Fluid circulation and carbonate vein precipitation in the footwall of an oceanic core complex, Ocean Drilling Program Site 175, Mid‐Atlantic Ridge

Abstract

Carbonate veins recovered from the mafic/ultramafic footwall of an oceanic detachment fault on the Mid‐Atlantic Ridge record multiple episodes of fluid movement through the detachment and secondary faults. High‐temperature (∼75–175°C) calcite veins with elevated REE contents and strong positive Eu‐anomalies record the mixing of up‐welling hydrothermal fluids with infiltrating seawater. Carbonate precipitation is most prominent in olivine‐rich troctolite, which also display a much higher degree of greenschist and sub‐greenschist alteration relative to gabbro and diabase. Low‐temperature calcite and aragonite veins likely precipitated from oxidizing seawater that infiltrated the detachment fault and/or within secondary faults late or post footwall denudation. Oxygen and carbon isotopes lie on a mixing line between seawater and Logatchev‐like hydrothermal fluids, but precipitation temperatures are cooler than would be expected for isenthalpic mixing, suggesting conductive cooling during upward flow. There is no depth dependence of vein precipitation temperature, indicating effective cooling of the footwall via seawater infiltration through fault zones. One sample contains textural evidence of low‐temperature, seawater‐signature veins being cut by high‐temperature, hydrothermal‐signature veins. This indicates temporal variability in the fluid mixing, possibly caused by deformation‐induced porosity changes or dike intrusion. The strong correlation between carbonate precipitation and olivine‐rich troctolites suggests that the presence of unaltered olivine is a key requirement for carbonate precipitation from seawater and hydrothermal fluids. Our results also suggest that calcite‐talc alteration of troctolites may be a more efficient CO2 trap than serpentinized peridotite.

Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

“…Micrite-filled fractures with similar petrographic characteristics as the micrite veins studied in this investigation have been discovered in numerous drill cores recovered during IODP Expeditions and on land (e.g., Christie et al, 2001;Clerc et al, 2014;Quandt et al, 2018;Schroeder et al, 2015). Their origin and formation, however, is often not discussed in detail and remains vague.…”

“…Note that the x-axis is compressed in order to show the full range of 87 Sr/ 86 Sr ratios. the oceanic crust, fluid-mediated injection or infill of calcareous sediments into fractures or cavities, that is, neptunian dykes (e.g., Lehner, 1991, and references therein), is often suggested (Christie et al, 2001;Clerc et al, 2014;Schroeder et al, 2015).…”

“…The volcanic units of the Izu-Bonin forearc and rear arc are substantially and similarly pervaded by veins that document extensive fluid flow through the oceanic crust and subsequent secondary mineral precipitation (Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015;Christeson et al, 2016;Reagan et al, 2015). Numerous previous studies on calcite veins pervading the oceanic crust used δ 18 O, δ 13 C, and 87 Sr/ 86 Sr isotopic compositions, rare earth element and yttrium (REE+Y) concentrations, and fluid inclusions to investigate the geochemical composition of circulating fluids, and the age and temperature of secondary mineral precipitation (e.g., Alt & Teagle, 2003;Alt et al, 1998;Brandstätter et al, 2016;Brandstätter et al, 2018;Hart & Staudigel, 1978;Quandt et al, 2018;Quandt, Micheuz, Kurz, Kluge, et al, 2019;Schroeder et al, 2015). However, studies on calcite veins from the Izu-Bonin-Mariana trench in particular are rare.…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…Ophicarbonates and related carbonate-silicate rocks with variable bulk major element compositions-notably SiO 2 , CaO and MgO-may form in oceanic and subduction settings (Figure 14), by leaching of alkali earth elements from silicate minerals and precipitation of carbonate upon mixing with dissolved inorganic carbon. In slow spreading mid-ocean ridges (Figure 14a), serpentinite-hosted carbonate-silicate rocks occur in the stockwork of submarine low-T hydrothermal fields (Lafay et al, 2017;Ludwig, Kelley, Butterfield, Nelson, & Früh-Green, 2006;Schwarzenbach, Früh-Green, Bernasconi, Alt, & Plas, 2013), in detachment faults of oceanic core complexes (Bach et al, 2011;Picazo, Manatschal, Cannat, & Andréani, 2013;Schroeder et al, 2015) and in transform faults (Alt et al, 2018;Bonatti, Lawrence, Hamlyn, & Breger, 1980). In these settings, carbonates are heterogeneously distributed at different scales in veins, replacing serpentine mesh textures and as matrix cement of tectonic or sedimentary serpentinite breccias (Figure 14a; Bonatti et al, 1980;Grozeva, Klein, Seewald, & Sylva, 2017;Klein et al, 2015;Lafay et al, 2017;Picazo et al, 2013;Schroeder et al, 2015).…”

“…the outer rise of subduction zones (Ranero, Phipps Morgan, McIntosh, & Reichert, 2003) might also result in serpentinization and carbonation of sub-crustal peridotite (Figure 14b). Serpentinite-hosted ophicalcites are symptomatic of seawater carbonation because the relatively low aCO 2,aq and Mg/Ca ratio of seawater and serpentinization fluids-in comparison to metamorphic fluids-promote crystallization of calcite and aragonite instead of Mg-carbonates (Bonatti et al, 1980;Eickmann, Bach, Rosner, & Peckmann, 2009;Grozeva et al, 2017;Schroeder et al, 2015;Schwarzenbach et al, 2013). In subduction zones, serpentinite-hosted carbonate-silicate rocks may also form in the subducting slab, the slab interface and in the overlying mantle wedge (Figure 14c).…”