Evaluation of potential monazite reference materials for oxygen isotope analyses by SIMS and laser assisted fluorination

Abstract

Monazite can record several episodes of fluid/rock interaction within a single grain. Coupled in situ determination of age and oxygen isotope composition by SIMS (secondary ion mass spectrometer) may characterize such events. Yet, monazite reference materials for oxygen isotope analyses are scarce. Ion probe (CAMECA IMS 1280) and laser fluorination measurements of δ18O values from four monazites are presented. Monazites (YREEPO4) with variable cheralite [CaTh(PO4)2] and huttonite (ThSiO4) substitutions, which span a wide compositional range covering most magmatic and metamorphic, as well as hydrothermal monazites were selected. Three monazites, Moacyr, Manangoutry and UNIL-Mnz1, have a homogeneous chemical (determined by electron microprobe analysis and back-scattered imaging) and oxygen isotopic composition with uncertainties (2 SD) in the range of 0.3 to 0.4‰ for ion probe measurements. These three samples are thus potentially suitable as reference materials, and sample UNIL-Mnz1 is available for inter-laboratory comparison. Sample UNIL-Mnz2 is less homogeneous, but it is still useful for internal monitoring. Uncertainties (2 SD) on the ion probe data are in the range of 0.5 to 0.6‰.Using the investigated monazites as well as literature data, a new calibration curve has been established to account for instrumental mass fractionation (IMF) due to the solid solution in natural monazites. The data illustrate that the IMF is primarily a function of the YREEPO4 content, while the influence of the Th-content seems to be less important.The δ18O values for the four studied monazites have been determined by laser fluorination (LF). They are as follows (1 SD): 1.5±0.1‰ for Moacyr, 10.2±0.1‰ for Manangoutry, 8.5±0.2‰ for UNIL-Mnz1, and 9.5±0.3‰ for the slightly less homogenous UNIL-Mnz2. The analytical protocol adopted here results in consistently high yields (>90%) and reproducible values, and hence it is argued that LF is a viable method for the calibration of phosphate reference materials.