Variation of Fe, Al, and F Substitution in Titanite (Sphene)

Abstract

Titanite is an important mineral in petrochronology studies. Understanding chemical signatures of titanite from different environments can provide significant data in unraveling the complex histories recorded in their textures and compositions. Using a database of over 8500 titanite analyses from both the literature (3829) and our own data (4900), we found that the ratio of Fe/Al is useful for separating igneous titanite (Fe/Al is typically close to 1:1 and almost always > 1:2) from metamorphic titanite (Fe/Al ratio is 1:2 and include titanite with the highest Fe/Al ratios. Although they overlap the field for normal igneous 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 +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 lesser contribution from F−.

Keywords

• titanite
• sphene
• composition
• iron
• aluminum
• fluorine