Oscillatory Zoning of La- and Y- doped Titanite During Thermal Cycling Experiments

Titanite is an important accessory mineral in silicic igneous rocks because it can hold a large proportion of the rare earth element (REE) budget compared to its modal abundance. It typically exhibits strong oscillatory zoning, suggesting variable thermal conditions or open-system addition during crystallization. We performed a set of thermal cycling experiments on La- and Y- (proxy for MREE) doped titanite in a basaltic melt and observed oscillatory zoned rims forming on preexisting, low-REE titanite seeds. Thermal cycling promotes crystal coarsening through precipitation-dissolution processes which may contribute to phenocrystic textures commonly seen in igneous rocks. Using a 1-atmosphere gas-mixing furnace, experiments were cycled ±10°C from 1220°C with a 96-minute period for 1-60 cycles. In these experiments, the number of oscillatory zones in titanite rims equaled the number of thermal cycles. Rims are typically 4-5 mm in wavelength and are euhedral compared to the core anhedral seeds. Oscillatory zones exist within sectored zones of titanite and consist of a bright and dark section determined by backscattered electron images. REE crystal/glass partition coefficients (D-values) determined by FE-microprobe reached 3 in the titanite rims and varied by 0.4 wt% across zones. D-values for Y are 2 orders of magnitude lower in experimentally grown titanite than in natural igneous titanite from silica-rich glass. Titanite incorporates large amounts (103-104 times chondrite) of REE by the coupled substitutions of Ca2+ + Ti4+ = (REE, Y)3+ + (Fe, Al) 3+. D-values negatively correlate with temperature and positively correlate with SiO2 glass content. Thus, high run temperatures and silica-poor glass ( 40 wt%) likely diminish experimental D-values. Although thermal cycling does not appear to increase REE uptake in the titanite-basalt system, it does correlate to the oscillatory zoning patterns common in igneous titanite.