Contrasting Cu isotopes in mid-ocean ridge basalts and lower oceanic crust: insights into the oceanic crustal magma plumbing systems

Magma plumbing systems exert strong controls on the formation and evolution of the oceanic crust during crust accretion. However, plumbing system dynamics based largely on mid-ocean ridge basalts (MORBs) are extremely difficult to constrain because the MORBs are aggregated melts and original information may have been blurred. Copper isotopic compositions of lower crustal gabbroic cumulates effectively archive early-stage histories because sulfides constitute the dominant Cu budget of gabbroic cumulates and are largely isolated from subsequent magma filtering processes. Here, we present Cu isotopes of a suite of gabbroic cumulates from the ultraslow-spreading Southwest Indian Ridge (Hole U1473A), and MORBs from the South Mid-Atlantic Ridge and East Pacific Rise. The δ⁶⁵Cu of the MORBs from this study (+0.04 ‰ to +0.19 ‰) match those of previous MORB analyses, indicating uniform δ⁶⁵Cu at various spreading rates. In contrast, the δ⁶⁵Cu of gabbroic rocks vary significantly (−1.14 ‰ to 0.87 ‰), exceeding by far the range known for peridotites. The Cu budget of these gabbroic rocks is hosted in sulfides and the mantle-like S isotopic compositions of sulfides indicate their origin of igneous processes. These results thus suggest that magma accumulation and sulfide segregation would lead to notable Cu isotopic fractionation in the lower oceanic crust. We propose that repeated recharging of primitive melts and efficient mixing of melts within the plumbing system can buffer the removal of early ⁶³Cu-rich sulfides from sulfide-saturated MORBs. The discrepancy in δ⁶⁵Cu between lower oceanic crust and MORBs is a natural consequence of continuous replenishment that occurs concurrently with melt-rock interaction, mixing, crystallization and extraction, irrespective of the oceanic settings. The consistent δ⁶⁵Cu in MORBs (0.09 ± 0.08 ‰) and komatiites (0.06 ± 0.06 ‰) further indicates that their Cu isotopes reflect the mean mantle source composition, providing a robust δ⁶⁵Cu for bulk silicate Earth (BSE) of 0.08 ± 0.08 ‰ (2sd). Accordingly, we propose that in open sub-ridge plumbing systems, other incompatible element isotopes and ratios of elements with similar incompatibility in MORBs as Cu isotopes suggest, could also represent mean mantle source compositions.