A deep crustal perspective on the origin of continental crust: the message from cumulates
The calc-alkaline and tholeiitic magmatic series are the two most important igneous differentiation trends on Earth and are expressed by Earth’s crustal dichotomy of andesitic continental crust and basaltic oceanic crust, respectively. There is still debate over whether Fe depletion in arc magmas is caused by early fractionation of iron oxides due to convergent margin magmas being more oxidized or by crustal contamination with evolved end members rich in silica and poor in iron. To see through crustal processes, we examine deep crustal cumulates – the complementary crystalline solids fractionated from evolving magmas. We find that mid-ocean ridge cumulates are Fe-poor and complementary to Fe-enriched MORBs that evolve along the tholeiitic trend. In contrast, arc cumulates are Fe-rich and complementary to Fe-depleted arc magmas evolving along the calc-alkaline trend. The Fe-rich nature of arc cumulates is apparent in early-formed cumulates (Mg# > 80), suggesting that several factors unique to arcs (e.g., higher fO2, higher water, and/or thicker crust) probably play a role in the early evolution of arc magmas and that crustal assimilation is not likely the dominant control on iron depletion. Both continental and island arc cumulates are substantially Fe- and Ti-enriched relative to mid-ocean ridge cumulates suggesting the primary role of Fe-Ti oxide (magnetite, ilmenite) fractionation on the Fe-depletion of arc magmas and ultimately the Fe-depleted nature of the upper continental crust.
Date： 31th March 2017 3:00
Place：Three Gorge Research Center 210
Date： 7th April 2017 3:30
Place：Main building 308