Papers by Esther Schwarzenbach
Contributions to Mineralogy and Petrology, 2015
Proceedings of the National Academy of Sciences of the United States of America, Jan 29, 2015
Subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy... more Subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle (i.e., serpentinite). However, geosphere-biosphere interactions in serpentinite-hosted subseafloor mixing zones remain poorly constrained. Here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a Cretaceous Lost City-type hydrothermal system at the magma-poor passive Iberia Margin (Ocean Drilling Program Leg 149, Hole 897D). Brucite-calcite mineral assemblages precipitated from mixed fluids ca. 65 m below the Cretaceous paleo-seafloor at temperatures of 31.7 ± 4.3 °C within steep chemical gradients between weathered, carbonate-rich serpentinite breccia and serpentinite. Mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity. Dense microbial colonies are fos...
Serpentinization is a common mineralogical process that occurs in ultramafic rocks when water int... more Serpentinization is a common mineralogical process that occurs in ultramafic rocks when water interacts with the primary minerals olivine and pyroxene to form a rock dominated by serpentine. Temperature of hydration and the primary mineralogy of the peridotite strongly influence the alteration products. We studied peridotites with low to extremely low degrees of serpentinization from two settings, the Santa Elena ophiolite in Costa Rica and the Gakkel Ridge, to test how temperature and primary mineralogy control reactions, reaction sequences, and subsequent vein textures during peridotite hydration. Serpentinization of a harzburgite from Costa Rica occurred at around 250°C, resulting in olivine-hosted veins that are a mixture of brucite and serpentine – with a bulk composition approximately equivalent to hydrated host olivine – and later stage veins that also contain magnetite. Serpentinite that formed by hydration of orthopyroxene is accompanied by talc, with a bulk composition (se...
Contributions to Mineralogy and Petrology, 2014
formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic seque... more formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water-rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems.
Geochemistry, Geophysics, Geosystems, 2014
Serpentinization is a planetary process that has important consequences on geochemical cycles, su... more Serpentinization is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H 2 and CH 4 and having the potential to sequester atmospheric CO 2 . We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1 C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0-5.9 mg/L) and K (1.0-5.5 mg/L) and relative high Ca (29-167 mg/L), Na (16-27 mg/L), Cl (26-29 mg/L), hydroxide (41-63 mg/L), and carbonate (31-49 mg/L). Active CH 4 (24.3% v/v) vents coupled with carbonate deposits (d 13 C CO2 5227 to 214&; d 18 O CO2 5217 to 2 6&) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (d 18 O 5 27.9&, d 2 H 5 251.4&) and groundwater (d 18 O 5 27.6&; d 2 H 5 248.0&) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.
Active serpentinization systems are abundant on present-day Earth and are of increasing interest ... more Active serpentinization systems are abundant on present-day Earth and are of increasing interest because waterrock reactions lead to alkaline, Ca-OH fluids that have the potential to sequester CO 2 and form reduced chemical species (e.g. CH 4 and H 2 ) that can support chemolithoautotrophy. We present a study of alkaline (pH 10-12) springs in the Voltri Massif (Italy) that focuses on the sources and cycling of inorganic carbon in the basement rocks, the interacting fluids, and the resulting surface carbonate deposits. Most springs are located in mantle rocks that underwent varying degrees of ocean-floor serpentinization and incorporation of carbon (as carbonate and organic carbon) in the Jurassic, which is preserved during subsequent subduction and uplift onto the continent. The springs are fed by meteoric water that evolves into alkaline, Ca-rich spring waters that have 2-3 times higher Ca concentrations than the adjacent rivers. Our study is consistent with previous reaction path modeling and identifies the formation of clay minerals in serpentinites that have been altered by alkaline fluids. These strongly altered basement rocks contain up to 2 wt.% C in the shallow subsurface, also documented by the presence of late calcite veins. Concentrations of dissolved inorganic carbon (DIC) of the alkaline fluids are generally low (b 16 μmol/L) and δ 13 C DIC is between −24.2‰ and +1.3‰. We argue that the concentrations and isotopic composition of the DIC in the alkaline waters provide evidence for 1) the precipitation of calcium carbonate under closed-system conditions with respect to atmospheric CO 2 and 2) removal of DIC by microbial activity in the subsurface. Late-stage uptake of atmospheric CO 2 in the shallow subsurface or at the exit sites subsequent to water-rock-microbe interactions in the basement result in fluids with lower pH and Ca, and enrichment in DIC and 13 C. At the surface, interaction of the high pH, Ca-OH fluids with atmospheric CO 2 causes precipitation of carbonates as travertines or crusts on the basement rocks, storing CO 2 as calcium carbonate. Carbonate precipitation at the exit sites is strongly dominated by kinetic processes leading to carbon and oxygen isotope compositions as low as −27.2‰ and −18.7‰, respectively. With increasing distance from the springs, the 13 C and 18 O content of the carbonate increases and the fluid pH changes towards neutral. Our study shows that surficial carbonate precipitation plays a subordinate role in carbon sequestration, but that the evolution from Mg-HCO 3 spring waters to Ca-OH waters removes significant amounts of carbon, presumably in the subsurface. We calculate that the serpentinites have the capacity to sequester up to 0.50 to 2.05 × 10 9 mol carbon per year and conclude that they can take up significantly more CO 2 than they currently contain.
