Partial Dissolution after Thermal Annealing: Towards a New Approach of Measurement of Sr Isotopic Composition In Apatites

Sr-isotopes play a key role in studies of granite petrogenesis. Determining accurate initial ⁸⁷Sr/⁸⁶Sr from whole-rock isotopic analysis requires that the system has remained closed, and that no post-crystallization mobilization of Sr and Rb has occurred. In most cases, however, micro-textural analysis suggests otherwise. We present a new method to help circumvent such problems. Our approach utilizes apatite from a series of ~ 300 Ma granites. An aliquot of ~ 20 clear, inclusion-free apatite crystals were annealed at 300ºC for 3 hours which annealed fission and alpha tracks without disturbing the Sr concentration profile within the grains. The grains were treated with 3 M HNO3 to dissolve rim material. The residue cores were dissolved separately. Sr was separated from the acid-leach and from the dissolved core and analyzed via TIMS for ⁸⁷Sr/⁸⁶Sr. As an independent control another batch of apatite from the same rocks were physically abraded to remove rims, allowing apatite cores to be analyzed for their Sr isotopic composition. ⁸⁷Sr/⁸⁶Sr ratios of apatite cores from the Harbison Granite derived by chemical-dissolution and by physical-abrasion were similar (0.70473 and 0.70477 respectively) and were lower than that obtained from the rims which ranged from 0.70490 to 0.70501. However, both chemical dissolution and physical abrasion of apatites from Danburg pluton yielded cores with ⁸⁷Sr/⁸⁶Sr higher than those from apatite rims. Consistently higher core ratios were also obtained from acid-treated apatite from the Lexington Pluton. LAICPMS trace element concentration measurements of a single apatite crystal from Lexington Pluton show differences in the chemical compositions of core and rim. Similar differences in the Hf-isotopic ratios between the core and rim of zircon (Kemp et al 2007) and Sr-isotopic ratios of feldspar core-rims (Davidson et al 2007) have been interpreted as reflecting syn-crystallization magma evolution. Our data suggests similar syn-crystallization evolutionary complexities of magma.