A TALE OF TWO ROCKS: LOW-TEMPERATURE ALTERATION OF ULTRAMAFIC AND GABBROIC ROCKS AND CONSEQUENCES FOR FLUID COMPOSITION

Submarine chimneys famously venting black smoker hydrothermal fluids are well known consequences of high-temperature alteration of mafic and ultramafic rocks common in the seafloor. The variabilities in lithological settings found in the seafloor yield diverse compositions of hydrothermal fluids that support varied chemolithotrophic communities. Less known, but likely far more widespread, are the consequences of alteration of these rocks at the ambient conditions persistent in most of the Earth’s oceanic and continental aquifers. Opportunities to easily access, study, and compare fluids generated through low-temperature alteration of rocks of variable compositions exist in the Oman Ophiolite, which hosts one of the largest exposures of ultramafic and gabbroic rocks on Earth’s continents. In the Oman Ophiolite, fluid/rock reactions occur actively at the ambient conditions (≤40°C) present in the ophiolite’s aquifer and produce some of the most reduced and alkaline fluids on Earth. To explore how variabilities in host rock compositions and extents of the overall alteration process control fluid compositions, we sampled and analyzed >100 fluids from various locations in the ophiolite, encompassing a wide range fluid compositions and lithological settings. Many fluids associated with ultramafic rocks approach compositions predicted by equilibrium calculations despite reactions occurring at low temperatures. In contrast, fluids associated with gabbroic rocks deviate from equilibrium predictions, and plot between those predicted by models simulating ultramafic alteration and those simulating gabbro alteration. Ultramafic rocks often underlie gabbroic bodies in the ophiolite and fluids seeping out from gabbroic outcrops may have followed subsurface transport paths involving both rock types. Some aqueous species (Si, Mg, sulfide, H₂) can be diagnostic for the lithological setting and the extent of fluid/rock interactions. This study provides a framework to explore the diverse outcomes on fluid compositions as a result of the varied composition of the rocky interiors of ocean worlds in our solar system and their consequences for fueling habitable environments beyond our own planet.