Titanium isotopic fractionation of arc derived melts and cumulates

Mechanisms such as crystallization differentiation, subduction erosion, delamination, or relamination that are responsible for the formation and modification of modern crust with an on average andesitic composition are actively debated (Hacker et al. 2015). Isotope fractionation associated with igneous processes is documented for many non-traditional stable isotope systems, making them promising tools to advance our understanding of modern arc crust formation. Titanium isotopes are especially promising, as volcanic and plutonic arc rocks show a trend from light to heavy isotope values with increasing SiO₂ concentration due to the fractionation of minerals with light Ti isotopes.

We present new Ti isotope data on medium K calc-alkaline to shoshonitic magmatic differentiation suites from the Adamello Batholith (N-Italy), Kos (Agean arc), Torres del Paine (Patagonia) and the Dolomites (N-Italy) in addition to crust-derived mafic cumulates. The Ti isotopic composition of dacites and granites range between δ⁴⁹Ti_OL-Ti ≈ 0.3 to 1.1‰, with heavier values for more alkaline granitic melts in agreement with published data (Hoare et al. 2020). Mafic cumulates from related and additional localities are overall isotopically lighter than (their) granitic counterparts ranging between δ⁴⁹Ti_OL-Ti ≈ -0.15 and +0.08‰. Cumulates of studied crustal sections enriched in Fe-Ti oxides (>5 modal %) show δ⁴⁹Ti values lighter than the depleted MORB mantle (DMM, δ⁴⁹Ti_OL-Ti ≈ +0.002 ± 0.007‰) and counterbalance the isotopically heavy composition of felsic rocks. The occurrence of cumulates heavier than DMM may have several reasons: (i) “heavy” cumulates may represent late-stage relicts of progressive magma differentiation containing trapped intercumulus melt or (ii) they experienced overprinting, e.g., by mafic rejuvenation.

We therefore find that the Ti isotopic composition of cumulate rocks and likely also the magmatic lower continental crust is influenced by their mineralogical composition. How this impacts the Ti isotopic composition of the bulk continental crust in the light of delamination and relamination processes needs further work.

 

REFERENCES

Hacker, B. R., Kelemen, P. B., & Behn, M. D. (2015). Continental lower crust. Annual Review of Earth and Planetary Sciences, 43, 167-205.

Hoare, L., Klaver, M., Saji, N. S., Gillies, J., Parkinson, I. J., Lissenberg, C. J., & Millet, M. A. (2020). Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation. Geochimica et Cosmochimica Acta, 282, 38-54.