Abstract
Recent work on spinel-facies mantle assemblages has shown that Amph/CpXDTi (a) are always higher than 1; (b) may vary in the range 2.7-8.9, and (c) are significantly higher than those of REEs and Zr (Vannucci et al. , 1995). Therefore, in amphibolebearing systems Ti can strongly fractionate relative to REEs and Zr, with important implications for the modelling of geochemical processes. The behaviour of Ti 4§ in amphibole is rather complex, and has recently been the subject of a number of systematic studies based on complete (EMP+SIMS) chemical characterization and X-ray single-crystal structure-refinement (SREF) of calcic, sodic-calcic and sodic amphiboles (Oberti e t al., 1992, Hawthorne et al. , 1993,1998). These studies have shown that Ti 4§ can enter the amphibole structure according to three different crystalchemical mechanisms: (a) T2Ti4+ T2Si4+ 1; (b) M1Ti4+ O302MI(Mg,Fe)2_~ ~ (c) M2'M3Ti4+
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