Dr. Katie Kelley,
Rhode Island Graduate School of Oceanography
Thursday, January 19th
233 Advanced Research Complex
25 Templeton Street
University of Ottawa
Early Fe depletion of magmas, often termed “calc-alkaline” affinity, drives them towards the composition of bulk continental crust. Subduction zone magmas may develop early Fe depletion due to high magmatic H2O, which suppresses silicates, high oxygen fugacity (fO2), which promotes early Fe-oxide crystallization, or primary Fe-poor magmas may emerge via slab melting and mix with Fe-rich basaltic magmas, causing an apparent Fe-depletion trend that is unrelated to differentiation of a single parent magma. Yet, the relative importance of these key factors in generating calc-alkaline trends in natural arc magmas remains under-constrained.
We use innovative measurements of dissolved volatiles and Fe3+/ΣFe ratios in natural volcanic glasses to assess the H2O contents and fO2 conditions of magmas that have emerged from Earth’s mantle in a variety of tectonic settings, to test the relationships between magmatic volatiles, fO2, and calc-alkaline affinity. The effects of H2O and fO2 on arc magmatic differentiation are challenging to isolate in nature, but experimental data suggest that fO2 exhibits stronger control than H2O on the relative appearance of magnetite vs. silicates on the liquidus, thus exerting greater leverage on the calc-alkaline trend.