USE OF SURFACE-CONTROLLED REE SECTORAL ZONING IN APATITE FROM LLALLAGUA, BOLIVIA, TO DETERMINE A SINGLE-CRYSTAL SM-ND AGE

Surface-structure controlled partitioning of REEs between symmetricall y nonequivalent crystal faces leads to sectoral zoning between the {00 01} and {10 (1) over bar 0} sectors of vein-filling apatites from Llal lagua, Bolivia. Absolute differences in the concentration of the analy zed 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 concen trations of Sm and Nd as well as significantly different Sm/Nd ratios (Delta(147)Sm/Nd-144 = 0.3558). A Sm-Nd isochron determined from petro graphically constrained coeval portions of different sectors gives an age of 43.8 +/- 4.7 Ma (95% confidence limits). We believe that this c alculated age represents the time of crystallization of the apatite. T he application of intracrystalline heterogeneities, that formed due to differential partitioning at the crystal surface, to dating of minera ls is germane for any mineral in which sectoral or intrasectoral zonin g is exhibited, and may be used with other isotopic systems in which s ufficient segregation of parent and daughter nuclides occurs and in wh ich concentrations rue sufficient for analysis of small intracrystalli ne samples.