Simultaneous in situ determination of U-Th-Pb and Sm-Nd isotopes in monazite by laser ablation ICP-MS

Abstract

Results are presented for the in situ simultaneous determination of U-Pb and Sm-Nd isotopes in monazite using the Laser Ablation Split-Stream (LASS) method. This method uses a laser ablation system coupled to both a magnetic-sector inductively coupled plasma mass spectrometer (HR) (ICP-MS) for measuring U-Pb isotopes and multicollector (MC) ICP-MS for measuring Sm-Nd isotopes. The ablated material is split using a glass Y-connector and transported simultaneously to both mass spectrometers. In addition to Sm and Nd isotopes, the MC-ICP-MS is configured also acquire Ce, Nd, Sm, Gd, and Eu elemental abundances. This approach provides age, tracer isotope, and trace element data in the same ablation volume, thus reducing but not eliminating sampling problems associated with fine-scale zoning in accessory minerals. The precision and accuracy of the U-Pb method (along with the precision of the Sm-Nd method) is demonstrated by analysis of six well-characterized monazite reference materials. The LASS results agree within uncertainty with previously determined isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. Accuracy of the Sm-Nd method is assessed by comparing the LA-MC-ICP-MS results with ID-TIMS determinations on a well-characterized, in-house monazite reference material. The LASS method is then applied to monazite from the Birch Creek Pluton (BCP) in the White Mountains of southeastern California as a case study to illustrate the utility of this method for solving geologic problems. The U-Pb ages and Sm-Nd isotopic data determined using the LASS method support the conclusions drawn from previous results that monazite can record both timing and potential sources of hydrothermal fluids.