Modern Vent Research Articles

Fluid chemistry of Archean seafloor hydrothermal vents: Implications for the composition of circa 3.2 Ga seawater

Seafloor hydrothermal vents of mid-Archean age (ca. 3230 Ma) have been identified and mapped in the Barberton greenstone belt, South Africa and are known as the Ironstone Pods. Fluid inclusion homogenization temperature data, when combined with gas chromatographic data, provide a minimum calculated water depth for the pods of 982 m. Ironstone Pod hydrothermal fluid endmember concentrations (Mg = 0) of various dissolved components derived from bulk fluid inclusion crush-leach experiments, include: Cl(730 mmol/L), Br (2.59), I (0.058), Na (822), NH 4 (11.4), K (21.5), Ca (42.6), and Sr (0.15). This hydrothermal fluid also contains up to 1.07 mol% CO 2, 0.03 mol% N 2, 0.02 mol% CH 4, 262 ppm COS, and minor amounts of C2–C4 hydrocarbons. Hydrothermal endmember Ca, Sr, and NH 4, in particular, and to a lesser degree K, I, and CO 2, commonly plot on, or very close to, modern vent fluid trends. By contrast, endmember Na and Br concentrations are distinct (higher) from modern vent fluids. High I and NH 4 concentrations are consistent with contributions from sediments and/or organic matter. Calculated δ 18O H 2O values for the pod hydrothermal endmember fluid define a narrow range from 0.9 to 1.6‰ very similar to that of modern vent fluid values (0.4–2.1‰). A best estimate for the Ironstone Pod seawater endmember composition is Cl (920 mmol/L), Br (2.25), SO 4 (2.3), I (0.037), Na (789), NH 4 (5.1), K (18.9), Mg (50.9), Ca (232), and Sr (4.52). Barberton seawater components are commonly within an order of magnitude of modern seawater values, with the exception of significantly higher 1, NH 4, Ca, and Sr in the inclusions. Sulfate concentrations are minimum estimates for Barberton seawater. Fluid inclusion samples containing the greatest amount of seawater component have higher N 2 (up to 0.1 mol%) and low CO 2, when compared to samples dominated by the hydrothermal endmember fluid. Barberton ambient seawater is considered to have been an evaporative brine of NaClCaCl 2 composition during the time of pod deposition. Ironstone Pod fluid inclusion seawater endmember Br/Cl and I/Cl values of 2.45 × 10 −3 and 40.2 × 10 −6, respectively, are within error of bulk Earth (2.38 × 10 −3 and 190 × 10 −6) and are consistent with the chemistry of 3.23 Ga Barberton seawater being buffered by the mantle.

The Cl−[sbnd]Br−[sbnd]I− composition of ∼3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry

Fluid inclusion leachates obtained from vug and vein quartz samples from an Archean (∼3.23 Ga) Fe-oxide hydrothermal deposit in the west-central part of the Barberton greenstone belt, South Africa, were analyzed by ion chromatography for chloride, bromide, and iodide. The deposit, known as the ironstone pods, formed by seafloor hydrothermal activity and fluid discharge. Quartz is dominated by type I liquid-vapor, aqueous inclusions with a bimodal salinity distribution (0–0.25 MCl− and 0.9–1.8 MCl−). Bulk analytical salinities range from 0.45 to 0.99 MCl− represent averages of type I inclusions. Bulk fluid inclusion bromide and iodide concentrations are 1.44–3.32 mM and 0.01–0.12 mM, respectively. For comparison, modern seawater has halogen contents of 590 mM chloride, 0.9 mM bromide, and 0.5 μM total iodine. In the fluids from the ironstone pods, bromide and iodide are enriched relative to chloride, when compared with modern seawater.Approximate Br−Cl− and I−Cl− ratios of 3.2 Ga Barberton seawater are 2.5 × 10−3 and 40 × 10−6, respectively. Dispersion to higher values was caused principally by reaction with organic sediments whose trends are similar to those seen for modern vent fluids at unsedimented and sedimented ridges, relative to modern seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromine and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values.

Silurian hydrothermal-vent community from the southern Urals, Russia

MODERN hydrothermal-vent communities are remarkable for being dependent on bacterial chemosynthetic primary production and for having a high percentage of endemic taxa (95% at the species level)1–3. Based on phylogenetic analyses, it has been suggested that some of these taxa are Mesozoic or even Palaeozoic relicts, and that the vent environment has thus acted as a refuge against evolutionary pressures, such as mass extinctions, that affect other ecosystems1,2,4. However, little is known about ancient vent communities because fossils have been reported from very few5–11 of a thousand or so documented vent deposits12. Here we describe a macrofossil assemblage of monoplacophoran molluscs, inarticulate brachiopods, vestimentiferan tube-worms and other tubes, probably of polychaete origin, from the Silurian Yaman Kasy deposit12. The assemblage represents the oldest, and most diverse, fossil hydrothermal-vent community known, and shares vestimentiferan and polychaete tube-worms with both modern vent communities1,2 and other ancient vent assemblages7–12, but is unique in having brachiopods and monoplaco-phorans. Modern vent communities are not refuges for these Silurian shelly vent taxa, a finding that may have implications for the refuge hypothesis.

Early Carboniferous low-temperature hydrothermal vent communities from Newfoundland

LIVING vent communities, dependent on microbial chemosynthesis rather than photosynthesis, were discovered only in the 1970s1. Since then, six fossil vent communities have been identified; they range in age from early Carboniferous to early Tertiary2–7. Here we describe fossil tubes, an abundant low-diversity fauna and sulphide mineralization in 340-Myr-old Carboniferous carbonate mounds in Newfoundland (Fig. 1a). These features, together with evidence for microbial activity, point to the existence of a seventh chemosynthetic community, clustered around low-temperature hydrothermal vents. The remarkable preservation of this Newfoundland community allows a direct comparison to be made with the composition and structure of modern vent communities.