Controls on the Fluid Compositions of Hyperalkaline Springs: Clues from the Samail Ophiolite in Oman

The Samail Ophiolite in Oman hosts dozens of highly reduced and high-pH springs, manifestations of an alteration process involving the interaction of water with rocks rich in ferromagnesian silicates. We sampled a number of these hyperalkaline fluids and coupled field and laboratory measurements with thermodynamic calculations to decipher various reaction paths occurring during low-temperature serpentinization. Using major and trace element as well as isotopic (87Sr/86Sr) measurements of fluids sampled from various locations in the ophiolite, we observed that numerous factors can influence the composition of hyperalkaline fluids such as the type of host rock (peridotite vs gabbro), geological setting of the springs (basal thrust vs inner part of the ophiolite), fluid-mineral equilibria, and influences from the shallow subsurface. Controls imposed by fluid-mineral equilibria are evident especially for Si, Mg and Ca. Equilibrium thermodynamic calculations simulating serpentinization predict high-pH fluids controlled by the serpentine-brucite-diopside equilibria. However, we observed that spring chemistry does not coincide with this prediction. For instance, silica measured from peridotite-hosted springs lies between that controlled by serpentine-brucite precipitation and that governed by forsterite dissolution. Brucite seems to buffer the Mg content as it never reaches a value where brucite is undersaturated. Total Ca and dissolved inorganic carbon content are in equilibrium with calcite, an indication that these elements are already controlled by surficial and/or shallow subsurficial processes. Moreover, a diverging trend in the Na/Cl ratio observed between fluids sampled from surface seeps and those from wells suggests surficial and/or shallow subsurficial influences. An upcoming attempt to drill into deeper aquifers will allow us to determine if fluid chemistry predicted by equilibrium thermodynamic calculations was attained at deeper levels. Furthermore, this will also aid us in quantifying how much the shallow subsurface can influence the composition of a deep-seated hyperalkaline fluid on its way up to the surface.