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Constant Cu/Ag in upper mantle and oceanic crust: Implications for the role of cumulates during the formation of continental crust

2018; Elsevier BV; Volume: 493; Linguagem: Inglês

10.1016/j.epsl.2018.04.008

1385-013X

Zaicong Wang, Harry Becker, Yongsheng Liu, J. Elis Hoffmann, Chunfei Chen, Zongqi Zou, Yuan Li,

earthquake and tectonic studies

Delamination of dense, sulfide-bearing pyroxenitic cumulates was proposed to explain the low Cu content and the evolved major element composition of the continental crust, yet evidence for this hypothesis has been circumstantial. In this study, we present Cu and Ag contents of mantle pyroxenites and associated peridotites from the Balmuccia peridotite massif (Italian Alps) and mantle xenoliths from Hannuoba (North China Craton) to constrain the fractionation behavior of Cu and Ag during melt–peridotite reaction and magmatic accumulation in the mantle. Furthermore, we reexamine the behavior of these elements during arc magmatism and in other reservoirs to highlight the fractionation of Cu and Ag as an important tracer for processes that led to the formation of continental crust. Melt–peridotite reaction and magmatic accumulation led to highly variable sulfide abundances, Cu and Ag contents in the mantle pyroxenites (e.g., Balmuccia, 87–484 μg/g Cu; Hannuoba, 15–116 μg/g Cu). The mean Cu/Ag of the pyroxenite suites (Balmuccia: 3800 ± 1100; Hannuoba: 3100 ± 900, 1s) are indistinguishable from those of their host peridotites and other lherzolites (3500 ± 1200), MORBs (3600 ± 400), and Hawaiian basalts (3200 ± 100). These results reflect the limited fractionation of Cu from Ag during magmatic processes in the upper mantle and during the formation of oceanic crust, and indicate a similar mean Cu/Ag (3500 ± 1000, 1s) in these reservoirs. Experimental data indicate that similar partitioning of Cu and Ag between sulfide melt and silicate melt is responsible for the limited variation of Cu/Ag. Magmatic processes in the convecting mantle and in the oceanic crust, however, cannot explain the low Cu/Ag ratio of the continental crust (about 500). Experimental constraints on sulfide stability in oxidized mantle and data from back-arc magmatic series suggest that formation of primitive back-arc and island arc basalts inherit mantle-like Cu/Ag and thus also cannot explain the low Cu/Ag of the continental crust. In contrast, previous data on evolved back-arc magmas suggest that the low Cu/Ag coupled with a considerable depletion of Cu in the continental crust might be ascribed to the segregation of monosulfide solid solution (MSS) during fractional crystallization of evolving arc magmas. These results thus support the model that accumulation of sulfide-bearing mafic cumulates in the lower crust of magmatic arcs has been a critical process in defining some of the chemical characteristics of the continental crust.