Experimental study of the partitioning of Cu during partial melting of Earth's mantle

Primitive basaltic glasses from mid-ocean ridges (MORB), ocean islands (OIB) and arcs contain three to five times the Cu as the currently accepted primitive upper mantle (PUM) value, suggesting a bulk partition coefficient DCumantle/melt of ∼0.20. Sulfide, with a DCusulfide/melt of 250–960 is presumed to be ubiquitous in the mantle. The Cu abundances in basalts are unable to be achieved if Cu behaves compatibly, or if a high enough modal abundance of sulfide exists during the onset of melting to sequester significant Cu. To address this conundrum we determined DCu in olivine and orthopyroxene at 1250–1525 °C and 1.0 GPa in a hydrous basalt and KLB1 peridotite, at ƒO2 of FMQ-1, near to melting conditions of the upper mantle. The measured DCuol/liq of 0.06–0.21 and DCuopx/liq 0.15–0.82 do not vary with melt fraction, or significantly with ƒO2, and can be combined with estimates for DCu for clinopyroxene into melting models to examine the Cu contents of mantle-derived melts. The Cu abundances for MORB, OIB, and arc glasses are all explicable by up to 15% melting of the silicate—only portion of the mantle in which Cu behaves as a mildly incompatible element (DCumantle/melt∼0.26). For Cu to be enriched in basaltic melts in the presence of sulfide, the melt/sulfide ratio, or the oxidation state of the mantle during melting, must be significantly high to diminish the potential of sulfide to sequester any Cu. Our results suggest the role of sulfide during mantle melting may be insignificant with regards to controlling the concentration of moderately chalcophile elements like Cu, and confirm that the previously estimated Cu content for PUM of 20 ppm is correct.

► New experimental measurements for mineral–melt partitioning for Cu in olivine and orthopyroxene at mantle conditions. ► The Cu contents of primitive basaltic glasses explicable by mantle melting not involving sulfide. ► Sulfide plays no or a limited role, in chalcophile element geochemistry of the mantle melts, contrary to canonical view. ► The Cu contents of the primitive upper mantle are likely correct. ► The Arc mantle, which sources magmas for porphyry Cu deposits, is not endowed in Cu relative to other tectonic settings.