Zircon 4He/3He fractional loss step-heating and characterization of parent nuclide distribution

Abstract

The radiogenic ingrowth and diffusive loss of 4He in accessory minerals is used to determine the timing of low-temperature thermal processes such as shallow tectonic, magmatic, hydrothermal, and geomorphic events. While the thermal history experienced by a sample cannot be directly constrained by a single (UTh)/He total gas age, 4He concentration profiles, normalized against irradiation-induced spallogenic 3He, have been shown to reliably recover continuous thermal history constraints if the parent nuclide distribution is either spatially uniform or can be determined. As such, 4He/3He thermochronometry has been successfully applied to apatite. However, individual zircon grains can contain complex parent nuclide zonation. Using moderately zoned, euhedral zircon grains from the rapidly cooled, distal Fish Canyon Tuff, we investigated the potential of the zircon 4He/3He thermochronometer through combining 4He/3He diode laser fractional loss step-heating with detailed analysis of parent element zonation. We present a routine method for the quantitative characterization of parent nuclide zonation by LA-ICP-MS depth-profiling. This study also demonstrates the importance of thermal control and avoidance of intra-sample thermal gradients during laser step-heating fractional loss experiments. Accurate and stable thermal control was achieved with an optimized sample-holder design, laser defocusing, and PID control loops to minimize temperature overshoot, failure to reach set-point, and temperature drop-offs. These analytical developments illustrate both the pitfalls and promise of zircon 4He/3He thermochronometry and create a reproducible methodology for 4He/3He thermochronometric applications to zoned and slowly cooled zircon.