SECULAR VARIATION IN CU ISOTOPE COMPOSITION OF MORB-SOURCE MANTLE ALONG A 61-MILLION-YEAR-LONG MANTLE FLOW LINE: BASALTS FROM THE SOUTH ATLANTIC TRANSECT

The Cu isotopic compositions of various Earth reservoirs remain poorly constrained. Bulk silicate Earth is estimated to have a δ⁶⁵Cu value of +0.06 ± 0.20 ‰, and mid-ocean ridge basalts (MORBs) are thought to be similar (+0.09 ± 0.13 ‰) [1]. However, the MORB-source reservoir value is based on fewer than 25 samples of ‘fresh’ mid-ocean ridge basalt (whether whole rock or glass was analyzed is not clear in all cases), and only six of these analyses are accompanied by data on Cu concentrations [1,2]. IODP Expeditions 390 and 393 recovered basaltic basement from five sites along a single mantle flow line on the western flank of the Mid-Atlantic Ridge (~ 30^oS) that range in age from 61.2 to 6.6 Ma, i.e., the South Atlantic Transect (SAT). In this study we examine the temporal variation in Cu isotope composition of MORB-source mantle along the transect. The samples analyzed are hand-picked basaltic glasses, free of alteration, yet they show considerable variation in Cu concentration (51 to 113 ppm); this is interesting, because the differences among average values for depleted, normal, and enriched MORB [3] are less than 2 ppm, suggesting that magmatic processes common to most MORBs do not produce large variations in average Cu contents. SAT basalt glasses also show a significant range in δ⁶⁵Cu values of +0.09‰ to +0.34‰, a range that extends to greater enrichment in ⁶⁵Cu than previously observed among fresh basaltic glasses. Within this range, basalts from the oldest sites in the transect (61.2 & 60.7 Ma) have the lowest values (+0.09‰ to +0.16‰), whereas those from sites younger than 50 Ma have higher values (+0.24‰ to +0.34‰), and a plot of δ⁶⁵Cu vs Cu produces a broad, negative array. Average Cu contents also form two broad negative correlations when plotted against Zr/TiO₂, one array reflecting a plume influence but the other, shown by depleted MORB tholeiites, overlaps the composition of normal MORBs at one end, extending to higher Cu and lower Zr/TiO₂ at the other. Thus, higher Cu contents and lower δ⁶⁵Cu are observed in basalts that are otherwise the most depleted geochemically (with lower Zr/TiO₂).

[1] Liu et al. (2015) EPSL 427, 95-103; [2] Savage et al. (2015) Geochem. Persp. Let. I, 53-64; [3] Gale et al. (2013) Geochem. Geophys. Geosyst 14, doi:10.1029/2012GC004334