EPSL ELSEVIER Earth and Planetary Science Letters 146 (1997) 329-336 Use of surface-controlled REE sectoral zoning in apatite from Llallagua, Bolivia, to determine a single-crystal Sm-Nd age John Rakovan *, Diane K. McDaniel, Richard J. Reeder zyxwvutsrqponmlkjihgfed Department of Earth and Space Sciences, State Unioersity of New York, Stony Brook, NY 11794-2100, USA Received 1 July 1996; revised 19 September 1996; accepted 14 October 1996 zyxwvutsrqponmlkjihgfedcbaZYXW Abstract Surface-structure controlled partitioning of REEs between symmetrically nonequivalent crystal faces leads to sectoral zoning between the (0001) and {lOiO} 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 ( A147Sm/ l”Nd = 0.3558). A Sm-Nd 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. Keywords: absolute age; Sm/Nd; apatite; Llallagua, Bolivia; crystal zoning 1. Introduction Apatite [Ca,(PO,),(F,OH)] crystals from Llal- lagua, Bolivia, show a differential distribution of rare earth elements (REEs) between symmetrically nonequivalent sectors (regions of a single crystal that have grown by incorporation on specific crystal faces). The differences in trace element concentra- tion between the sectors are extreme, reaching al- most an order of magnitude in the case of La. We present a novel application of this sectoral zoning by determining the age of the apatite using different intracrystalline Sm and Nd concentrations and ratios from these sectors. We also propose a detailed mech- anism for the observed sectoral zoning. Samples used to obtain an isochron are taken from portions of different sectors that can be shown to have formed at the same time. Hence, this technique unambiguously constrains the points on the isochron to be time equivalent. * Corresponding author’s current address: Department of Geo- logical Sciences, Virginia Polytechnic Institute and State Univer- sity, Blacksburg, VA 24061 USA. E-mail: rakovan@vt.edu Differential incorporation of REEs between sym- metrically nonequivalent growth steps on the zyxwvutsrqponm {loio) face of apatite leads to compositional heterogeneities 0012-821X/97/$12.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO O l2- 821X(96)00226- 9