THERMOCHRONOLOGICAL AND ISOTOPIC CHARACTERIZATION OF SINGLE APATITE CRYSTALS: FROM MAGMA SOURCE TO EXHUMATION

Sediment sources are commonly characterized to better our understanding of the evolution of surficial and crustal processes. Various geochemical and thermochronological techniques provide information on changes in source terrains, exhumation rates, and orogenic evolution. Although single-grain dating of accessory minerals (e.g. U-Pb, fission track, and (U-Th)/He systems), is now common, multiple characterization of single crystals is advantageous as it provides significantly more information about the history of the mineral. Here, we present the development and its associated obstacles of the double characterization of single apatite crystals, specifically Durango apatite, using both the ⁸⁷Sr/⁸⁶Sr isotopic composition and the (U-Th)/He date. These two isotopic signatures provide independent characterization of the composition of the magma and the low-temperature exhumation of history of the apatite host-rock. Isolation of Sr and U + Th + Sm was achieved following He extraction from the apatite. The average (U-Th)/He age obtained was 31.5 ± 5.7 Ma (2SD), which is within published values. The ⁸⁷Sr/⁸⁶Sr composition of undegassed Durango apatite was 0.70634 ± 0.00001 (2SE; N=18), consistent with published high-precision analyses from other laboratories. However, Durango apatite ⁸⁷Sr/⁸⁶Sr composition after He degassing is more radiogenic, with values ranging from 0.70634 to 0.70977 (N = 18). Evidently, the Sr isotopic composition of apatite is affected by the degassing stage. To find the source of excess of radiogenic Sr we performed a variety of tests: using both Nb and Pt tubes during degassing; extracting the apatite from the tubes prior to dissolution (to evaluate if the tubes themselves were the source); and using degassed quartz to test for a Sr coating following degassing. Despite these tests we have not yet identified the source(s) of excess radiogenic Sr. Given the potential utility of dual characterization of apatite for a better understanding of orogenic evolution, however, warrants further work to overcome the Sr isotope modification.