LOW-TEMPERATURE HYDROTHERMAL ALTERATION OF SLOW-SPREADING, NON-MORB OCEANIC CRUST AND ITS EFFECTS ON THE SEAWATER POTASSIUM BUDGET

Low-temperature chemical exchanges between oceanic crust and seawater play a significant role in the biogeochemical cycle of many elements and help to regulate atmospheric CO₂ concentrations over geological timescales. Understanding the timing, style and extent of these reactions is thus crucial to our knowledge of global climate feedback. Potassium (K) is a highly mobile element during low-temperature hydrothermal alteration and tends to accumulate in altered crust as it exchanges with seawater. As a result, K concentrations in altered oceanic crust (AOC) provide a direct window into the extent of reaction between crust and seawater. Recently, K-isotopes have been paired with K concentrations to shed light into the heterogeneous nature of oceanic crust alteration. However, existing data are limited to intermediate to fast spreading ridges with MORB-like lithologies. In this study, we expand on our knowledge of K cycling in AOC through the analysis of K-isotopic compositions of oceanic crust samples from a nearly 60-million-year-old piece of slow-spreading crust from the South Atlantic Ocean (~30^oS). These rocks span compositions varying from MORB-like to enriched tholeiites and alkali basalts, thus covering a large range of initial K2O (wt%) contents. Our K-isotope data reinforce the dynamic and heterogeneous nature of low-temperature seawater-crustal exchange and indicate both gain and loss of K during crustal alteration. Depending on the global extent of these non-MORB lithologies, results from this study also suggest that our current picture of the effects of low-temperature hydrothermal alteration on the chemistry of seawater might need revision.