At slow and ultraslow mid-oceanic ridges, tectonic extension and crustal thinning lead to the exp... more At slow and ultraslow mid-oceanic ridges, tectonic extension and crustal thinning lead to the exposure of ultramafic rocks on the ocean floor resulting in serpentinization and precipitation of carbonates in open fractures. Serpentinization processes play a major role in the global marine bio-geochemical cycle and account for an important part of the exchange of sulfur and carbon between seawater and
Chemical Geology, 2013
Fluid circulation in peridotite-hosted hydrothermal systems influences the incorporation of carbo... more Fluid circulation in peridotite-hosted hydrothermal systems influences the incorporation of carbon into the oceanic crust and its long-term storage. At low to moderate temperatures, serpentinization of peridotite produces alkaline fluids that are rich in CH 4 and H 2 . Upon mixing with seawater, these fluids precipitate carbonate, forming an extensive network of calcite veins in the basement rocks, while H 2 and CH 4 serve as an energy source for microorganisms. Here, we analyzed the carbon geochemistry of two ancient peridotite-hosted hydrothermal systems: 1) ophiolites cropping out in the Northern Apennines, and 2) calcite-veined serpentinites from the Iberian Margin (Ocean Drilling Program (ODP) Legs 149 and 173), and compare them to active peridotitehosted hydrothermal systems such as the Lost City hydrothermal field (LCHF) on the Atlantis Massif near the Mid-Atlantic Ridge (MAR). Our results show that large amounts of carbonate are formed during serpentinization of mantle rocks exposed on the seafloor (up to 9.6 wt.% C in ophicalcites) and that carbon incorporation decreases with depth. In the Northern Apennine serpentinites, serpentinization temperatures decrease from 240°C to b 150°C, while carbonates are formed at temperatures decreasing from~150°C to b 50°C. At the Iberian Margin both carbonate formation and serpentinization temperatures are lower than in the Northern Apennines with serpentinization starting at~150°C, followed by clay alteration at b100°C and carbonate formation at b 19-44°C. Comparison with various active peridotite-hosted hydrothermal systems on the MAR shows that the serpentinites from the Northern Apennines record a thermal evolution similar to that of the basement of the LCHF and that tectonic activity on the Jurassic seafloor, comparable to the present-day processes leading to oceanic core complexes, probably led to formation of fractures and faults, which promoted fluid circulation to greater depth and cooling of the mantle rocks. Thus, our study provides further evidence that the Northern Apennine serpentinites host a paleo-stockwork of a hydrothermal system similar to the basement of the LCHF. Furthermore, we argue that the extent of carbonate uptake is mainly controlled by the presence of fluid pathways. Low serpentinization temperatures promote microbial activity, which leads to enhanced biomass formation and the storage of organic carbon. Organic carbon becomes dominant with increasing depth and is the principal carbon phase at more than 50-100 m depth of the serpentinite basement at the Iberian Margin. We estimate that annually 1.1 to 2.7 × 10 12 g C is stored within peridotites exposed to seawater, of which 30-40% is fixed within the uppermost 20-50 m mainly as carbonate. Additionally, we conclude that alteration of oceanic lithosphere is an important factor in the long-term global carbon cycle, having the potential to store carbon for millions of years.
Lithos, 2013
We summarize the uptake of carbon and sulfur during serpentinization of seafloor peridotites, and... more We summarize the uptake of carbon and sulfur during serpentinization of seafloor peridotites, and discuss the fate of these volatiles during subduction of serpentinite. We use a simplified classification to divide seafloor serpentinization into high-temperature and low-temperature processes. High-temperature serpentinization typically involves heat and mass transfer from gabbro intrusions, leading to addition of hydrothermal sulfide sulfur (up to >1 wt.%) having high δ 34 S values (+5 to +10‰). Total carbon contents of bulk rocks are elevated (0.008-0.603 wt.%) compared to mantle values and δ 13 C Total C values of −3‰ to −17.5‰ result from mixtures of organic carbon and seawater-derived carbonate. Low-temperature serpentinization is generally characterized by microbial reduction of seawater sulfate, which leads to addition of sulfide sulfur (up to 1.4 wt.%) having negative δ 34 S values (down to −45‰), although local closed-system conditions can lead to reservoir effects and positive δ 34 S values (up to +27‰). Extensive circulation of cold seawater can cause oxidation, loss of sulfide, and addition of seawater sulfate resulting in high δ 34 S Total-S values. High total carbon contents (0.006-7.2 wt.%) and δ 13 C values of −26 to +2.2‰ result from addition of variable proportions of organic carbon and seawater-derived carbonate to serpentinite. We estimate that serpentinization at mid ocean ridges is a sink for 0.35-0.64×10 11 mol C y −1 and 0.13-1.46×10 11 mol S y −1 , comparable to the sinks of these elements per unit volume of mafic oceanic crust. Serpentinization in the subducting plate at subduction zones may further affect chemical budgets for serpentinization. During subduction metamorphism, sulfur and carbon contents remain unaffected by recrystallization of seafloor lizardite and chrysotile to antigorite, and formation of minor olivine. Dehydration of antigorite-serpentinites to chlorite-harzburgites at higher pressure and temperature results in loss of 5 wt.% water, and an average of 260 ppm sulfur is lost as sulfate having δ 34 S=14.5‰, whereas carbon is unaffected. These volatiles can induce melting and contribute to 34 S enrichments and oxidation of the sub-arc mantle wedge. Serpentinized oceanic peridotites carry isotopically fractionated water, carbon and sulfur into subduction zones. Up to 0.49×10 11 mol sulfur y −1 and 1.7×10 11 mol carbon y −1 are subducted in serpentinites, less than 3% of the total subduction budgets for each of these elements. Isotopically fractionated carbon, sulfur, and water remain in serpentinite dehydration products, however, and can be recycled deeper into the mantle where they may be significant for volatile budgets of the deep Earth.
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Papers by Esther Schwarzenbach