Titanium isotopic fractionation during alkaline magma differentiation at St. Helena Island.

To better understand the behavior of Ti isotope fractionation during alkaline magma differentiation, we studied well characterized alkaline lavas from St. Helena Island (South Atlantic), as well as their titanomagnetite separates. The lavas are classified into three groups according to petrographic observations and major element composition. Group 1 and Group 2 samples (5 to > 13 wt.% MgO) have a narrow δ^49/47Ti range (− 0.02 to 0.05‰), suggesting that Ti isotopic fractionation is insignificant in less evolved basaltic lavas. Conversely, Group 3 samples (MgO < 5 wt.%) are saturated with titanomagnetite and display a wide range in δ^49/47Ti (− 0.02 to 1.96‰). The δ^49/47Ti values for Group 3 samples show significant correlation with TiO₂ and SiO₂ content, as well as with Mg and Fe isotopic values. Moreover, titanomagnetite phenocrysts from Group 3 rocks have remarkably lighter δ^49/47Ti values (− 0.54 to 0.01‰) relative to the corresponding whole rock (− 0.02 to 0.21‰), indicating that titanomagnetite crystallization exerts significant control over the δ^49/47Ti of Group 3 samples. These observations are further supported by modeling calculations. Together with published Ti isotope data, the results demonstrate that the range in Ti isotopic evolution in alkaline, calc-alkaline and tholeiitic magmatic systems is controlled by fractional crystallization of diverse Fe-Ti oxides with contrasting Ti isotopic compositions. This makes Ti stable isotopes an important geochemical tracer for magma evolution. [ABSTRACT FROM AUTHOR]