|Title||On the development of the calc-alkaline and tholeiitic magma series: A deep crustal cumulate perspective|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Chin E.J, Shimizu K., Bybee G.M, Erdman M.E|
|Journal||Earth and Planetary Science Letters|
|Type of Article||Article|
|Keywords||arc; arcs; calc-alkaline trend; colorado plateau; continental; cumulates; Geochemistry & Geophysics; hosted melt inclusions; Iron; island-arc; mid-ocean ridge; new-zealand; ocean-floor basalts; oxidation-state; sierra-nevada; silicic magmas; subduction zones; tholeiitic trend|
Two distinct igneous differentiation trends - the tholeiitic and calc-alkaline - give rise to Earth's oceanic and continental crust, respectively. Mantle melting at mid-ocean ridges produces dry magmas that differentiate at low-pressure conditions, resulting in early plagioclase saturation, late oxide precipitation, and Fe-enrichment in mid-ocean ridge basalts (MORBs). In contrast, magmas formed above subduction zones are Fe-depleted, have elevated water contents and are more oxidized relative to MORBs. It is widely thought that subduction of hydrothermally altered, oxidized oceanic crust at convergent margins oxidizes the mantle source of arc magmas, resulting in erupted lavas that inherit this oxidized signature. Yet, because our understanding of the calc-alkaline and tholeiitic trends largely comes from studies of erupted melts, the signals from shallow crustal contamination by potentially oxidized, Si-rich, Fe-poor materials, which may also generate calc-alkaline rocks, are obscured. Here, we use deep crustal cumulates to "see through" the effects of shallow crustal processes. We find that the tholeiitic and calc-alkaline trends are indeed reflected in Fe-poor mid-ocean ridge cumulates and Fe-rich arc cumulates, respectively.,A key finding is that with increasing crustal thickness, arc cumulates become more Fe-enriched. We propose that the thickness of the overlying crustal column modulates the melting degree of the mantle wedge (lower F beneath thick arcs and vice versa) and thus water and Fe3+ contents in primary melts, which subsequently controls the onset and extent of oxide fractionation. Deep crustal cumulates beneath thick, mature continental arcs are the most Fe-enriched, and therefore may be the "missing" Fe-rich reservoir required to balance the Fe-depleted upper continental crust. (C) 2017 Elsevier B.V. All rights reserved.
|Short Title||Earth Planet. Sci. Lett.|
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.