Vein distribution in ultramafic basement of the Wadi Lawayni drill sites

Veins in core from Holes BA1B (400 m deep), BA3A and BA4A (both 300 m deep) represent fractures that provided pathways for fluids migrating through mantle peridotite of the Samail ophiolite, starting in the late Cretaceous and continuing today. Characterization of vein distribution was conducted onboard D/V Chikyu, using visual core description, thin section microscopy, and XRD analyses.

In the topmost 50-100 m of core, carbonate veins are abundant and make up between 1 and 3 % of the core by volume. Throughout the core, serpentine veins are abundant, and they are dominant in the lower 200-300 m of the core, comprising several percent by volume. Several serpentine vein types were distinguished: (1) early paragranular black serpentine+magnetite veins that are typically anastomosing or branched and commonly dip steeply. Also early and commonly paragranular are dark green and brown serpentine veins. Later veins comprise para- to transgranular white chrysotile veins and waxy-green serpentine veins that are amorphous to cryptocrystalline (picrolite). These later veins occur throughout, but the white veins appear to dominate in the deeper sections of the cores. In core with abundant gabbroic dikes, white to pale green waxy serpentine veins are common. In the deeper sections of all cores, these intervals also feature irregular veins of xonotlite, a hydrous Ca-silicate, and rare wollastonite. The gabbroic rocks also host small veinlets of chlorite and hydrogrossular.

As expected from the compositions of the hyperalkaline groundwaters in the Samail Ophiolite, Ca is mobile. It is trapped in Ca carbonate veins that form close to the surface where atmospheric CO2 can penetrate into the basement. At greater depth, Ca mobility is restricted where Ca-silicates such as xonotlite and wollastonite form. Silica activities of the fluids are low, as we observed rodingite assemblages, such as chlorite-hydrogrossular in veins. The presence of brucite in veins could not be confirmed, however. Although some veins show halos that may have more or less magnetite, Fe mobility was also low. The veins appear to represent fracture networks that allowed water to penetrate into the basement, where chemical potential difference between different lithologies drove reactions and minor mass transfer.