Thorium isotopic measurements on silicate rock samples with a multi-collector inductively coupled plasma mass spectrometer

Abstract

Chemical and mass spectrometric procedures for thorium isotopic analysis using a highly sensitive multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a retardation lens were developed. They were applied to thorium isotopic measurements in volcanic rock samples. Rock samples were digested in screw-capped Savillex beakers with HF–HClO₄. Thorium was then isolated from the rock matrix by a conventional anion-exchange procedure. Purified thorium was analyzed using an MC-ICP-MS (Micromass, IsoProbe) with a micro-concentric nebulizer (Cetac Aridus). The sensitivity for thorium was about 80 V ppm⁻¹, which enabled us to obtain a strong enough ion current to measure ²³⁰Th/²³²Th ratios with a 10 ppb solution. With these procedures, we were able to reduce the amount of thorium required for precise isotopic analysis to <10 ng. Mass-fractionation correction and Daly/Faraday multiplier gain calibration were applied using natural uranium standard solution. The procedures were applied to standard rock samples whose thorium isotopic ratios had been reported using thermal ionization mass spectrometry (TIMS). Thorium isotopic compositions measured for JB-1 and AGV-1 gave coincident values with the TIMS values. In addition, the ²³⁰Th/²³²Th radioactivity ratios of the three standard rocks including JR-2, with eruption ages of older than 350 kyr, were in equilibrium with those of ²³⁸U/²³²Th, confirming the accuracy of our analysis.