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Durham Research Online Durham Research Online Deposited in DRO: 25 November 2019 Version of attached le: Published Version Peer-review status of attached le: Peer-reviewed Citation for published item: Albers. E., and Bach, W. and Klein, F. and Menzies, C.D. and Lucassen, F. and Teagle, D.A.H (2019) 'Fluidrock interactions in the shallow Mariana forearc : carbon cycling and redox conditions.', Solid Earth discuss., 10 (3). pp. 907-930. Further information on publisher's website: https://doi.org/10.5194/se-10-907-2019 Publisher's copyright statement: c Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Additional information: Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details. Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk Solid Earth, 10, 907–930, 2019 https://doi.org/10.5194/se-10-907-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions Elmar Albers1, Wolfgang Bach1, Frieder Klein2, Catriona D. Menzies3,a, Friedrich Lucassen1, and Damon A. H. Teagle3 1Department of Geosciences and MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, 28359, Germany 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 3School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, SO14 3ZH, UK anow at: Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3FX, UK Correspondence: Elmar Albers ([email protected]) Received: 12 March 2019 – Discussion started: 22 March 2019 Revised: 19 May 2019 – Accepted: 24 May 2019 – Published: 24 June 2019 Abstract. Few data exist that provide insight into pro- to C4 ‰) and likely derived from the decarbonation of cal- cesses affecting the long-term carbon cycle at shallow fore- careous sediment and/or oceanic crust. These findings pro- arc depths. To better understand the mobilization of C in vide evidence for the mobilization of C in the downgoing sediments and crust of the subducting slab, we investigated slab at depths of < 20 km. Our study shows for the first time carbonate materials that originate from the subduction chan- in detail that a portion of this C forms carbonate precipitates nel at the Mariana forearc (< 20 km) and were recovered in the subduction channel of an active convergent margin. during International Ocean Discovery Program Expedition This process may be an important asset in understanding the 366. Calcium carbonates occur as vein precipitates within deep carbon cycle since it highlights that some C is lost from metavolcanic and metasedimentary clasts. The clasts repre- the subducting lithosphere before reaching greater depths. sent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the fore- arc seafloor by serpentinite mud volcanism. Euhedral arag- 1 Introduction onite and calcite and the lack of deformation within the veins suggest carbonate formation in a stress-free environ- Subduction of oceanic lithosphere plays a crucial role in geo- ment after peak metamorphism affected their hosts. Inter- chemical cycling on Earth, as subduction zones are a primary growth with barite and marked negative Ce anomalies in car- locus of transfers between exogenic and endogenic reser- bonate attest the precipitation within a generally oxic envi- voirs. During subduction of H2O- and C-rich sediments as ronment, that is an environment not controlled by serpen- well as hydrated and carbonated lithosphere, a continuum of tinization. Strontium and O isotopic compositions in car- prograde diagenetic and metamorphic reactions in the down- 87 86 18 bonate ( Sr= Sr D 0.7052 to 0.7054, δ OVSMOW D 20 to going slab drive its devolatilization and dehydration (e.g., 24 ‰) imply precipitation from slab-derived fluids at tem- Kerrick and Connolly, 2001; van Keken et al., 2002). The peratures between ∼ 130 and 300 ◦C. These temperature es- nature and extent of these processes determine the solubil- timates are consistent with the presence of blueschist facies ity of components in the slab-derived fluids and hence de- phases such as lawsonite coexisting with the carbonates in termine if elements are transported into the suprasubduction 13 some veins. Incorporated C is inorganic (δ CVPDB D −1 ‰ lithosphere or remain in the slab and are subducted deeply into Earth’s mantle. Published by Copernicus Publications on behalf of the European Geosciences Union. 