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Nb-, Zr- and LREE-rich titanite from the ALnö alkaline complex: Crystal chemistry and its importance as a petrogenetic indicator
Stockholm University, Faculty of Science, Department of Geological Sciences.
2005 (English)In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 83, no 1-2, 128-142 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier B.V. , 2005. Vol. 83, no 1-2, 128-142 p.
Keyword [en]
Alnö; Titanite; Niobium; Zirconium; LREE; Silica activity
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:su:diva-23914DOI: 10.1016/j.lithos.2005.01.008OAI: diva2:195301
Part of urn:nbn:se:su:diva-546Available from: 2005-05-26 Created: 2005-05-26 Last updated: 2010-12-08Bibliographically approved
In thesis
1. The Alnö alkaline and carbonatitic complex, east central Sweden - a petrogenetic study
Open this publication in new window or tab >>The Alnö alkaline and carbonatitic complex, east central Sweden - a petrogenetic study
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Alnö complex on the central Swedish east coast is composed of a main composite intrusion (the main intrusion) and four smaller satellite intrusions (Söråker, Sälskär, Långharsholmen and Båräng) distributed around the main intrusion on Alnö Island and on the mainland north of the island. The major rock types exposed within the complex are melilitolite, pyroxenite, ijolite series (melteigite-ijolite-urtite), nepheline syenite, carbonatite and alnöite dykes. Melilitolite is only exposed within the Söråker intrusion. The intrusive sequence is melilitolite → pyroxenite → ijolite series → nepheline syenite → carbonatite → alnöite.

Mineralogical, whole rock geochemical and radiogenic isotope (Nd-Sr-Pb) studies of exposed rocks from the Alnö alkaline complex, east central Sweden, were performed in order to investigate the genetic relationships between the diverse rock-types, and to evaluate the contributions from mantle and crustal components in the genesis of the complex. Most analysed samples fall within the depleted quadrant in a eNd-eSr diagram, similar to carbonatites and alkaline silicate rocks from other complexes, indicating derivation of parental magma(s) from a source that had experienced time-integrated depletion in LIL elements. Contamination by local crust is indicated by Sr and Pb isotope data, but is geographically restricted to samples collected from the outer parts of the main intrusion and from satellite intrusions. This localized contamination is attributed to selective hydrothermal element leaching of surrounding bedrock during fenitization. Nd- and Sr-isotope data separates the carbonatites into two groups (group I and II), each related to a specific set of silicate rock types. The overlap of group II carbonatites with ijolite and nepheline syenite could indicate a common origin through liquid immiscibility but this hypothesis cannot be confirmed by trace element data because initial concentrations are obscured by fractionation processes. Interestingly, results from AFC-modelling suggest that production of ijolite residual magma requires addition of a small volume (2.4 %) of carbonatite component to the parental magma, whereas formation of nepheline syenite residuals requires removal of an almost equal amount of carbonatite (1.5 %) to yield a statistically significant result. AFC-modelling further suggests that the various silicate rock types exposed within the complex are related to the same parental olivine-melilitite magma through crystal fractionation of olivine, melilite, clinopyroxene, nepheline, Ti-andradite and minor phases. These results agree with compositional trends exhibited by clinopyroxene and Ti-andradite from the silicate rocks of the main intrusion, which suggests co-genesis of pyroxenite, ijolite series rocks and nepheline syenite. Production of ijolite-like residual liquids can be achieved by <40% fractionation whereas production of nepheline syenite residuals requires >80% fractionation.

An investigation of the origin of silicate minerals in carbonatites suggest that most silicate minerals observed in the carbonatites on Alnö Island are derived from surrounding wall-rock and/or produced through corrosive interaction between carbonatite liquid and assimilated phases. This leads to ambiguities when addressing the possible genetic link between carbonatites and associated silicate rocks as occurrences of identical “liquidus” phases in inferred immiscible liquids may not actually be such.

Place, publisher, year, edition, pages
Stockholm: Institutionen för geologi och geokemi, 2005. 130 p.
Meddelanden från Stockholms universitets institution för geologi och geokemi, ISSN 1101-1599 ; 324
carbonatite, alkaline, Alnö, geochemistry, isotope
National Category
Earth and Related Environmental Sciences
urn:nbn:se:su:diva-546 (URN)91-7155-097-6 (ISBN)
Public defence
2005-06-17, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Available from: 2005-05-26 Created: 2005-05-26Bibliographically approved

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