Sulfur belongs among H
O, CO
, and Cl as one of the key volatiles in Earth's chemical cycles. High oxygen fugacity, sulfur concentration, and δ
S values in volcanic arc rocks have been attributed to ...significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ
S values of approximately -8‰, -1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30-230 km depth, and the predominant sulfur loss takes place at 70-100 km with a net δ
S composition of -2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver
S-enriched sulfur to produce the positive δ
S signature in arc settings. Most sulfur has negative δ
S and is subducted into the deep mantle, which could cause a long-term increase in the δ
S of Earth surface reservoirs.
The hydrothermal alteration of mantle rocks (referred to as serpentinization) occurs in submarine environments extending from mid-ocean ridges to subduction zones. Serpentinization affects the ...physical and chemical properties of oceanic lithosphere, represents one of the major mechanisms driving mass exchange between the mantle and the Earth's surface, and is central to current origin of life hypotheses as well as the search for microbial life on the icy moons of Jupiter and Saturn. In spite of increasing interest in the serpentinization process by researchers in diverse fields, the rates of serpentinization and the controlling factors are poorly understood. Here we use a novel in situ experimental method involving olivine micro-reactors and show that the rate of serpentinization is strongly controlled by the salinity (water activity) of the reacting fluid and demonstrate that the rate of serpentinization of olivine slows down as salinity increases and H
O activity decreases.
Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose ...of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ53Cr values falling within a narrow range of −0.12±0.13‰ (2SD, n=30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ53Cr values of −0.13±0.13‰, −0.11±0.13‰, and −0.07±0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ53Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ53Cr values up to ~0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ53Cr (−0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust.
•Undetectable δ53Cr variation in high temperature rocks•Undetectable δ53Cr variation in a metacarbonate transect that is representative of fluid-rich regional metamorphism•Up to 1‰ δ53Cr variation in a low pH black shale weathering profile•Undetectable δ53Cr variation in mildly altered upper oceanic crust•High δ53Cr and low Cr in serpentinized peridotites
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 fossilized in brucite–calcite veins that are strongly enriched in organic carbon (up to 0.5 wt.% of the total carbon) but depleted in 13C (δ13CTOC= −19.4‰). We detected a combination of bacterial diether lipid biomarkers, archaeol, and archaeal tetraethers analogous to those found in carbonate chimneys at the active Lost City hydrothermal field. The exposure of mantle rocks to seawater during the breakup of Pangaea fueled chemolithoautotrophic microbial communities at the Iberia Margin, possibly before the onset of seafloor spreading. Lost City-type serpentinization systems have been discovered at midocean ridges, in forearc settings of subduction zones, and at continental margins. It appears that, wherever they occur, they can support microbial life, even in deep subseafloor environments.
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 >1wt.%) having high δ34S values (+5 to +10‰). Total carbon contents of bulk rocks are elevated (0.008–0.603wt.%) compared to mantle values and δ13CTotal 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 1.4wt.%) having negative δ34S values (down to −45‰), although local closed-system conditions can lead to reservoir effects and positive δ34S values (up to +27‰). Extensive circulation of cold seawater can cause oxidation, loss of sulfide, and addition of seawater sulfate resulting in high δ34STotal-S values. High total carbon contents (0.006–7.2wt.%) and δ13C 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 0.35–0.64×1011molCy−1 and 0.13–1.46×1011molSy−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 5wt.% water, and an average of 260ppm sulfur is lost as sulfate having δ34S=14.5‰, whereas carbon is unaffected. These volatiles can induce melting and contribute to 34S enrichments and oxidation of the sub-arc mantle wedge. Serpentinized oceanic peridotites carry isotopically fractionated water, carbon and sulfur into subduction zones. Up to 0.49×1011molsulfury−1 and 1.7×1011molcarbony−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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► We summarize data for carbon and sulfur in oceanic and metamorphosed serpentinites. ► Carbon and sulfur sinks in oceanic serpentinite are similar to those in mafic crust. ► No change in sulfur and carbon during metamorphism to antigorite and minor olivine. ► Serpentinite dehydration oxidizes and enriches the mantle in 18O, D, 13C, and 34S.
Serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. ...Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H
2
O) and silica (SiO
2
) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + brucite ± magnetite veins occur in olivine, Al-rich serpentine + talc veins occur in orthopyroxene, and Al-rich serpentine ± talc ± brucite veins occur at the boundary of orthopyroxene and olivine. Bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene- and olivine-dominated areas. Specifically, the sample records (1) varying brucite composition depending on whether or not it is associated with magnetite, (2) formation of magnetite from Fe-rich brucite (±Fe-rich serpentine) during olivine hydration, where magnetite coexists with brucite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxene- and olivine-hosted veins, and 4) local (different) equilibrium assemblages within different zones of veins. The studied sample preserves rarely observed textures documenting continuous replacement of olivine, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization.
Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from ...the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cu-bearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copper-bearing sulfides (Cu-pentlandite, sugakiite Cu(Fe,Ni)
8
S
8
, samaniite Cu
2
(Fe,Ni)
7
S
8
, chalcopyrite, chalcocite, bornite and cubanite), native copper and copper–iron–nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides 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.
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 CH4 and H2. Upon mixing with seawater, these fluids precipitate carbonate, forming an extensive network of calcite veins in the basement rocks, while H2 and CH4 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 peridotite-hosted 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 9.6wt.% C in ophicalcites) and that carbon incorporation decreases with depth. In the Northern Apennine serpentinites, serpentinization temperatures decrease from 240°C to <150°C, while carbonates are formed at temperatures decreasing from ~150°C to <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 <100°C and carbonate formation at <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 50–100m depth of the serpentinite basement at the Iberian Margin. We estimate that annually 1.1 to 2.7×1012g C is stored within peridotites exposed to seawater, of which 30–40% is fixed within the uppermost 20–50m 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.
•We compare the carbon geochemistry of peridotite-hosted hydrothermal systems.•The Ligurian ophiolites are likely an ancient analog to the Atlantis Massif.•Up to 2.7×1012g C is stored within peridotites exposed to seawater.•The extent of carbonate uptake is mainly controlled by the presence of fluid pathways.•Serpentinization plays an important role in the long-term global carbon cycle.
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