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

• Published in: Geosciences (Basel) 2022-06, Vol.12 (6), p.229
• Main Authors: Kowallis, Bart J., Christiansen, Eric H., Dorais, Michael J., Winkel, Anthony, Henze, Porter, Franzen, Lauren, Mosher, Haley
• Format: Article
• Language: English
• Subjects: aluminum; Calcium ions; Composition; Crystallization; Earth science; Eclogite; Fluids; fluorine; Grains; Iron; Metamorphic rocks; Ratios; Silica; sphene; Substitutes; Titanite; Titanium; Trends
• ISSN: 2076-3263; 2076-3263
• DOI: 10.3390/geosciences12060229

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) with few exceptions. Volcanic titanite grains can also be separated from plutonic titanite grains due to their shorter crystallization histories with compositions clustered more tightly in terms of Fe, Al, and F. Compositions of titanite from plutonic rocks often have later metamorphic or hydrothermal overgrowths that are not found on volcanic titanite. Fe/Al ratios in titanite from silica-undersaturated volcanic and plutonic rocks are typically > 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 < 1:2 except for a few metamorphic titanite grains that formed in mafic rocks. Titanite from ultrahigh pressure metamorphic rocks (eclogite facies) 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 lesser contribution from F−.