908 E. Albers et al.: Fluid–rock interactions in the shallow Mariana forearc The largest portion of C on Earth is stored in the litho- shallow levels, given that the conditions such as the fluid flux sphere, and because subduction is the main route for re- and the proportion of calcite in the slab are favorable. In the cycling of this shallow C, a thorough understanding of Costa Rican margin, for instance, Barry et al. (2019) suggest (de-)carbonation processes in the subduction channel is cru- that more than 10 % of the subducted C may be mobilized at cial to understanding Earth’s carbon cycle (e.g., Sleep and forearc depths, as derived from studying C and He isotopes Zahnle, 2001). Geochemical flux estimates of C in conver- in hot springs from two transects across the forearc. gent plate boundary settings have thus gained much attention Slab-derived fluids that upwell in a number of serpenti- in the recent past (e.g., Kerrick and Connolly, 2001; New- nite mud volcanoes at the deeply faulted Mariana forearc af- ton and Manning, 2002; Gorman et al., 2006; Dasgupta and firm the suggestion of carbonate breakdown at shallow sub- Hirschmann, 2010; Füri et al., 2010; Frezzotti et al., 2011; duction depths. The compositions of these fluids reflect pro- Sverjensky et al., 2014; Ague and Nicolescu, 2014; Kelemen grade metamorphic processes in the subducting lithosphere and Manning, 2015; Piccoli et al., 2016; Barry et al., 2019). and hence provide a direct, natural window into an active Yet, the fate of C in subduction zones remains unconstrained subduction system (e.g., Fryer et al., 1985, 1995; Hulme with recent mass balance estimates being extremely broad et al., 2010). In particular fluids upwelling at the deeper- and suggesting that between 1 and ∼ 80 % of the 40 to rooted mud volcanoes exhibit high carbonate alkalinities 2− 66 Mt C subducted per year is transported back into the con- (CO3 ) and elevated concentrations of dissolved hydrocar- vecting mantle (Kelemen and Manning, 2015). bons (CH4 and C2H6), indicative of decarbonation reactions Some insight into the deep carbon cycle is derived from in the downgoing plate at depths of < 30 km (Mottl et al., studying magmatic products of arc volcanism. It has been 2003; Hulme et al., 2010; Fryer et al., 2018b). shown that C is mobilized below volcanic arcs and trans- In peridotite–water systems, carbonate minerals can pre- ported into the upper plate’s mantle and/or back into the at- cipitate over a wide range of pressure and temperature con- mosphere as CO2 in volcanic emissions (e.g., Kerrick and ditions and in various natural settings. In shallow parts of Connolly, 1998; Sleep and Zahnle, 2001; Blundy et al., the basement, carbonates can form during heating of seawa- 2010). However, these insights only reflect processes occur- ter, cooling of hydrothermal fluids, mixing of hydrothermal ring at deep levels of subduction below depths of 70 km, fluids with seawater, and metasomatic replacement reactions and the amount of CO2 emitted from arc volcanoes does (e.g., Bach et al., 2011; Klein et al., 2015; Schroeder et al., not balance the amount of C initially being subducted (e.g., 2015; Grozeva et al., 2017); carbonation of peridotite may Dasgupta and Hirschmann, 2010; Kelemen and Manning, also occur under conditions similar to those of intermediate 2015). A significant portion of subducted C must hence ei- depths of forearc regions, as indicated by laboratory experi- ther be released from the downgoing slab before reaching ments at 1 to 2 GPa and 500 to 600 ◦C (Sieber et al., 2018). these depths or it is transported to even greater depths (e.g., However, to date no carbonated mantle wedge peridotites Kelemen and Manning, 2015). Geologic processes at both have been identified from the serpentinite mud volcanoes in shallower (i.e., forearc) and greater depths (i.e., backarc) re- the Mariana forearc. Carbonate precipitates from serpentinite main poorly understood since obtaining natural samples from mud volcanoes so far described have mostly formed where these regions is not technologically possible. Insight into the upwelling fluids mixed with seawater near/at the seafloor shallow system, however, comes from the investigation of (Haggerty et al., 1991; Gharib, 2006; Tran et al., 2014), ex- exhumed high-pressure metamorphic units and/or ophiolite cept for few samples that are thought to have formed at tem- sequences that commonly include obducted forearc materi- peratures of up to 250 ◦C from slab-derived fluids (Alt and als (e.g., Frezzotti et al., 2011; Cook-Kollars et al., 2014; Shanks III, 2006). The latter have not been studied in detail.
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