Dissolved inorganic carbon isotopic composition of low-temperature axial and ridge-flank hydrothermal fluids of the Juan de Fuca Ridge

摘要

We analyzed stable carbon (δ~(13)C) and radiocarbon (Δ~(14)C) isotopes of ocean crustal fluid samples from two low-temperature environments on and near the Juan de Fuca Ridge (JDFR), a seafloor spreading center in the northeastern Pacific Ocean. The major goals of this work were to resolve relative dissolved inorganic carbon (DIC) sources and removal processes, and characterize the isotopic signatures of DIC vented to the overlying ocean. DIC was isolated from diffuse vents on the Main Endeavour Field (MEF), on zero age seafloor, and from two ridge-flank sites located 100 km to the east, on 3.5 Ma seafloor; the Baby Bare outcrop and Ocean Drilling Program (ODP) Hole 1026B. Low-temperature MEF fluids were enriched in DIC (3.13 to 5.51 mmol kg~(-1)) relative to background seawater (2.6 mmol kg~(-1)), and their Δ~(14)C and δ~(13)C values are consistent with simple two-endmember mixing of pure high-temperature (≥ 300 ℃) hydrothermal fluid and bottom seawater (secondary recharge). These data suggest that no major sedimentary or biological DIC sources are present, however our results do not preclude a minor sedimentary influence on low-temperature MEF vent fluids. DIC △~(14)C and μ~(13)C values of ridge-flank fluids from this area are consistent with an off-axis recharge source, followed by water-rock interaction at moderate temperatures (60-70 ℃) during flow through basement. An observed offset in radiocarbon ages between fluids from Baby Bare outcrop and Hole 1026B (～1100 yr) is consistent with crustal flow from south to north, at rates similar to those inferred from other geochemical and thermal tracers. The ridge-flank hydrothermal fluids are strongly depleted in DIC and δ~(13)C relative to bottom seawater, suggesting more extensive carbon removal in this setting (～5.7×10~(12) mol C yr~(-1)) than has been previously suggested. DIC isotopic depletion is consistent with carbonate vein precipitation in conjunction with a minor addition of CO_2 from basalt vesicles, and suggests that ridge-flank systems may be an important sink for seawater inorganic carbon, and comprise an important global reservoir of isotopically depleted and "pre-aged" DIC.