Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite – Improvements, potential reference materials, and application to the Archean Saglek Block gneisses

Abstract

Continued improvements in both ICPMS (inductively-coupled plasma mass spectrometry) and laser ablation technologies are fueling advancements in accessory mineral investigations and their related isotope systems of interest, and are now being applied to a wide range of geological applications. In this contribution we present an updated methodology for laser ablation split-stream (LASS) analysis of the light rare earth element (LREE) enriched minerals monazite, titanite, and apatite for simultaneous analysis of the U-Th-Pb age (or trace element content) and the Sm-Nd isotope system. The data were collected with the current generation of high-sensitivity ICPMS and laser systems (ThermoFinnigan NeptunePlus MC-ICPMS and Element XR SF-ICPMS- coupled to a RESOlution 193 nm ArF excimer laser system). The increased sensitivity of these ICPMS instruments allows for improved spatial resolution and the ability to target minerals which previously contained insufficient concentrations of elements of interest (e.g., Sm-Nd in apatite), making their analysis difficult, if not impossible, using less sensitive instruments. Furthermore, the higher sensitivity allows less aggressive ablation parameters that facilitate thin section sampling and reduces inter-element and isotopic fractionation.To assess and improve the accuracy, precision, and efficiency of the technique, three new potential LASS reference materials (RMs) are evaluated for dual U-Pb and Sm-Nd analysis (Tory Hill apatite, Tory Hill titanite, and Steenskralkamp monazite). The homogeneity of these materials was first assessed using reconnaissance laser ablation analyses, with final characterization of U-Pb age and Sm-Nd isotope composition using isotope-dilution thermal ionization mass spectrometry (ID-TIMS). The precision and accuracy of the LASS method is explored using secondary mineral reference materials of known age and Sm-Nd isotope composition. The utility of the technique is evaluated with a case study of monazite and apatite from the Eoarchean Uivak Gneiss complex of Labrador, Canada. Finally, we suggest that there is a wide variety of geological applications for this methodology, such as multi-mineral detrital provenance, crustal growth, and petrogenic studies.