Journal Article

Hydrothermal Seepage of Altered Crustal Formation Water Seaward of the Middle America Trench, Offshore Costa Rica

Hutnak, Michael, Parsons, Patrice K. F., Silver, Eli A., Wheat, C. Geoffrey, Fisher, Andrew T.

2024

Overview

Chemical compositions of sediment pore waters are presented from 13 piston and gravity cores that were collected on ∼24 Ma crust of the Cocos Plate seaward of the Middle America Trench and near the onset of crustal faulting from subduction. Cores were collected mainly within a 1.75 km2 area overlying a buried basement topographic high that supports an elevated heat flux, consistent with seawater transport within the upper volcanic crust. Systematic variations in pore water chemical profiles indicate upward seepage speeds (up to 1.7 cm yr−1 providing a net flux of 0.1 L s−1), constrain the chemical composition of the formation water within the underlying upper basaltic basement, and elucidate diagenetic reactions in the sediment. Relative to seawater, formation water has an elevated temperature (70–80°C) and concentrations or values of Ca, chlorinity, Sr, Ba, Li, Fe, Mn, Si, Cs, D/H, and Mo, and lower concentrations or values of Mg, Na, sulfate, alkalinity, TCO2, K, B, F, phosphate, 87Sr/86Sr, δ13C, δ18O, U, and Rb. Although this site is located only 30 km from the trench axis, there is no chemical evidence for subduction‐related hydrologic influences. Instead, the data are explained by a combination of seawater‐basalt reactions within the upper basement and diffusive exchange with overlying sediment, as part of a shallow, ridge‐flank hydrothermal system. It is unclear why this site has an elevated heat flux relative to neighboring crust, but this may result from variations in crustal properties or modification related to flexural faulting outboard of the trench. Plain Language Summary An area of elevated volcanic rock buried below sediment west of the Middle American Trench is unusually warm, with temperatures of 70–80°C compared to more regional‐distributed crust that is vigorously cooled (10–20°C) from circulating seawater (formation water) within the upper volcanic crust. Because of the proximity to subduction‐related crustal faults, sediment cores were collected from this area of warm crust and interstitial sediment pore waters were extracted and analyzed to assess if and how this area with a high heat flux is linked to subduction processes, potentially providing a source of water that serpentinizes the underlying mantle. Chemical data from sediment pore waters confirm upward seepage, the composition of crustal formation water in underlying volcanic rocks, and the extent of reactions as the formation water ascended the sediment column. Crustal formation water appears to be unaffected by nearby plate subduction. Instead, this warm and chemically altered formation water defines a ridge‐flank hydrothermal system, which is distinct from hydrothermal systems to the west and south. How this hydrothermal system became isolated is unknown, but it could be a result of aging and evolving crustal properties or of plate faulting related to subduction processes. Key Points Pore water chemical profiles reveal sediment diagenesis, seepage speeds up to 1.7 cm yr−1, and discharge from 1.75 km2 of 0.1 L s−1 Crustal formation water is warm (∼75°C) and chemically altered, stemming from water‐basalt reactions and diffusive exchange with pore water This ridge‐flank hydrothermal system is hydrologically isolated from the ventilated crust to the west and the trench to the east

• Publisher: John Wiley & Sons, Inc
• Subject: Alkalinity / Basalt / Chemical composition / Chlorinity / Cores / Diagenesis / Fluctuations / formation water / Gravity / Heat flux / Heat transfer / High temperature / hydrogeology / hydrothermal / Manganese / Middle American Trench / Offshore / Phosphates / Plates / Pore water / ridge flank / Seawater / Sediment / Sediments / Strontium isotopes / Subduction / Sulfates / Volcanic rocks