Compositional Variation of Fe, Al, and F in Titanite (Sphene) (original) (raw)
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Variation of Fe, Al, and F Substitution in Titanite (Sphene)
Geosciences
Titanite is an important mineral in petrochronology studies. Understanding chemical signatures of titanite from different environments can provide significant data in unraveling the complex histories recorded in their textures and compositions. Using a database of over 8500 titanite analyses from both the literature (3829) and our own data (4900), we found that the ratio of Fe/Al is useful for separating igneous titanite (Fe/Al is typically close to 1:1 and almost always > 1:2) from metamorphic titanite (Fe/Al ratio is < 1:2) with few exceptions. Volcanic titanite grains can also be separated from plutonic titanite grains due to their shorter crystallization histories with compositions clustered more tightly in terms of Fe, Al, and F. Compositions of titanite from plutonic rocks often have later metamorphic or hydrothermal overgrowths that are not found on volcanic titanite. Fe/Al ratios in titanite from silica-undersaturated volcanic and plutonic rocks are typically > 1:2 ...
Lithos, 2011
ABSTRACT 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.