Active near-surface mobilisation of slab-derived geochemical signatures by hyperalkaline waters in brecciated serpentinites

Abstract

Unusual hyperalkaline meteoric groundwaters on Troodos massif, Cyprus, issue from highly deformed and completely serpentinized ultramafic brecciated rock masses of the Artemis Diapir and have high salinity (25–30% seawater total dissolved solids) and some of the highest recorded pH values (11−13) for natural waters. These waters have elevated dissolved Na, K, Li, B, Ba, Rb, Cs, Cl and SO4 with ion/chloride substantially above seawater ratios, overprinting minor contributions from marine aerosols. For example, K concentrations are similar to seawater values. Water stable isotope ratios imply extensive water-rock interaction and 87Sr/86Sr is rock-dominated (0.705) and significantly lower than seawater. These rare fluids contrast with surface and shallow ground waters draining the ultramafic Troodos Mantle Sequence that have pH of 8.5–9, meteoric stable isotope ratios, 87Sr/86Sr similar to early Miocene seawater (0.7085) and low dissolved salts albeit with ion/chloride ratios also greater than seawater. The combination of high pH and salinity is unusual in ophiolites and these hyperalkaline waters are most similar to end-member fluids emanating from serpentine mud volcanoes in the Mariana forearc. Rainwater rapidly transiting terraces of asbestos mine tailings on Troodos mostly resemble the surface waters but show slight contamination by a saline component. We propose that inclusions within the serpentinite that contain highly soluble salts have been made accessible by tectonic deformation during uplift and diapirism, or comminution during mining activity. A multistage process is proposed whereby alkali and other signatures have been released from the stalled subducting slab beneath the Troodos massif and have infused the mantle wedge. Some of these rocks have been carried to the surface by diapiric uplift and erosion, where tectonic deformation or mining activities have enabled the near-surface mobilisation of slab-derived geochemical signatures by modern meteoric waters.