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On the development of the calc-alkaline and tholeiitic magma series: A deep crustal cumulate perspective

Using a “crystal” perspective provided by 500+ global cumulates, we show that primitive arc cumulates are Fe-rich whereas mid-ocean ridge cumulates are Fe-poor, reflecting the dichotomy of the calc-alkaline and tholeiitic magma series. Our results provide clear evidence that fractional crystallization, rather than crustal assimilation/mixing, is the main driver of the calc-alkaline trend and therefore of andesite (and continental crust) formation. An Fe-rich reservoir, in the form of deep crustal cumulates, is required to balance the Fe-depleted upper continental crust. Analysis of arc cumulates shows that with increasing crustal thickness, deep arc cumulates increase in Fe and Ti, suggesting earlier oxide fractionation beneath thicker arcs. We speculate that arc crustal thickness modulates melting degree, which in turn governs the initial H2O content and Fe3+/ΣFe ratio of primitive arc melts, and which ultimately controls the degree of Fe enrichment in arc cumulates and the complementary degree of Fe depletion in arc magmas. In contrast, the low H2O and Fe3+/ΣFe in MORBs generate low Fe ridge cumulates and the complementary Fe enrichment in MORBs. Our observation shows that, compared to cumulates and magmas from thick-crust arcs, those from thin-crust arcs are more similar to cumulates and magmas from mid-ocean ridges, suggesting that the cause of greater oxidation state in arc magmas is more complicated than being solely due to subduction of oxidized oceanic crust.

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