230Th isotope measurements of femtogram quantities for U-series dating using multi ion counting (MIC) MC-ICPMS

Results of Th isotope measurements on Harwell Uraninite (HU) solution aliquots and carbonates such as corals and speleothems using multi ion counter (MIC) procedures are presented. A multi ion counter array available for the ThermoFinnigan Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) is used for simultaneous measurements of 230Th and 229Th ion beams. A static collection of low-level ion beams improves the efficiency of isotope measurements which in turn reduces the required sample size and measurement time. The static measurement also circumvents problems with ion beam intensity fluctuations often observed with a plasma source. Instrumental bias corrections are based on a sample bracketing in-house Th standard solution. The accuracy and precision of the MIC array procedures for 230Th/229Th measurements are investigated by comparison to a MC-ICPMS peak jump procedure using just a single ion counting channel. The results show that similar precisions for 230Th/229Th measurements can be achieved using the MIC procedure with less than half total 230Th consumed compared to a peak jump routine. Thus, the MIC setup allows to use less than half the sample sizes for 230Th/U dating of carbonates compared to peak jump methods enabling higher spatial resolution sampling.