Zircon water content: reference material development and simultaneous measurement of oxygen isotopes by SIMS

Abstract

Zircon water content is an important physicochemical parameter for many geological processes, yet its measurement by the secondary ion mass spectrometry (SIMS) technique is hampered by the lack of suitable reference materials and high water background, especially if large-geometry (LG)-SIMS is used. Here we have described a suite of newly developed reference materials for SIMS zircon water content analysis and a modified micro-analytical technique (using a CAMECA IMS 1280-HR SIMS) that can simultaneously measure the zircon water content and oxygen isotopes. A total of 20 natural zircon grains/sherds were analyzed via Fourier transform infrared spectroscopy (FTIR), among which 8 (with good water content result reproducibility) were further analyzed by SIMS. Before the SIMS analysis, FTIR analyzed sample blocks were mounted with a Sn-based alloy to minimize degassing and background water. As in routine SIMS oxygen isotope measurement, ¹⁶O⁻ and ¹⁸O⁻ were collected using two Faraday cups, and in addition ¹⁶O¹H⁻ was simultaneously measured using an electron multiplier. The measured ¹⁶O¹H/⁶O ratio was converted into water content, using a calibration line established based on SIMS ¹⁶O¹H⁻/¹⁶O⁻ ratios vs. the FTIR water content. Both the internal and external precisions of corrected δ¹⁸O are <0.4 permil (2SE or 2SD). The internal precision of ¹⁶O¹H⁻/¹⁶O⁻ ratios follows a Poisson error theoretical trend and is generally better than 0.3%. The external precision (reproducibility) of ¹⁶O¹H⁻/¹⁶O⁻ ratios is better than 5% (2SD) for homogenous samples, and uncertainty of the calibrated water content is ∼10%.