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

Chemical and mass spectrometric procedures for thorium isotopic analysis us ing a highly sensitive multiple-collector inductively coupled plasma mass s pectrometer (MC-ICP-MS) equipped with a retardation lens were developed. Th ey 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-exch ange procedure. Purified thorium was analyzed using an MC-ICP-MS (Micromass , IsoProbe) with a micro-concentric nebulizer (Cetac Aridus). The sensitivi ty for thorium was about 80 V ppm(-1), which enabled us to obtain a strong enough ion current to measure Th-230/Th-232 ratios with a 10 ppb solution. With these procedures, we were able to reduce the amount of thorium require d for precise isotopic analysis to <10 ng. Mass-fractionation correction an d Daly/Faraday multiplier gain calibration were applied using natural urani um standard solution. The procedures were applied to standard rock samples whose thorium isotopic ratios had been reported using thermal ionization ma ss spectrometry (TIMS). Thorium isotopic compositions measured for JB-1 and AGV-1 gave coincident values with the TIMS values. In addition, the Th-230 /Th-232 radioactivity ratios of the three standard rocks including JR-2, wi th eruption ages of older than 350 kyr, were in equilibrium with those of U -238/Th-232, confirming the accuracy of our analysis.