Minimizing Variable Downhole Fractionation in U–Pb Zircon Geochronology by LA-MC-ICP-MS at Smaller Spot Size

Abstract

The development of LA-ICPMS and LA-MC-ICPMS resulted in analytical methods for zircon geochronology using a 20- to 60-μm laser spot size. A high amount of complexly zoned zircons promotes the requirement of U–Pb dating at smaller spot sizes. When spot size reduces, downhole fractionation (DHF) increases, increasing the DHF discrepancy between zircon grains and the primary reference zircon standard and resulting in inaccurate results. With MC-ICPMS's high sensitivity and multi-collector capabilities, this work attempts to accurately determine age with spatial resolutions below 20 μm. Three well-characterized zircon standards (91500, GJ-1, and Plešovice) were tested at spot sizes of 35, 20, 15, and 10 μm. Laser and mass spectrometry tuning, laser shot count, and ablation time have been optimized to reduce the DHF effect on measurement accuracy. Static ablation of 35- to 15-μm spots with 150 laser shot counts (30 s) yielded precision of less than 1.5% and age offset of less than 2%. The DHF differs significantly from the reference standard and two test zircon samples used for validation, with an age offset of 4% at 10-μm spot sizes. Masking shot counts from the end enhanced accuracy, notably for lower laser shot counts and shorter ablation times. At 75 laser shot counts, precision reached 1.4% and age offsets reduced to 1.6% for ²⁰⁶Pb/²³⁸U age. This method minimizes laser shot counts to avoid sampling two age groups. Higher zircon DHF variability at smaller spot sizes may influence a well-calibrated, sensitive LA-MC-ICP-MS analytical figure of merit.