Applications Two broad applications have been developed for the ims 1270, a large-radius, triple-focusing mass spectrometer: U-Th-Pb measurements of accessory minerals, and stable isotopic analysis of insulators.

U-Th-Pb analysis U-Th-Pb analysis is conducted using a beam of negative oxygen ions that can be focused to spots from 1 to 30 µm on 1" polished thin sections or grain mounts. A slit-lens transfer optical system and the 585mm radius magnet achieve high secondary ion transmission at the mass resolving power of ~5,000 required for these analyses. Isotopic ratios are measured by rapid peak-switching in to a pulse-counting electron multiplier. Measurements of U-Th-Pb systematics of zircon [e.g. X. Quidelleur et al., The thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet. J. Geophys. Res. 1997] and monazite [e.g., Harrison et al., Earth Planet. Sci. Lett. 133, 271, 1995] yield high Pb sensitivities (up to 20 cps/ppm/Pb/nA) and typical age accuracy of ±2%.

Oxygen and Carbon Stable Isotope Analysis Stable isotope measurements of oxygen and carbon in insulating specimens are undertaken using a Cs+ primary beam which can be focused to a diameter between 0.2 and 30µm. Carbonates and silicates, for example, have yielded typical delta 18O precisions of 1.0 per mil for 10 min analysis times and, typically, 15µm diameter analysis areas [e.g. L.A. Leshin et al., The oxygen isotopic composition of olivine and pyroxene from CI chondrites. Geochim. Cosmchim. Acta 1997 (in press)]. The same precision has been obtained for delta 13C in carbonates under similar analytical conditions.