Non-matrix-matched 9 μm u-pb dating of zircon using excimer laser ablation icp-ms

Abstract

U-Pb dating geochronology by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been widely applied in geological research as a low-cost and high-efficient in situ micro-analytical tool. Currently, quantitative analysis using LA-ICP-MS requires matrix-matched external standards to correct for elemental fractionation. Since natural standard minerals are often limited in supply or not available, non-matrix-matched calibration methods using synthetic glasses as the calibration standards have attracted attention. In addition, access to a higher spatial resolution capability of <10 μm spot size has been driven by the interest in studying the temporal records of fine mineral grains or rims compared to the conventional >20 μm laser ablation spots used in most LA-ICP-MS laboratories for zircon dating. In this study, we present an LA-ICP-MS analytical procedure for non-matrix-matched U-Pb zircon dating using a 9 μm laser ablation spot size and a synthetic silicate glass (NIST SRM 610) as the external standard. The elemental fractionation between the NIST SRM 610 glass and zircons can be significantly reduced by using a low laser frequency (2 Hz) and energy density (3 J/cm2). To minimize the matrix effect on matrix-mismatched calibration between samples and standards, the effect of adding a small amount of water vapor or hydrogen into the carrier gas was investigated. A small amount (about 1.9 μL/min) of water vapor added into the sample cell can reduce the Pb/U ratios of the NIST SRM 610 up to 14% during a 40 s ablation time at a small spot size. However, by adding water vapor after the sample cell can achieve an accuracy and precision of better than 2.3% for the weighted average206Pb/238U ages using a 9 μm spot size. In addition, the laser-ablated crater depth needs to be less than 4.3 μm to control the down-hole fractionation during laser ablation.