ALBERT

Description: 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 9.6 wt.% 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 50-100 m depth of the serpentinite basement at the Iberian Margin. We estimate that annually 1.1 to 2.7 × 1012 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.

Description: © The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Chemical Geology 423 (2016): 19-33, doi:10.1016/j.chemgeo.2016.01.003.

Description: 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) 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.

Description: Funding for this research was provided by Agouron Institute to XLW, National Science Foundation (NSF) EAR-0105927 and EAR-1250269 to JJA, and NSF EAR-1324566 to ES. NJP and CTR acknowledge funding from the Alternative Earths NAI.

Description: 2017-01-12

Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 15517, doi:10.1038/s41598-018-33610-9.

Description: Subduction zones impose an important control on the geochemical cycling between the surficial and internal reservoirs of the Earth. Sulphur and carbon are transferred into Earth’s mantle by subduction of pelagic sediments and altered oceanic lithosphere. Release of oxidizing sulphate- and carbonate-bearing fluids modifies the redox state of the mantle and the chemical budget of subduction zones. Yet, the mechanisms of sulphur and carbon cycling within subduction zones are still unclear, in part because data are typically derived from arc volcanoes where fluid compositions are modified during transport through the mantle wedge. We determined the bulk rock elemental, and sulphur and carbon isotope compositions of exhumed ultramafic and metabasic rocks from Syros, Greece. Comparison of isotopic data with major and trace element compositions indicates seawater alteration and chemical exchange with sediment-derived fluids within the subduction zone channel. We show that small bodies of detached slab material are subject to metasomatic processes during exhumation, in contrast to large sequences of obducted ophiolitic sections that retain their seafloor alteration signatures. In particular, fluids circulating along the plate interface can cause sulphur mobilization during several stages of exhumation within high-pressure rocks. This takes place more pervasively in serpentinites compared to mafic rocks.

Description: This project was supported by NSF-EAR grant 1324566 to E.M.S. and B.C.G., and NSF EAR award 1250470 to M.J.C. We acknowledge support by the German Research Foundation and the Open Access Publication Fund of the Freie Universität Berlin.

Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 8 (2017): 16107, doi:10.1038/ncomms16107.

Description: 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 H2O activity decreases.

Description: This material is based upon work supported by the National Science Foundation under Grant OCE-1459433 to R.J.B. and E.M.S. Consejo Nacional de Ciencia y Tecnología (CONACyT), the Virginia Tech Department of Geosciences and Virginia Tech Graduate School provided partial funding to HML during this study. F.K. was supported by The Andrew W. Mellon Foundation Endowed Fund for Innovative Research.

Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Codillo, E., Klein, F., Dragovic, B., Marschall, H., Baxter, E., Scambelluri, M., & Schwarzenbach‬, E. Fluid‐mediated mass transfer between mafic and ultramafic rocks in subduction zones. Geochemistry Geophysics Geosystems, 23, (2022): e2021GC010206, https://doi.org/10.1029/2021gc010206.

Description: Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass-transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high-pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk-rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg-metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg-rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si-metasomatism of serpentinite in subduction zones is limited by pressure-dependent increase in the silica activity buffered by the serpentine-talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P-T conditions. Alternatively, a different set of mineral assemblages, such as serpentine- or chlorite-rich rocks, may be controlling the coupling-decoupling transition of the plate interface.

Description: M. Scambelluri acknowledges the Italian Ministry of Research MUR for granting the PRIN project n. 2017ZE49E7. This research was funded by NSF-OISE (Office of International Science & Engineering, Petrology & Geochemistry) PIRE, Award #1545903, and the WHOI Ocean Ventures Fund.

Description: 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.