Diffusive Ti Isotope Fractionation in Haplobasalts

Isotope fractionation during diffusion is a fundamental process in magma systems. However, diffusive isotope fractionation has not been studied for many elements, and Ti is one of them. Being a high-field strength element, Ti diffusivity is not much different from that of Si in the melt. Therefore, the β factor (defined as ln(D2/D1)/ln(m1/m2), where D is diffusivity, m is atomic mass, and subscripts 1 and 2 indicate light and heavy isotopes) for Ti isotope diffusion may be close to zero based on the empirical relation between β and ratio of diffusivity of the element to that of Si [1], and whether diffusive Ti isotope fractionation is observable in natural magmatic processes is unknown. Here, we report the first study of diffusive Ti isotope fractionation in haplobasaltic melts using diffusion couples from [2]. The 49Ti/47Ti isotope ratio profiles in diffusion couple experiments have been measured at WiscSIMS Lab using IMS-1280. We pushed the limits of current SIMS to measure these non-traditional stable isotope ratios. The resulting data show that SIMS measurements using IMS-1280 can reach 0.10‰ (2σ) with effort at TiO2 concentration of ~3 wt% using multi-collection Faraday cup with 1012 ohm feedback resistors. However, at low TiO2 concentration of 0.015 wt%, the precision in measured 49Ti/47Ti is about 1.0‰ by single collection electron multiplier with magnetic field scan. For initial Ti concentration contrast of about 200 in a diffusion couple, the total variation in 49Ti/47Ti across a diffusion couple profile is about 3.2‰, which is much larger than equilibrium Ti fractionation on magma [3]. By fitting the isotope ratio profiles, the β value is 0.030±0.002 for the MgO-TiO2 interdiffusion couples and ~0.036 for the SiO2-TiO2 interdiffusion couples both at 1500°C. Hence, the total Ti isotope ratio variation would be > 1‰ along a diffusion profile when the initial TiO2 concentration contrast is larger than 15. Such high concentration contrast may be realized for basalt-rhyolite magma mixing. These results will allow us to quantify the transport of Ti isotopes and help us to gain a deeper understanding of the diffusion mechanism on molecular level.

Watkins et al. (2017) RiMG 82, 85; 2. Guo & Zhang (2016) GCA 195, 126; 3. Greber et al. (2021) Chem Geol 578, 120303.