Clues From Ab Initio Calculations On Titanium Isotopic Fractionation In Tholeiitic And Calc-Alkaline Magma Series

Magmatic differentiation produces positive correlations between delta Ti-49 and SiO2. The equilibrium Ti isotope fractionation factors of Ti-bearing minerals are essential for understanding the mechanisms driving this isotopic fractionation. We present ab initio-derived mean force constants of Ti-bearing minerals (barium orthotitanate, potassium titanium oxide, fi-esnoite, diopside, geikielite, karrooite, titanite, pseudobrookite, anatase, and titanium oxide) based on density functional theory (DFT) to calculate equilibrium isotopic fractionation factors. We find that the main driver for Ti isotopic fractionation is its coordination, with four-, five-, and sixfold-coordinated Ti characterized by mean force constants of 547, 462, and 310 N/m, respectively. The coordination number of Ti in silicate melts is thought to be lower than in minerals, driving magmas toward higher delta Ti-49 values by fractional crystallization. The mineral-melt fractionation factors allow modeling of the observed Ti isotope trends in tholeiitic and calc-alkaline rocks. Our model results indicate that to first order, the steeper delta Ti-49 trend observed in tholeiitic versus calc-alkaline magmas is most likely due to enhanced removal of Ti into sequestered minerals at low SiO2 concentration in tholeiitic series compared to calc-alkaline series. The delta Ti-49-SiO2 differentiation trends, however, depend on Ti coordination in the melt and the strengths of Ti bonds in diverse Fe- and Ti-oxides, which are still uncertain. Our results show that Ti isotopes can be used to reconstruct the crystallization history and identify the magmatic series parentage of magmas that otherwise lack context, but further work is needed to identify the drivers behind Ti isotopic fractionation in igneous rocks.