U-Pb systematics and trace element characteristics in titanite from a high-pressure mafic granulite

Along the outer ~ 50-100 μm, U, Th, Zr, and REE concentrations are variably elevated relative to the crystal interiors, with profiles taking one of two forms: 1) sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge, or 2) gradually increasing to highest concentrations nearest the crystal edge. High-contrast BSE imaging shows that the former profiles are associated with ~ 1-2 μm wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is (or formerly was) in contact with matrix feldspar (i.e. crystallized melt), and are inferred to represent trace-element-enriched dissolution-precipitation reaction fronts. The latter profiles are associated with diffuse, locally wispy, brightness gradients adjacent to resorbed crystal boundaries, subgrain boundaries, or thicker bright bands formed in contact with matrix hornblende crystals, and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro-porosity pathways at some point in the recrystallization process.