Clues fromAb InitioCalculations on Titanium Isotopic Fractionation in Tholeiitic and Calc-Alkaline Magma Series

Abstract

Magmatic differentiation produces positive correlations between δ⁴⁹Ti and SiO<inf>2</inf>. The equilibrium Ti isotope fractionation factors of Ti-bearing minerals are essential for understanding the mechanisms driving this isotopic fractionation. We presentab initio-derived mean force constants of Ti-bearing minerals (barium orthotitanate, potassium titanium oxide, fresnoite, 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 δ⁴⁹Ti 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 δ⁴⁹Ti trend observed in tholeiiticversuscalc-alkaline magmas is most likely due to enhanced removal of Ti into sequestered minerals at low SiO<inf>2</inf>concentration in tholeiitic series compared to calc-alkaline series. The δ⁴⁹Ti-SiO<inf>2</inf>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.