In situ boron isotope measurements of natural geological materials by LA-MC-ICP-MS

Abstract

There are two main methods to determine boron isotopic composition. One is the solution method, in which boron is purified after the samples are dissolved in solution and the boron isotope ratios are determined by thermal ionization mass spectrometry (P-TIMS and N-TIMS) or multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The other is an in-situ analysis method, in which the in-situ boron isotopic ratios in minerals are analyzed directly using secondary ion mass spectrometry (SIMS) or laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). In the in-situ analysis method for boron isotopes, the multifarious chemical purification and separation processes of the solution method are avoided, with increased work efficiency. In addition, the microzones and microbeddings of minerals can be analyzed in-situ to reveal the fine processes and conditions of mineral formation. In this study, using the standard-sample-bracketing (SSB) method, mass bias of the instrument and the fractionation of isotopes were calibrated, and the in-situ determination method of LA-MC-ICP-MS for boron isotopes was established. Through detailed analyses on a series of boron isotope standards and samples, a matrix effect was assessed but not detected, and the analysis results were in accordance with the formerly reported values or P-TIMS determined values, within the error range. The analytical results for IAEA B4 and IMR RB1 with relatively high boron contents were δ11B = −(8.36±0.58)‰ (2σ, n=50) and δ11B = −(12.96±0.97)‰ (2σ, n=57), respectively; the analytical result for IAEA B6 with relatively low boron content was δ11B = −(3.29±1.12)‰ (2σ, n=35). In-situ measurements for B isotopes were performed on geological samples such as tourmaline, ulexite, ludwigite, inyoite and ascharite, with the results consistent with those determined by P-TIMS, within the error range.