm\Nd isotopic compositions for synthetic LREE glass and natural monazite samples are all consistent with the values obtained by isotope dilution thermal ionization mass spectrometry (ID-TIMS), which indicates the reliability and robustness of our method. Our results demonstrate that the Manangoutry monazite from Madagascar and the Namaqualand monazite from South Africa are relatively uniform and homogeneous in their Sm-Nd isotopic compositions, and hence can serve as standards for external calibration during in situ Sm-Nd isotopic analysis.