Use of surface-controlled REE sectoral zoning in apatite from Llallagua, Bolivia, to determine a single-crystal Sm[sbnd]Nd age

Abstract

Surface-structure controlled partitioning of REEs between symmetrically nonequivalent crystal faces leads to sectoral zoning between the {0001} and {101̄0} sectors of vein-filling apatites from Llallagua, Bolivia. Absolute differences in the concentration of the analyzed REEs between these sectors range from a factor of 9.5 in the case of La to a factor of 1.4 for Yb. The two sectors have different concentrations of Sm and Nd as well as significantly different Sm/Nd ratios ( Δ147Sm/144Nd= 0.3558). A SmNd isochron determined from petrographically constrained coeval portions of different sectors gives an age of 43.8 ± 4.7 Ma (95% confidence limits). We believe that this calculated age represents the time of crystallization of the apatite.The application of intracrystalline heterogeneities, that formed due to differential partitioning at the crystal surface, to dating of minerals is germane for any mineral in which sectoral or intrasectoral zoning is exhibited, and may be used with other isotopic systems in which sufficient segregation of parent and daughter nuclides occurs and in which concentrations are sufficient for analysis of small intracrystalline samples.