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Abstract The Von Damm vent field (VDVF) on the Mid-Cayman Rise in the Caribbean Sea is unique among modern hydrothermal systems in that the chimneys and mounds are almost entirely composed of talc. We analyzed samples collected in 2020 and report that in addition to disordered talc of variable crystallinity, carbonates are a major class of mineral at VDVF. The carbonate minerals include aragonite, calcite, magnesium-rich calcite, and dolomite. Talc and carbonate mineral textures indicate that, rather than replacing volcanic host rock, they precipitate from the mixing of hydrothermal fluids and seawater at the seafloor, occurring in chimneys and surrounding rubble. Alternating precipitation of this mineral assemblage is pervasive, with carbonate minerals typically being succeeded by talc, and with indications that in some cases talc and carbonate minerals replace one another. Stable carbon isotopic data indicate the carbonate minerals originate from the mixing of seawater and hydrothermal fluid, which is supported by U-Th data. Radiocarbon calcite ages and talc 234U-230Th isochron ages indicate mineral ages spanning over thousands to tens of thousands of years. Analyses of these samples illustrate a dynamic system that transitions from carbonate-dominated to Mg-silicate–dominated precipitation over time scales of thousands of years. Our observations raise questions regarding the eventual fate of seafloor precipitates and whether carbonate and silicate minerals in such settings are sequestered and represented in the rock record.
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Fluid Mixing and Spatial Geochemical Variability in the Lost City Hydrothermal Field Chimneys
Abstract Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (87Sr/86Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ13C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower87Sr/86Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate.
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- Award ID(s):
- 1536702
- PAR ID:
- 10492578
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 25
- Issue:
- 2
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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