Neodymium isotopic compositions of the standard monazites used in U[sbnd]Th[sbnd]Pb geochronology

Abstract

As an important carrier of thorium, uranium and trace elements, monazite is widely used in UThPb geochronology and SmNd isotopic studies. The combination of UThPb age and SmNd 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 UThPb and SmNd isotopes, as well as trace element determinations, of several monazite standards using a 193nm excimer laser ablation system coupled with an Agilent 7500a quadruple (Q)-ICP-MS and a Neptune multi-collector (MC)-ICP-MS. For UThPb 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 SmNd 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 SmNd isotopic composition. The obtained SmNd 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 SmNd isotopic compositions, and hence can serve as standards for external calibration during in situ SmNd isotopic analysis.