TESTING THE LIMITS OF UNCONVENTIONAL U-PB GEOCHRONOMETERS WITH APPLICATIONS TO HIGHLY-FRACTIONATED MAGMATIC SYSTEMS

A variety of U-bearing minerals from well-dated, traceable, locations is been systematically triaged from archival collections at UNB Earth Sciences. The goal of this study is to identify potential natural reference materials for in situ (e.g. LA ICPMS) geochronology in peralkaline (miaskitic) igneous suites, evolved LCT and NYF pegmatites, kimberlites, uranium deposits, and carbonatites. Although some of these rock types can contain zircon, monazite, and apatite facilitating more routine dating methods, these geochronometers are sometimes inherited, xenocrystic, or altered by late-stage magmatic-hydrothermal fluids; ambiguous dates are a hallmark of these systems. In such cases alternative mineral groups such as complex oxides and fluoro-carbonates might be targeted and unambiguously related to crystallization from fractionated late-stage liquids or hydrothermal fluids. These include bastnaesite and parasite, the pyrochlore group, microlite, betafite, uraninite (and uranothorite), fergusonite, euxenite, zirconolite, and columbite-group-minerals. These minerals are normally considered ‘leaky’ to Pb-loss owing to high actinide concentrations and, thus, high amorphous fractions. Our approach has been to empirically test these minerals for isotopic homogeneity and robustness. The minerals are mounted and polished and subject to compositional mapping by micro-XRF and SEM-BSE imaging. The time-dependent behavior during ablation at different laser fluence and pulse rate is assessed, as is common-Pb content and optimal crater sizes for intrinsic U and Th concentrations. Once these conditions are established, the material is ‘dated’ using NIST610 glass as an external standard. This step establishes whether the material is isotopically homogeneous. In some Phanerozoic age minerals we have explored (e.g., uraninite and bastnaesite) standardization against NIST610 yields concordant data within error of the assumed age of the material. Demonstrating primary external standardization using NIST610 relieves us of finding an additional natural material to act as a secondary standard. Thus, in addition to uncovering a range of potential new reference materials, this study ultimately seeks to establish the full bounds of non-matrix-matched LA ICPMS U-Pb geochronology.