Nb-, Zr- and LREE-rich titanite from the Alnö alkaline complex: Crystal chemistry and its importance as a petrogenetic indicator

Titanite occurs as an accessory phase in melteigite, ijolite and nepheline syenite from the Alnö alkaline intrusion and is typically associated with Ti-andradite and calcite±perovskite. Titanite from 7 samples was investigated using microanalytical techniques including EMPA, 57Fe Mössbauer spectroscopy, powder X-ray diffractometry and FTIR-spectroscopy. A common feature of the analysed titanite is high amounts of Nb2O5 (up to 7.3 wt.%), ZrO2 (up to 2.8 wt.%) and LREE2O3 (up to 3.9 wt.%), and significant amounts of Al2O3+Fe2O3 (up to 4.3 wt.%). Important substitutions are consequently (i) 2[7]Ca→[7]Na++[7]LREE3+, (ii) [7]Ca2++[6]Ti4+→[7]LREE3++[6](Al, Fe3+), (iii) 2[6]Ti4+→[6]M5++[6](Al, Fe3+). Zr is most likely accommodated into the structure through simple direct isovalent octahedral substitution for Ti, contrasting with other proposed, more complex models for Zr-incorporation in titanite. Stoichiometric calculations indicate all iron to be present as Fe3+ but 57Fe Mössbauer analyses show a small fraction (10%) of ferrous iron to be present as well. FTIR-spectroscopy indicate 0.14-0.26 wt.% H2O in the investigated titanites and X-ray diffraction data yield cell parameters close to the ideal synthetic end-member values. Chondrite normalised LREE patterns for the analysed titanites show (Ce+Pr)N>LaN similar to patterns reported from other occurrences and enrichment factors are typically in the order of 103-104. Comparison of whole rock Nb-, Zr- and LREE-contents with variations in modal amounts of trace element enriched titanite shows a significant effect which needs to be addressed when modeling magma evolution. The use of the prv-ttn assemblage for estimating silica-activity during crystallization of silica undersaturated alkaline rocks is discussed.