IMPACT OF SEAWATER ALTERATION ON CU ISOTOPE COMPOSITION OF OCEANIC BASALTS ALONG THE SOUTH ATLANTIC TRANSECT: IODP EXP 390/393

Cu isotope fractionations during ocean crust – seawater interactions are investigated using basalt samples from IODP Exp 390/393, the South Atlantic Transect (SAT). The SAT includes five drill sites located on the western flank of the mid-Atlantic Ridge at ~ 30^oS on crust that ranges in age from 61.2 to 6.6 Ma. Whole rock (WR) powders analyzed are part of a ‘pooled’ sample set taken during each cruise for collective research on material deemed representative of the core. Preliminary results show two contrasting trends. Alkali basalt WR samples from the oldest SAT site (U1556) have δ⁶⁵Cu values that extend to both higher and lower ratios than fresh glass from the same unit (+0.16‰). Indeed, one sample pair from U1556B-3R-3, 66-84 cm, which includes an example of pervasive background alteration and an adjacent alteration halo, exhibits values of +0.03‰ and +0.51‰, respectively. This suggests a multi-stage history that involves both loss of ⁶⁵Cu during the pervasive stage of alteration but subsequent addition of ⁶⁵Cu during formation of the alteration halo. In contrast, tholeiitic basalts from the younger sites (<50 Ma; U1558, U1583, U1560) all show a significant shift to lower δ⁶⁵Cu values in the WR samples (−0.25‰ to −0.15‰) relative to fresh basaltic glasses from the same sites (+0.27‰ to +0.34‰) for both pervasive background alteration and alteration halos. The shift to lower δ⁶⁵Cu is consistent with dissolution of Cu-bearing sulfides and release of ⁶⁵Cu to seawater. These results contrast markedly with those observed in Hole 1256D, where only limited variations in δ⁶⁵Cu were observed downhole for the volcanic section, with values remaining similar to fresh MORB (0.06 ± 0.20‰) [1]. This result was attributed to the low degree of alteration of the ~ 15 Ma crust at Site 1256, a ‘super-fast’ spreading ridge (110 mm/yr) [1]. In contrast, spreading rates for the SAT sites are much slower (i.e., ~13 – 26 mm/yr), and the transect extends to older crust (~ 61 Ma), providing not only more time for hydrothermal alteration at the ridge crest but also more time for off-axis, low temperature alteration. Interpretation of these results will be presented in the context of petrographic and geochemical analyses in progress.

Huang et al. (2016), J. Geophys. Res. 121, 7086–7100