Fluid Circulation Along an Oceanic Detachment Fault: Insights From Fluid Inclusions in Silicified Brecciated Fault Rocks (Mid‐Atlantic Ridge at 13°20′N)

Abstract

The MAR 13°20′N corrugated detachment fault is composed of pervasively silicified mafic cataclastic breccias, instead of ultramafics and gabbros commonly found at other detachments. These breccias record overplating of hangingwall diabases, with syntectonic silicification due to important influx of silica‐iron‐rich fluids, able to leach alkalis and calcium. Fluids trapped in quartz inclusions show important salinity variations (2.1–10 wt.% NaCl eq.) indicating supercritical phase separation. Fluid inclusions also contain minor amounts of H2 ± CO2 ± CH4 ± H2S, with high H2/CO2 and H2/H2S ratios, signatures typical of ultramafic‐hosted vent fluids. We propose that seawater infiltrated the hangingwall upper crust at the axis adjacent to the active detachment, reaching a reaction zone at the dyke complex base (∼2 km). At >500°C, fluids become Si‐rich during diabase alteration (amphibolite‐facies alteration in clasts), and undergo phase separation. Brines, preferentially released in the nearby detachment fault during diabase brecciation, mix with serpentinite‐derived fluids bearing H2 and CH4. Cooling during detachment deformation and fluid upward migration triggers silica precipitation at greenschist‐facies conditions (quartz + Fe‐rich‐chlorite ± pyrite). Important variations in fluid inclusion salinity and gas composition at both sample and grain scales record heterogeneous fluid circulation at small spatial and short temporal scales. This heterogeneous fluid circulation operating at <2 km depth, extending both along‐axis and over time, is inconsistent with models of fluids channeled along detachments from heat sources at the base of the crust at the fault root. Present‐day venting at detachment footwall, including Irinovskoe, is instead likely underlain by fluid circulation within the footwall, with outflow crossing the inactive detachment fault near‐surface.

Detachment-parallel recharge explains high discharge fluxes at the TAG hydrothermal field

“…A corollary is that beneath TAG high temperature fluid-rock interaction within the detachment mainly occurs close to the heat source in the footwall and not because of longdistance channelized flow along it. This would also be consistent with recent findings based on fluid inclusion data from a corrugated detachment fault on the MAR at 13 • 20 ′ N 45 , where a clear link between deformation and high temperature fluid rock interaction was established. However, the conclusion was drawn that this interaction happened within a reaction zone at depth, which was later exhumed by faulting.…”

“…The geometry of the detachment fault is based on high-resolution bathymetric data 32 and P-wave velocity analysis 36 . According to the estimation of the detachment fault thickness (70 − 100 m) in previous studies one TAG 36,45 and seismic evidence for detachment fault thickness (33.4 ± 5.7m) in Wooodlark basin 54 , we set it to be 50 m. In addition, the numerical model is based on the hypothesis that the TAG hydrothermal system is driven by shallow intrusion(s).…”

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

“…Near-ridge high-temperature (> 500 °C) hydrothermal alteration of gabbros in reaction zones is classically accompanied by phase separation leading to the formation of brines (up to ~40-50 wt.% NaCl eq.) and vapor phases (e.g., Delaney et al, 1987;Kelley & Delaney 1987;Vanko, 1988;Nehlig, 1991;Kelley et al, 1992;Alt et al, 2010;Castelain et al, 2014;Verlaguet et al, 2020). In contrast to the upward migration of low-density vapor phases, lessbuoyant brines may be stored at depth in gabbro porosity (Fontaine and Wilcock, 2006;Fontaine et al, 2007) and trapped as fluid inclusions in newly-formed amphibole (and epidote).…”

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

“…Research on detachment faulting at OCCs implies that oceanic spreading is closely linked to the development of hydrothermal circulation patterns and encompasses a wide variety of fluid flow and hydrothermal regimes (McCaig et al, 2007;Escartin et al, 2008). High-temperature fluid circulation both within the footwall and along the fault zone is well documented on the basis of mineralogy and geochemistry (Schroeder and John, 2004;Boschi et al, 2006a;McCaig et al, 2010, Picazo et al, 2012, Verlaguet et al, 2021. Importantly, uplift along detachment faults appears to promote circulation and alteration within the footwall.…”

“…Ultimately, hydrothermal circulation transitions to off-axis ultramafic-hosted systems within the footwall McCaig et al, 2007;Fouquet et al, 2010). In ultramafic and mafic systems, metasomatic assemblages of talc-tremolite-chlorite or quartz form at > 350 °C (Boschi et al, 2006a(Boschi et al, , 2008McCaig et al, 2010;Verlaguet et al, 2021), typical of black smoker discharge zones, whereas serpentine-prehnite-hydrogarnet assemblages form at lower temperatures (Frost et al, 2008;Bach and Klein, 2009).…”