Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation

Abstract

Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

Double-layer structure of the crust beneath the Zhongdian arc, SW China: U–Pb geochronology and Hf isotope evidence

“…Unlike normal arc magmas, the Cu-bearing porphyries in the Zhongdian arc are potassic, relatively oxidized, and adakitic, and they were derived from a juvenile lower crust formed by underplating of former arc basaltic magmas, as discussed above. Re-melting and remobilization of underplated mafic material (including sulfide-bearing metal-rich cumulates at the base of the crust that formed during the earlier arc magmatism) probably provided the Cu, Au, and S for the porphyry Cu systems (Richards, 2009;Lee et al, 2012;R. Wang et al, 2014;Hou et al, 2015).…”

“…The high H 2 O content of these porphyries can probably be attributed to the melting of H 2 O-bearing amphibolite cumulates that were the residues of underplated arc magmas, and which then saw the breakdown of hornblende, thus releasing H 2 O (Richards, 2009). Metal-rich cumulates in the early-stage juvenile crust probably contributed to the enrichment of ore-forming elements (Lee et al, 2012), and subsequently the slab break-off or slab-tearing may have provided enough heat to fuse the juvenile lower crust. Thus, in the Zhongdian arc area, conditions were optimal for the formation of porphyry copper systems, in contrast to other parts of the Yidun arc.…”

Triggers for the formation of porphyry ore deposits in magmatic arcs

“…Melting also occurs at lower temperatures if the mantle has been oxidized and hydrated by volatile species derived from the downgoing slab; this is potentially one of the reasons why subduction-related arc magmatism favours the formation of ore deposits in contrast to other parts of the Earth. However, a recent study has suggested that the source region for typical arc magmas is not unusually oxidized, nor enriched in economic elements of interest like copper 42 . Consequently, there is, as yet, little evidence to suggest that selective metal enrichment to form fertile porphyry magmas occurs in the mantle.…”

“…It is generally believed that copper and gold are likely to be derived from the mantle, although an anomalously enriched lower crust due to the presence of pre-existing ore deposits 46 or copperrich cumulates from an earlier subduction cycle 42 would have a major impact on the fertility of the magmas formed. Although molybdenum is generally considered to have a crustal origin 47 , derivation from lithospheric mantle ( Fig.…”

The Subduction Factory: Geochemical perspectives

“…In addition to arc volcanic rocks and BABB, both MORB and OIB contain recycled components derived from subduction zones (Sun et al, 2008). During plate subduction (right), the subducting slab undergoes dehydration, metamorphism, partial melting of sediments (Schmidt and Poli, 1998) and even slab melting (Defant and Drummond, 1990), forming arc magmas (Arculus, 1994), adakites (Defant and Drummond, 1990) and related ore deposits (Iizasa et al, 1999;McInnes et al, 1999;Lee et al, 2012;Li et al, 2013b). It also triggers backarc extensions and hydrothermal mineralization (Yang and Scott, 1996;Sun et al, 2004).…”