Element and Sr isotope signatures of titanite as indicator of variable fluid composition in hydrated eclogite

The trace element signature (Sr, common Pb, U, Th, REE, and HFSE) of titanite gives important information about the formation process of metamorphic titanite. We investigated titanite from different textural positions of a strongly retrogressed eclogite from the Tromsø (Norway) high-P area. It shows characteristic titanite rims around partially absorbed rutile, interpreted as grown during retrogression and hydration of the high-P assemblage. The titanite rims around rutile are polycrystalline and in contact with the plagioclase–amphibole matrix. Large idioblastic single crystal titanite grew in veins without relics of precursor rutile. Compositional zoning of Al–Ti–F in titanite of both textural positions is irregular and not related to core or rim position in the crystals. There is no evidence for systematic growth sequences or diffusion reflecting changing P–T-conditions or systematic changes of fluid composition. Trace element patterns of titanite (LA-ICP-MS data), however, show systematic variations in the coupling and decoupling behaviour of strongly fluid-mobile (Sr, U), moderately fluid-mobile (REE), and little fluid-mobile (Nb, Ta, Zr, Hf) elements. The trace element signatures of titanite from the two different fabric positions reflect contrasting characteristic fluid compositions: Single crystal titanite equilibrated with fluid that was channelled in the vein. Polycrystalline titanite on rutile records the composition of a fluid that had migrated along grain boundaries in the amphibole–plagioclase matrix. Fluid from the matrix also contributed to the geochemical signature of vein titanite, whereas influence of fluid from the vein on the composition of polycrystalline titanite in the matrix is not evident. 87Sr/86Sr isotope ratios of the different titanite types support the inference from trace elements of spatially and temporally variable fluid sources.