Clues from Ab Initio Calculations on Titanium Isotopic Fractionation in Tholeiitic and Calc-Alkaline Magma Series

Abstract

Magmatic differentiation produces positive correlations between  49 Ti 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, fresnoite, diopside, geikielite, karooite, 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 4-, 5-, and 6-fold 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 towards higher  49 Ti 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  49 Ti trend observed in tholeiitic vs. 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  49 Ti-SiO2 differentiation trends however depend on Ti coordination in the melt and the strengths of Ti bonds in diverse Fe,Ti-oxides, which are still uncertain. Our resultsshow 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.

Redox-dependent Ti stable isotope fractionation on the Moon: implications for current lunar magma ocean models

“…The ß clinopyroxene of 1.12 ± 0.04 ‰ from our study is within the range of other clinopyroxene data from ab initio modelling (see Supplementary Table 6), which range from 0.92 to 1.74 ‰ (Leitzke et al 2018;Wang et al 2020;Aarons et al 2021). The same applies for our ß orthopyroxene of 1.39 ± 0.04 ‰, which overlaps with the range of 0.92 to 1.53 ‰ by other studies (Wang et al 2020).…”

“…Overall, our Δ 49 Ti cpx-melt decreases from + 0.146 ± 0.033 ‰ at FMQ -0.5 to -0.035 ± 0.013 ‰ at IW-1 at 1100 °C. Thus, our Δ 49 Ti cpx-melt are within the values expected from ab initio modelling that yield Δ 49 Ti cpx-melt between -0.234 to + 0.251 ‰ at the same temperature depending on the amount of Ti 3+ in cpx (Wang et al 2020;Aarons et al 2021).…”

“…This is lower than indicated from ab initio modelling. For example, the Δ 49 Ti ilm-melt by Wang et al ( 2020) is -0.271 ‰ at the same temperature (1100 °C, calculated with ß melt by Aarons et al 2021). However, if we recalculate the Δ 49 Ti ilm-melt of Wang et al (2020) with our ß melt of 1.31 ‰, a Δ 49 Ti ilm-melt of -0.332 ‰ is obtained, which would be close to our estimated value especially considering that ab initio models provide no errors.…”

“…Indeed, the ability of Ti-oxides to strongly fractionate Ti stable isotopes was observed for natural terrestrial (Deng et al 2019;Hoare et al 2020;Zhao et al 2020) and lunar bulk samples (Millet et al 2016;Kommescher et al 2020). In addition, Ti stable isotope fractionation driven by the presence of Fe-Ti oxides has also been recently investigated in mineral separates (Johnson et al 2019;Mandl 2019;He et al 2020;Nie et al 2021;Rzehak et al 2021), as well as experimental studies (Rzehak et al 2021). However, Ti stable isotope fractionation can occur not only driven by Fe-Ti-oxides but also by silicate minerals as ab initio studies indicate (cf.…”

“…However, Ti stable isotope fractionation can occur not only driven by Fe-Ti-oxides but also by silicate minerals as ab initio studies indicate (cf. Wang et al 2020;Aarons et al 2021). Pyroxenes and olivine from experimental high-Ti compositions at fO 2 relevant to lunar magmatism can contain up to 25% trivalent Ti in VI-fold coordination, whereas the remaining Ti 4+ was shown to be present in IV-or VI-fold coordination (Leitzke et al 2018).…”