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CHARACTERIZATION OF FLUX-GROWN TRACE-ELEMENT-DOPED TITANITE USING THE HIGH-MASS-RESOLUTION ION MICROPROBE (SHRIMP–RG)

Frank K. Mazdab

U.S. Geological Survey – Stanford Ion Probe Laboratory, Stanford, California 94305, USA

Crystals of titanite can be readily grown under ambient pressure from a mixture of CaO, TiO₂ and SiO₂ in the presence of molten sodium tetraborate. The crystals produced are euhedral and prismatic, lustrous and transparent, and up to 5 mm in length. Titanite obtained by this method contains approximately 4300 ppm Na and 220 ppm B contributed from the flux. In addition to dopant-free material, titanite containing trace alkali and alkaline earth metals (K, Sr, Ba), transition metals (Sc, Cr, Ni, Y, Zr, Nb, Hf and Ta), rare-earth elements (REE), actinides (Th, U) and p-block elements (F, S, Cl, Ge, Sn and Pb) have been prepared using the same procedure. Back-scattered electron (BSE) imaging accompanied by ion-microprobe (SHRIMP–RG) analysis confirms significant incorporation of selected trace-elements at structural sites. Regardless of some zonation, the large size of the crystals and broad regions of chemical homogeneity make these crystals useful as experimental starting material, and as matrix-matched trace-element standards for a variety of microbeam analytical techniques where amorphous titanite glass, heterogeneous natural titanite or a non-titanite standard may be less than satisfactory. Trace-element-doped synthetic crystals can also provide a convenient proxy for a better understanding of trace-element incorporation in natural titanite. Comparisons with igneous, authigenic and high-temperature metasomatic titanite are examined. The use of high-mass-resolution SIMS also demonstrates the analytical challenges inherent to any in situ mass-spectrometry-based analysis of titanite, owing to the production of difficult-to-resolve molecular interferences. These interferences are dominated by Ca–Ca, Ca–Ti and Ti–Ti dimers that are significant in the mass range of 80–100, affecting all isotopes of Sr and Zr, as well as ⁸⁹Y and ⁹³Nb. Methods do exist for the evaluation of interferences by these dimers and of polyatomic interferences on the LREE.

Keywords: titanite, "sphene", ion microprobe (SHRIMP–RG), trace elements, flux-growth synthesis.