LA-ICP-MS apatite fission track dating: A practical zeta-based approach

The LA-ICP-MS method is becoming increasingly popular for uranium determinations in fission track dating of apatite, zircon or titanite. This is because the approach has several advantages over the classical external detector method (EDM), including faster sample throughput, simultaneous acquisition of additional data (such as U-Pb age information and trace element abundances), while removing the need for neutron irradiation. Two different approaches are used to determine U contents in LA-ICP-MS fission track dating: an absolute dating approach, or a zeta-based determination analogous to the classical EDM. Absolute age dating by LA-ICP-MS potentially suffers from small but systematic deviations in apatite U contents, which in turn propagate through to minor systematic deviations in the accuracy of absolute fission track age determinations. A zeta-based approach typically requires time-consuming counting of large numbers of zeta-standard grains (usually Durango apatite) so as to yield a precise zeta factor for every LA-ICP-MS session containing unknowns. The modification of the zeta-based approach proposed here has two major advantages. Firstly, it employs just one large primary LA-ICP-MS session to determine a precise primary zeta factor on a large number of counted Durango primary zeta grains. During subsequent secondary LA-ICP-MS sessions with unknowns, no further fission track counting of the primary zeta standard is required. This is because we reanalyse a subset of the primary zeta grains to calculate a session-specific zeta fractionation factor, which is related to variations in the instrumental operating conditions (primarily plasma tuning) between primary and secondary LA-ICP-MS sessions. This enables us to 'reuse' the primary zeta factor, and thus avail of its precision derived from the large spontaneous track count. The second advantage is that reusing the primary zeta grains by applying a session-specific zeta fractionation factor allows us to verify that background and drift corrections applied during the secondary LA-ICP-MS session were fully appropriate. This method has been successfully tested by dating samples of known apatite fission track age, by comparing EDM and LA-ICP-MS data from the same sample and by participating in a round robin test between international fission track laboratories where 'blind' fission track dating of two unknown samples was undertaken. Our LA-ICP-MS apatite fission track dating approach is also easily modifiable for fission track dating of zircon or titanite if suitable age standards are employed.