U-Pb dating of sub-ng/g U garnet by LA-MC-ICPMS

Abstract. Advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have largely focused on improving spatial resolution through progressively smaller laser spot sizes. Here, we explore the opposite end of the analytical limits by investigating the lower concentration limits of in-situ U–Pb geochronology in garnet. Using a Neptune Plus multi-collector ICP-MS equipped with seven ion counters, we developed an analytical workflow specifically designed for metamorphic garnet with ultra-low U concentrations. The method was used to date garnet from a wide range of ages and geological settings, including granulites, eclogites and hydrothermal demantoids. Uranium concentrations were exceptionally low, even below 1 ng/g in some cases. At these concentrations, the total amount of uranium ablated during a single analysis is at femtogram levels, more than three orders of magnitude lower than that of a typical in-situ analysis of zircon. Despite these extremely low signal intensities, geologically meaningful ages were obtained for the analysed samples. Analytical precision depends on the U and radiogenic Pb concentrations, but we have obtained precisions of ca. 5–6 % for garnet containing less than 1 ng/g U. In garnet with higher U concentrations or ages as old as the Archean, internal precision of ca. 1 % may be achieved. These results expand the applicability of in-situ garnet U-Pb geochronology to the vast majority of metamorphic garnet, providing a powerful new tool for constraining garnet growth, prograde metamorphism, and deep crustal evolution.