PRIMARY SULFIDE GLOBULES IN OCEANIC BASALTS AND THEIR IMPACT ON Cu, SAT 390/393

Preliminary work on the Cu isotope systematics of basalts along the South Atlantic Transect has produced surprising results, with altered samples extending to both lower and higher d⁶⁵Cu than associated fresh glass; the results suggest loss of ⁶⁵Cu during pervasive alteration but enrichment during later formation of alteration halos (Mindrup et al., 2023). To better understand the processes involved in Cu isotope fractionation during progressive alteration of ocean crust, two pillow basalts samples were systematically sampled along transects from the glassy chilled margins into the crystalline pillow interior. We report on sample petrography and Cu concentration data here.

Surprisingly, the transect samples show large variations in Cu concentrations (62 – 715 ppm) that contrast with previously measured Cu contents of handpicked glasses that range from 47.5-113.4 ppm (Kempton, unpublished). Even more surprising is that concentrations vary systematically from pillow rind to interior, with the highest concentrations in both samples (334 and 715 ppm) in the glassy margins and lowest concentrations in the pillow interior (62 and 65 ppm).

Petrographic analysis of ten available SAT glass samples has revealed common presence of primary sulfide globules within the glass (>1-90 µm in size). The globules occur as discrete masses within the glass, attached to the edges of vesicles and spherulites (>1-5 µm in size), trapped within phenocrysts and microcysts (>1-5 µm in size), and within interstitial spaces within the cryptocrystalline interior (>1-50 µm in size). They are most common within the glass and adjacent cryptocrystalline quench zone of pillow basalts. Microprobe analyses show that the globules are composed of three phases: Ni-rich monosulfide solid solution (avg. ~2 wt% Cu); Cu-rich intermediate sulfide solid solution (avg ~ 20 wt% Cu), and an Fe oxide phase.

Calculated rehomogenized compositions of sulfide globules yield estimates for the original sulfide melt composition of 8 – 15 wt% Cu. At these concentrations, it requires <1% of the analysed glassy material to account for the high Cu concentrations observed.

Mindrup, Q. et al. (2023). Impact of seawater alteration on Cu isotope composition of Oceanic Basalts along the SAT: IODP EXP 390/393. Annual GSA meeting, Pittsburg, PA 2023. https://doi.org/10.1130/abs/2023am-392408