TRACE-ELEMENT INCORPORATION IN TITANITE: CONSTRAINTS FROM EXPERIMENTALLY DETERMINED SOLID/LIQUID PARTITION COEFFICIENTS

Abstract

Titanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control.The exchange vectors Na1Ca−1 and Al1Ti−1 allow local charge balance to be achieved after incorporation of REE3+ and R5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard [the exchange mechanism being (OH)1O−1 at the O1 site]. The incorporation of U4+ and Th4+ at the Ca site is scarce, and is likely balanced by that of Mg2+ at the Ti site; both these substitutions are responsible for strong structural strain.Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U–Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na+ and Mg2+.