SYNTHESIS OF (TH, PB)- AND (U, TH, PB)-BEARING LA-PHOSPHATE: METHODOLOGY, CRYSTAL-CHEMICAL CONSIDERATIONS, AND APPLICATION AS ELECTRON MICROPROBE FICTIVE AGE STANDARDS

Electron microprobe (EMP) dates result from a concentration-time unit conversion, so use of a concentration-based (rather than isotope-ratio based) fictive age standard is warranted. This observation has motivated our mineral synthesis program, aimed at producing actinide-doped REE phosphate EMP dating standards that meet the following criteria: 1) known concentrations of U, Th, and Pb; 2) homogeneous intragrain distribution of all components; 3) of suitable size, either as a single-crystal or polycrystalline sintered ceramic.

Single-crystal synthesis of actinide-doped LaPO4 by flux-growth methods results in disproportionation of lanthanide and flux+alkali+actinide components into phosphate and oxide phases, respectively, and flux-growth methods were abandoned. Actinide-doped LaPO4 is successfully prepared by high-T annealing and hydrothermal processing of microcrystalline phosphate; both homogeneity and charge-balance of (Ca, Th, Pb)-bearing LaPO4 increase with increasing solvent acidity during cold-seal hydrothermal synthesis. A combination of pressing and high-T (~1400°C) sintering transforms fine-grained (0.1-10 mm) run-products to ceramic pellets with 90-95% theoretical density.

Our most recent results focused on a target composition of La80(CaTh)17(CaU)2(PbTh)1PO4 processed with 6% 2M HCl at 820°C, 0.75 kbar for 1 week. The run products are 0.1-2 mm crystals identified by XRD as La-actinide phosphate solid solution. ~2 mm grains (N=16) give a composition (mean ±2 sd) of La79.77±1.26(CaTh)17.87±1.00(CaU)1.53±0.42(PbTh)0.82±0.09PO4. Th (8.07-9.13 wt %) is homogeneous at the level of analytical precision, and the Pb concentration range (3500-4350 ppm) is restricted. Uranium concentration values are more variable (6500-10000 ppm); this apparent variability may be in part due variable UO3/UO2, as indicated by residual U cation charge diagrams. This run yields a fictive age of 702±4 Ma (mean±2 se), compared to the fictive age of 794 Ma for the target composition.