Geochemical indications of hydrothermal fluid through sediments within the Geolin Mounds and Mienhua Volcano hydrothermal fields, southernmost Okinawa Trough

Abstract

This study focused on the chemistry of sedimentary pore fluids to clarify hydrothermal fluid migrating within sediments at the Geolin Mounds (GLM) and Mienhua Volcano (MHV) hydrothermal fields, southernmost Okinawa Trough, where are characterized by covering of thick sediment. The significant downward decrease in Mg²⁺ (low to 23.3 mmol L⁻¹) and concurrent increase in Li⁺ (up to 2,269 μmol L⁻¹) in sedimentary pore fluids implied a substantial influence of hydrothermal fluid, which might be associated with high-temperature (>350 °C) rock/sediment-fluid interaction. The best fitting of the 1-D advection-diffusion equation to pore-fluid Cl⁻, Mg²⁺, and Li⁺ concentrations further evidenced the upward hydrothermal through sediments with rates of 0.13 ∼ 124 cm yr⁻¹. The apparently Cl-depleted and slightly Cl-enriched pore fluids in the GLM and MHV hydrothermal fields supported the occurrence of subseafloor phase separation and classified their hydrothermal fluids into vapor-rich and brine-rich phases, respectively. The low pH values (pH = 5.67 ∼ 6.21) with downward increasing trends of pore-fluid dissolved inorganic carbon (DIC, up to 60 mmol L⁻¹) and its heavy isotopic compositions (δ¹³C_DIC = +2.5 ∼ +7.0 ‰) inferred in-situ liquid CO₂-impregnated sedimentary circumstance in the GLM and MHV hydrothermal fields. This sedimentary environment, as demonstrated, enhanced the chemical weathering of silicate and carbonate minerals, resulting in the elevated concentrations of Ca²⁺ and K⁺ in pore fluids. It highlights the impact of acidic and in-situ CO₂-saturated fluids within sediments on geochemical alterations of bulk solids/sediments and interstitial fluids.