COM-1 and Hongcheon: New monazite reference materials for the microspot analysis of oxygen isotopic composition

Abstract

Background: Monazite, a moderately common light-rare earth element (LREE) and thorium phosphate mineral, has chemical, age, and isotopic characteristics that are useful in the investigation of the origin and evolution of crustal melts and fluid-rock interactions. Multiple stages of growth and partial recrystallization commonly observed in monazite inevitably require microspot chemical and isotopic analyses, for which well-characterized reference materials are essential to correct instrumental biases. In this study, we introduce new monazite reference materials COM-1 and Hongcheon for the use in the microspot analysis of oxygen isotopic composition. Findings: COM-1 and Hongcheon were derived from a late Mesoproterozoic (~1080 Ma) pegmatite dyke in Colorado, USA, and a Late Triassic (~230 Ma) carbonatite-hosted REE ore in central Korea, respectively. The COM-1 monazite has much higher levels of Th (8.77 ± 0.56 wt.%), Si (0.82 ± 0.07 wt.%) and lower REE contents (total REE = 49.5 ± 1.2 wt.%) than does the Hongcheon monazite (Th, 0.23 ± 0.11 wt.%; Si, < 0.1 wt.%; total REE, 59.9 ± 0.7 wt.%). Their oxygen isotopic compositions (δ18OVSMOW) were determined by gas-source mass spectrometry with laser fluorination (COM-1, 6.67 ± 0.08‰; Hongcheon-1, 6.60 ± 0.02‰; Hongcheon-2, 6.08 ± 0.07‰). Oxygen isotope measurements performed by a Cameca IMS1300-HR3 ion probe showed a strong linear dependence (R2 = 0.99) of the instrumental mass fractionation on the total REE contents. Conclusions: We characterized chemical and oxygen isotopic compositions of COM-1 and Hongcheon monazites. Their internal homogeneity in oxygen isotopic composition and chemical difference provide an efficient tool for calibrating instrumental mass fractionation occurring during secondary ion mass spectrometry analyses.