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doi: 10.7185/geochemlet.1916 Tectonically induced bending of incoming plates at subduction zones can result in normal faulting in the upper ocean crust. Seismic surveys and numerical models indicate enhanced permeability and fluid circulation when this occurs. Yet, direct geological evidence of such effects on the seafloor is lacking. Here we report Human Occupied Vehicle (HOV) based observations of the existence of fluid discharge features on the seafloor of the incoming plate of the Mariana subduction zone. These features include fluid discharge points and associated pockmarks, which are striking, and occur in abundance in several depth related fields. The existence of Galatheid crabs, a typical seep related organism, also indicates fluid discharge from the seafloor. Alteration of the coexisting basaltic ocean crust is extensive, with iddingsite-rich muds within and overlapping the apparent fluid discharge zones. Our findings are significant in that they suggest that structural deformation of the incoming plate could substantially influence chemical exchange between the upper ocean crust and seawater in a new way. We further suggest that these fluid discharge points may represent previously unknown niches for H2-based chemolithotrophic life and microbial ecosystems at deep trenches. Observations reported here contrast both chemically and physically with serpentine mud volcano formation associated with the shallower Mariana forearc region. Received 31 January 2019 | Accepted 9 May 2019 | Published 5 June 2019 1. Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya, Hainan 572000, China 2. Department of Earth Science, University of Minnesota, 310 Pillsbury Drive E, Minneapolis, Minnesota 55455, USA 3. 954 National Center, U.S. Geological Survey, Reston, Virginia 20192, USA * Corresponding author (email: xtpeng@idsse.ac.cn) Letter Subduction of the oceanic crust can result in large scale structural effects, especially at the hinge point, where extension and normal faulting tend to occur (Masson, 1991). Previous studies reveal that seawater derived fluids can penetrate deeply into the ocean crust owing to the enhanced permeability that these structures provide (Ranero et al., 2003, 2005; Faccenda et al., 2009). One fate for crustal fluids is to propagate downward, hydrating the lower ocean crust and upper mantle (Ranero et al., 2003; Grevemeyer et al., 2005, 2007; Faccenda et al., 2008, 2009), while another might involve upward flow and discharge at the seafloor, made possible by tectonically induced forces. Not surprisingly, however, little direct evidence exists in support of these sorts of phenomena on the seafloor of the incoming plates at subduction zones. Investigations of old and highly tectonised portions of the incoming plate of the southern Mariana trench were conducted using the HOV Jiaolong, which has an operating depth of 7000 m. The HOV study resulted in the discovery of a fluid discharge field (site M). The field lies on the base of a slope of a small scale fault at a water depth of 5448 m, making it the deepest fluid discharge field yet reported for this region of the ocean floor (Fig. 1, Table S-1, Video S-1). In contrast with earlier observations of serpentine mud volcanoes in the Mariana forarc region (Hulme et al., 2010; Fryer, 2012; Plümper et al., 2017), mineralisation reported here is directly associated with variably altered mafic rocks that crop out on the trench slope, and is dominated by iddingsite (hydrous ferric silicate) not serpentine. Initial surveys of the surrounding area indicate that the field extends for about 100 m across the slope and hosts at least 2 small fluid discharge points and 4 pockmarks (Fig. 2). Fluid discharge points are about 1 m in height and 2-5 m in diameter, while associated pockmarks have a diameter of 3-6 m. Galatheid crabs, which are common seep-associated consumers, were observed and recovered. Similar small pockmarks were also discovered at two dive sites N and P, at water depths of 6300 and 6669 m, respectively. Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2019) 11, 1-5 | doi: 10.7185/geochemlet.1916 2 Raman spectroscopy, X-ray powder diffraction and optical microscopy analysis show that unaltered basement rocks in the vicinity of site M contain abundant augite and labradorite, as is typical of basaltic protolith. When altered, however, iddingsite formation dominates (Figs. 3 and S-3). Iddingsite is a reddish-coloured alteration product of pyroxene and olivine and commonly exhibits variable chemical composition, while often coexisting with iron oxide and clay minerals (Edwards, 1938; Smith et al.,1987; Kuebler, 2013). Electron microprobe analyses reveal that iddingsite in altered rocks and surficial sediment recovered during the present study consists of SiO2 (39.8-54.3 %), Fe2O3 (24.6 to 34.5 %) and MgO (2.7-4.1 %) (Table S-2). Raman spectra show that iddingsite is intimately associated with goethite and two groups of peaks belonging to a polymerised silicate phase (Fig. S-1). The ubiquitous presence of iddingsite at site M and elsewhere, together with clear evidence of iddingsite association with augite (Fig. S-2), indicates a replacement origin. In light of the augite-dominated mineral composition of basement rocks, we suggest that iddingsite is mainly derived from the alteration reaction of pyroxene rather than olivine. Based on the oxygen isotope composition of carbonates extracted from altered rocks, we estimate that temperatures at which the alteration reaction occurs range from 93 to 130 oC (Table S-3). An overall representation of chemical exchange leading to iddingsitisation can be depicted as follows: 4CaMg0.5Fe0.5Si2O6 (augite) + 10H+ + 10H2O = MgFe2Si3 O10 .4H2O (iddingsite) + 5H4SiO4 (aq) + Mg2+ + 4Ca2+ + H2. The apparent removal of mobile alkaline earth cations and silica during iddingsite formation processes is evident (Tables S-2, S-4). The formation and release of dissolved H2, however, is of particular interest and highly significant (Table S-5). Indeed, the conspicuous abundance of dissolved hydrogen and methane in push core samples provides compelling evidence of oxidation of iron components by water reduction in the metalliferous muds on the seafloor and likely in broadly analogous chemical and mineralogical systems beneath the seafloor (Seewald et al., 2003; Seyfried et al., 2003), feeding surficial depositsa process in keeping with the existence of moderately reducing conditions and low temperatures. This is consistent with previous analyses of crustal fluids (Cowen et al., 2003; Lin et al., 2014) and thermodynamic calculations (McCollom and Bach, 2009; Bach, 2016), which corroborated that basaltic rocks of ocean crusts could interact with circulating seawater and generate nanomolar to micromolar H2 below a temperature of 120 oC. Recent laboratory experiments have also demonstrated that the transfer of electrons between Fe(II) in minerals and water promotes molecular hydrogen generation at 55 and 100 oC (Mayhew et al., 2013). The conspicuous abundance of dissolved hydrogen and methane in push core samples (Table S-4) provides compelling evidence of oxidation of iron components by water reduction in the metalliferous muds and more likely in broadly analogous chemical and mineralogical systems beneath the seafloor, feeding surficial deposits. Fluid discharge in the southern Mariana subduction zone might be driven by tectonic overpressure under compressional regimes (Westbrook et al., 1983; Moore, 1989; Shipley et al., 1990; Kopf, 1999; Kopf et al., 2001), in keeping with the effects of fault-induced bending (Figs. 1, S-1). During Jiaolong dives, strong deformation associated with faulting in the vicinity of fluid discharge points was commonly observed (Fig. S-1). Tectonic pressure variation due to deformation is an intrinsic property of fracture under geological conditions. Strong tectonic pressure gradients are instrumental in driving local pore fluid flow during deformation (Mancktelow, 2008; Faccenda et al., 2009). Due to the tectonic overpressure, pore fluid in the upper basaltic layer could be either pumped downward producing serpentinisation or be expelled upward into the overlying layers as the slab subducts (Faccenda et al., 2009). Emplacement and sudden release of excess pore pressures could ultimately lead to the eruption of fluids on seafloor Figure 1 (a) Location of the newly discovered fluid discharge field at site M and two pockmarks at sites N and P (red dots) on the incoming plate of Mariana trench. (b) Enlarged shaded-relief map of red box in (a). The sites M, N and P (green circles) are situated on the base of the slopes of bending-related faults that have been extensively developed and pervasively cross the entire trench slope of the incoming plate in response to plate flexure. White triangles show the axis of the southern Mariana trench. The known serpentinite mud volcanoes (black dots in (a)) are exclusively located on the overriding plate of Mariana trench. White bars in (b) show blank areas where no multi-beam bathymetric data are available.

Fluid discharge linked to bending of the incoming plate at the Mariana subduction zone