Titanium isotopic compositions of bulk rocks and mineral separates from the Kos magmatic suite: Insights into fractional crystallization and magma mixing processes

Abstract

Terrestrial and extraterrestrial rocks exhibit significant variations in their mass-dependent Ti isotopic compositions, with basalts being isotopically lighter than evolved lithologies. The observed trend from light to heavy Ti isotopic compositions from more primitive to more differentiated rocks agrees with theoretical predictions that light Ti isotopes are sequestered in Fe–Ti oxides. However, there are lingering questions about the exact extent of this fractionation and whether it is influenced by the nature of oxides and silicate melt. To improve on this matter, we measured the Ti isotopic compositions of mineral separates and bulk rocks from the calc-alkaline Kos volcano-plutonic system, Aegean arc, Greece. Bulk rock Ti isotopic compositions (δ⁴⁹Ti) increase with differentiation of the magmatic system, from δ⁴⁹Ti of +0.042 ± 0.033‰ in basalt to +0.654 ± 0.034‰ in rhyolite. We document two different Ti isotope trends produced by (i) fractional crystallization, and (ii) mixing between a basaltic melt and an evolved (rhyolitic) magma. Trend (i) can be explained by a melt-cumulate Ti isotopic fraction factor α of 0.9998 (i.e., the bulk cumulate is on average 0.20‰ lighter than the melt). The mineral separates reveal variable δ⁴⁹Ti values, with magnetite having the lightest ⁴⁹Ti/⁴⁷Ti isotopic composition, biotite being intermediate and neso- and tectosilicates (i.e., olivine, plagioclase and quartz) heaviest. Comparing the TiO<inf>2</inf> concentrations of the low-Ti minerals olivine, plagioclase and quartz determined with LA-ICP-MS and isotope dilution shows that the δ⁴⁹Ti values measured in these minerals reflect their isotopic compositions, and contamination by inclusions is minimal. The difference in δ⁴⁹Ti between different minerals is smallest in a basalt (Δ⁴⁹Ti<inf>olivine-magnetite</inf> = +0.426) and largest in two rhyolites (Δ⁴⁹Ti<inf>quartz-magnetite</inf> = +1.083; both ± 0.046‰). Our data agree with theoretical predictions that Fe–Ti oxides have a light δ⁴⁹Ti signature, and neso/tectosilicate minerals are heavy. Furthermore, the measured difference in δ⁴⁹Ti between magnetite-olivine, magnetite-plagioclase and magnetite-quartz agree to first order with theoretically predicted inter-mineral Ti isotopic fractionation factors, thus suggesting that the measured inter-mineral Ti isotopic variations are equilibrium in nature.