Nb-, Zr- and LREE-rich titanite from the ALnö alkaline complex: Crystal chemistry and its importance as a petrogenetic indicator
2005 (English)In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 83, no 1-2, 128-142 p.Article in journal (Refereed) Published
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) 2Ca→Na++LREE3+, (ii) Ca2++Ti4+→LREE3++(Al, Fe3+), (iii) 2Ti4+→M5++(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.
Place, publisher, year, edition, pages
Elsevier B.V. , 2005. Vol. 83, no 1-2, 128-142 p.
Alnö; Titanite; Niobium; Zirconium; LREE; Silica activity
Earth and Related Environmental Sciences
IdentifiersURN: urn:nbn:se:su:diva-23914DOI: 10.1016/j.lithos.2005.01.008OAI: oai:DiVA.org:su-23914DiVA: diva2:195301
Part of urn:nbn:se:su:diva-5462005-05-262005-05-262010-12-08Bibliographically approved