INTRACRYSTAL VARIATION OF SR AND ND ISOTOPIC COMPOSITION OF APATITE IN GRANITOIDS: IMPLICATIONS FOR THE MAGMATIC COMPLEXITY OF ‘SIMPLE' GRANITES

Low rates of diffusion of Sr and REE in apatite allow apatite from deformed granites to potentially retain finer petrogenetic details, otherwise altered in the whole rock system. We present a new method to extract such complex petrogenetic information from apatite from a number of ~ 300 Ma granites. An aliquot of ~ 20 clear, inclusion-free apatite crystals was annealed at 300ºC for 3 hours, which repaired fission and alpha tracks. The grains were then treated with 3 M HNO₃ to dissolve rim material. Residual core material was dissolved separately. Sr, Nd and Sm were separated from the rim and core solutions and analyzed via TIMS for Sr and Nd isotopic composition. As an independent control, another aliquot 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), but were lower than that obtained from the rims which ranged from 0.70490 to 0.70501. In contrast, both chemical dissolution and physical abrasion of apatite from the Danburg pluton yielded cores with ⁸⁷Sr/⁸⁶Sr higher than those from apatite rims. Consistently higher core Sr ratios were also obtained from apatite from the Lexington pluton. LAICPMS trace element concentration measurements of a single apatite crystal from the Lexington pluton show compositional difference between core and rim, suggesting presence of chemical zonations similar to that observed by Lux et al (2007). Initial ¹⁴³Nd/¹⁴⁴Nd were also measured for the same rim and core solutions. While apatite from plutons such as Steven's Creek granite yielded relatively more juvenile core (0.512004) than rim (0.51108), apatite cores from Lexington granite were slightly more evolved (0.512190) than rim (0.512227). Other authors have interpreted similar complexities of Hf and O isotopic composition in zircon and Sr isotopic ratios in feldspar from volcanic rocks as reflecting syn-crystallization magma evolution. Very few studies document similar phenomenon in plutonic rocks. Our data suggests such syn-crystallization evolutionary complexities of magma might be a far more common phenomenon in plutonic rocks than previously thought.