COMPOSITIONAL VARIATION OF FE, AL, AND F IN TITANITE (SPHENE)

Titanite has become an important mineral in petrochronology studies. Understanding the chemical signatures of titanite from different environments can provide significant help in unraveling the complex histories observed in many titanite grains. We have compiled a database of over 7000 titanite analyses from the literature and supplemented it with our own data. We find that the ratio of Fe/Al is useful for separating volcanic and plutonic titanite (Fe/Al typically close to 1:1 and almost always >1:2) from metamorphic titanite (Fe/Al ratio is <1:2) with a few exceptions. Volcanic titanites can also be separated from plutonic titanites due to their shorter crystallization histories causing them to cluster more tightly in terms of their Fe, Al, and F. Compositions of titanite from plutonic rocks are typically more scattered than volcanic titanite suggesting longer histories of crystallization and, in many cases, later metamorphic or hydrothermal overgrowths that are not found on volcanic titanite. Fe/Al ratios in titanites from silica-undersaturated volcanic and plutonic rocks (phonolites, nepheline syenites, ijolites, etc.) are typically >1:2 and include titanite with the highest Fe/Al values. Although they overlap the field for normal volcanic and plutonic titanite, other elements (particularly high levels of Nb and low levels of Y) allow them to be separated. In most metamorphic rocks the Fe/Al ratio is <1:2 with the exception of a few metamorphic titanites that formed in mafic rocks, such as those from metabasalts or metabasites. Titanite from high pressure metamorphic rocks (eclogites, blue schists, etc.) tend to have the lowest Fe/Al ratios, typically <1:8. Titanite from hydrothermal and pegmatitic environments scatter widely in terms of Fe/Al even within single grains due to crystallization from fluids with highly variable compositions. Charge balancing in metamorphic, hydrothermal, and pegmatitic titanite due to Fe⁺³ and Al⁺³ substitution into the Ti⁺⁴ site is largely accomplished by the coupled substitution of F^- for O^-2. However, in volcanic and plutonic titanite the charge imbalance due to Fe⁺³ and Al⁺³ substitution appears to be mainly coupled with REE⁺³ or Y⁺³ substitution into the Ca⁺² site with a more minor contribution from F^-.