The Role of Mafic Intrusions in Producing Alkaline Vent Fluids at the Lost City Hydrothermal Field: Evidence from Stable Potassium Isotopes

The alkaline, H₂-rich vent fluids of the Lost City Hydrothermal Field (LCHF) have generally been attributed to serpentinization of tectonically uplifted mantle peridotite. However, elevated concentrations of highly incompatible and fluid mobile trace alkali metals, Rb and Cs, indicate reaction with relatively unaltered mafic components (Seyfried et al., 2015). Here, we present high-precision stable-K isotope analyses of LCHF vent fluids, which provide new constraints on source-rock lithology and support more quantitative estimates of fluid:rock ratio, providing a new constraint on heat and mass transfer processes. We find that δ⁴¹K values of LCHF vent fluids (0.03–0.07 ‰) are distinct from local seawater (0.13 ± 0.02 ‰). In combination with near-seawater concentrations of K (10.4 ± 0.1 mmol/kg), these data are incompatible with hydrothermal alteration of peridotite alone, which is highly depleted in K. Instead, K-isotope and concentration data are consistent with hydrothermal alteration of mafic rocks (e.g., gabbro or diabase) at a fluid:rock ratio of 6–26, a range that agrees well with Rb, Cs, and ⁸⁷Sr/⁸⁶Sr-based estimates of fluid:rock ratio, updated here to reflect derivation from mafic source rocks.

Geochemical modeling of hydrothermal fluid-rock reactions indicates that LCHF vent fluids can be effectively reproduced by a two-stage reaction in which seawater initially reacts at 200–300 °C with mafic rocks at in a fluid:rock ratio of ∼ 10 —as constrained by the above isotopic and trace-element systematics— and subsequently reacts with peridotite at a fluid:rock ratio of ∼ 30 at roughly the same temperature, pressure conditions. We also note that LCHF vent fluids are ∼ 2 % depleted in Cl compared to seawater, which may reflect admixture of a vapor component derived from ongoing phase separation at higher (> 450 °C) temperatures. We thus propose that the LCHF represents waning-stage hydrothermal activity initiated by a discrete off-axis mafic intrusion that has subsequently cooled to intermediate temperatures conducive to olivine hydrolysis and production of alkaline vent fluids. In this interpretation, alkaline H₂-rich vent fields occupy a narrow thermal window in the expected evolution of hydrothermal activity associated with episodic off-axis mafic intrusions into tectonically exposed ultramafic host rocks and are thus likely to be rare on the modern seafloor.