Brucite chimney formation and carbonate alteration at theShinkaiSeepField, a serpentinite‐hosted vent system in thesouthernMariana forearc

Abstract

Brucite-carbonate chimneys were discovered from the deepest known ($5700 m depth) serpentinite-hosted ecosystem-the Shinkai Seep Field (SSF) in the southern Mariana forearc. Textural observations and geochemical analysis reveal three types (I-III) of chimneys formed by the precipitation and dissolution of constitutive minerals. Type I chimneys are bright white to light yellow, have a spiky crystalline and wrinkled surface with microbial mat and contain more brucite; these formed as a result of rapid precipitation under high fluid discharge conditions. Type II chimneys exhibit white to dull brown coloration, tuberous textures like vascular bundles, and are covered with grayish microbial mats and dense colonies of Phyllochaetopterus. This type of chimney is characterized by inner brucite-rich and outer carbonate rich zones and is thought to have precipitated from lower fluid discharge conditions than type I chimneys. Type III chimneys are ivory colored, have surface depressions and lack living microbial mats or animals. This type of chimney mainly consists of carbonate, and is in a dissolution stage. Stable carbon isotope compositions of carbonates in the two types (I and II) of active chimneys are extremely 13 C-enriched (up to 124.1&), which may reflect biological 12 C consumption under extremely low dissolved inorganic carbon concentrations in alkaline fluids. Type III chimneys have 13 C compositions indicating re-equilibration with seawater.Our findings demonstrate for the first time that carbonate chimneys can form below the carbonate compensation depth and provide new insights about linked geologic, hydrologic, and biological processes of the global deep-sea serpentinite-hosted vent systems.

Cool, alkaline serpentinite formation fluid regime with scarce microbial habitability and possible abiotic synthesis beneath the South Chamorro Seamount

“…Serpentinization-associated geofluid systems, typically characterized by an abundance of H 2 and/or highly alkaline fluids, have been discovered at a range of tectonic/ geological settings on the modern Earth where ultramafic rocks encounter water circulation. The most notable examples include slow-spreading mid-oceanic ridges (Charlou et al 2002;Konn et al 2015), ocean core complexs (Kelley et al 2001;Früh-Green et al 2003), ophiolites (Barnes et al 1978;Schrenk et al 2013), coastal springs (Barnes et al 1967), onshore volcanic hot springs (Homma and Tsukahara, 2008;Suda et al 2014), inner trench slope along the southern Mariana forearc (Ohara et al 2012;Okumura et al 2016a;Onishi et al 2018), and forearc serpentinite seamounts (Mottl et al 2004;Fryer, 2011).…”

“…Party 2002;Wheat et al 2008). When the observatory seal was opened on 20 March 2003, 2 years after its installation, over-pressurized fluids discharged from the CORK outlet Kelley et al 2001Charlou et al 2002Stern et al 2014Früh-Green et al 2003Okumura et al, 2016aOnishi et al 2018Proskurowski et al 2008Kawagucci et al Progress in Earth and Planetary Science (2018 pipe (this fluid is referred to as the 'CORK fluid' hereafter) at a flow rate of~0.2 L/s. The endmember chemical composition of the CORK fluid, estimated from two fluid samples diluted with entrained ambient seawater, was generally similar to those from the porewater samples from deeper zones of the ODP core (Wheat et al 2008).…”

Life is a verb, not a noun

“…Indeed, that alkaline springs may have played a part in some of these developments seems inescapable. The precipitates comprised the brucite-structured mixed-valence oxyhydroxide, green rust [~Fe 2 (OH) 5 ] in place of the true brucite [Mg(OH) 2 ] forming modern chimneys and mounds (Table 1; Arrhenius, 2003;Okumura et al, 2016;Halevy et al, 2017). And when combined with silica, greenalite would have been the Hadean/Archean phyllosilicate precipitated in place of the smectites comprising the mounds and chimneys in the Strytan field, although hisingerite appears to have been common to both the Strytan field and the banded iron formations (Table 1) (Badaut et al 1985;Meunier et al, 2010;Tosca et al, 2016;Rasmussen et al, 2017;Stanulla et al, 2017;Price et al, 2017).…”

Twenty Years of Subduction Zone Science: Subduction Top to Bottom 2 (ST2B-2)

“…• Increased appreciation of the role that fluids play in subduction margin mechanics and seismogenesis (Saffer and Tobin, 2011); • Seismological advances that better resolve earthquake structure and mechanisms in the downgoing plate (e.g., Rondenay et al, 2008;Shillington et al, 2015) and improved tomography to image the subducted slab at depths greater than the 670 km limit of earthquakes (van der Hilst et al, 1997); • Accelerating exploration of the deep oceanic trenches because of technological advances in manned submersibles, remotely operated vehicles, and autonomous undersea vehicles (e.g., Cui et al, 2013;Okumura et al, 2016 et al, 2012a, 2012b), the volumetric importance of subcrustal accretionary underplating (e.g., Bassett et al, 2010) versus frontal accretion, as well as providing insights about chemical cycling in and above subduction zones (see the review by Bebout, 2014, and references therein); • Studies of ultrahigh-pressure metamorphic rocks, coupled with thermomechanical models, demonstrating that oceanic and continental crust can be subducted to >100 km depth and returned to the surface (e.g., Gerya et al, 2002;Yamato et al, 2008); • Improved understanding of the nature of supercritical fluids, where they exist in and above subduction zones, and their mass transport capabilities (via experimental and theoretical approaches; e.g., Manning, 2004;Hermann et al, 2006), and appreciation of the tremendous amounts of subducted water that could be stored in the mantle transition zone; • Microanalytical advances allowing measurement of volatiles and trace element contents in minerals and melt inclusions, further constraining chemical cycling through subduction zones (Frezzotti et al, 2011) and the causes of explosive arc volcanism (e.g., Wallace, 2005;Zellmer et al, 2015); • Improved understanding of chemical recycling via subduction of oceanic crust, sediment, and uppermost mantle (e.g., Plank, 2005), especially the cycling of volatiles at convergent margins (e.g., Hilton et al, 2002;Mason et al, 2017) and technical advances enabling field measurements of arc volcanic gas emissions (e.g., Fischer and Lopez, 2016).…”