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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2021-01, Vol.22 (1), p.n/a
• Main Authors: Verlaguet, A., Bonnemains, D., Mével, C., Escartín, J., Andreani, M., Bourdelle, F., Boiron, M‐C., Chavagnac, V.
• Format: Article
• Language: English
• Subjects: Amphibolites; Brines; Calcium; Carbon dioxide; Chemical analysis; Chlorite; Circulation; Crystallization; Crystals; Deformation; fluid inclusions; Fluids; Gabbros; hangingwall overplating; Heat sources; Hydrogen; Hydrogen sulfide; Hydrothermal alteration; Isotopes; Life Sciences; mafic breccias; MAR 13°20′N; Methane; Mineralization; Outflow; phase separation; Pyrite; Quartz; Rock; Rocks; Salinity; Salinity variations; Seawater; Separation; Serpentinization; Silica; Silicification; silicified detachment; Sodium chloride; Water analysis
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2020GC009235

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