In situ analysis of 230Th–232Th–238U ratios in titanite by fs-LA-MC-ICPMS

Abstract

The potential of femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICPMS) for in situ analysis of U-Th disequilibria in titanite was investigated. The aim of the study was to resolve spatial variations in ( 230 Th/ 238 U) ratios (where parentheses denote activity) in titanite from slowly cooled magma bodies. An in-house titanite glass (TG2), determined to be in secular equilibrium by solution mode MC-ICPMS (i.e. ( 230 Th/ 238 U) ¼ 1), was used to correct for U-Th elemental fractionation by sample standard bracketing. The effect of instrument operating conditions on the accuracy and reproducibility of ( 230 Th/ 238 U), ( 232 Th/ 238 U) and ( 230 Th/ 232 Th) ratios was studied by analyses of titanite minerals with known composition and a secondary titanite glass standard. The ( 230 Th/ 232 Th) data were found to be accurate and reproducible, independent of the instrument setting used, suggesting that corrections made for SEM-Faraday gain and abundance sensitivity were appropriate. However, plasma conditions, laser ablation mode, laser energy and wavelength, and titanite material properties were all found to variably influence the U-Th elemental fractionation and compromise the accuracy of the ( 230 Th/ 238 U) data to different extents. Hot plasma conditions significantly reduce the fractionation between U and Th. A drift in elemental fractionation was observed during single spot analyses using NIR laser ablation and results in errors of up to 29% on the ( 230 Th/ 238 U) data. The magnitude of the drift in the elemental fractionation was different for different laser wavelengths and energies. Ablation using the UV single spot mode was significantly less affected by variable elemental fractionation compared to NIR spot analyses, but precision was limited by lower sample uptake. Scanning mode analyses were not compromised by temporal variation of the U-Th intensity ratios but the degree of elemental fractionation was variable between analyses of different materials (e.g. glass versus minerals). This observation suggests material-dependent differences in U-Th fractionation even for near identical titanite compositions. Analyses of the secondary titanite glass standard TG1 bracketed by TG2 yield the most reproducible and accurate ( 230 Th/ 238 U) data, indicating more adequate correction for elemental fractionation when the calibration standard is matched in terms of material composition and structure.

U-Series Dating

“…It was hoped that the development of the femtosecond laser would also limit the degree of fractionation (Horn and von Blanckenburg 2007). A recent study of U-Th disequilibria in sphene using a femtosecond laser has shown that while the 230 Th/ 232 Th ratios could be measured accurately, it was more difficult to obtain precise results on Th/U ratios if the standard compositions did not match the matrix of the natural samples (Koornneef et al 2012). Thus, an important conclusion from this research is that the production of high quality standards of equivalent composition and texture seems essential.…”

Evaluation of New Geological Reference Materials for Uranium-Series Measurements: Chinese Geological Standard Glasses (CGSG) and Macusanite Obsidian

“…Uranium-series analytical measurements are widely used in geochemistry − and geochronology , and can be applied to a wide variety of fields including paleoclimatology, , volcanology, , environmental risk assessment, , and archeology. , Recent advances in high-resolution, rapid, in situ, microanalytical techniques, for example, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS), and micro X-ray fluorescence (μ-XRF) present numerous opportunities for the analytical community. − Reference glasses are essential in microanalysis and are commonly used as samples for calibration, method development, quality control/assurance, and for inter-laboratory comparison . About 20–25 reference glasses are currently available for microanalysis .…”