MEASUREMENT OF TH-230/TH-232 RATIOS IN YOUNG VOLCANIC-ROCKS BY SINGLE-SECTOR THERMAL IONIZATION MASS-SPECTROMETRY

Techniques have been developed for the measurement of low Th-230/Th-232 ratios (approximately 5.10(-6)) in young volcanic rocks using a conventional single-focussing, multi-collector mass spectrometer without any hardware modifications to improve abundance sensitivity. The external precision, checked by replicate analyses of the Table Mountain Latite (TML) rock standard over a 2-yr period is better than +/-1.5% (2sigma). This technique therefore offers a three- to five-fold improvement in precision compared with alpha-counting, simple chemical separation techniques and a short analysis time (approximately 1.5 hr). The enhanced precision and reduced analytical time arises from the significantly more favourable counting statistics achieved by mass spectrometry, which in turn permits analysis of smaller samples (approximately 300 ng instead of approximately 1 mug Th) than required by alpha-spectrometry.