Serpentinization of mantle‐derived peridotites at mid‐ocean ridges: Mesh texture development in the context of tectonic exhumation

Abstract

At slow spreading ridges, axial detachment faults exhume mantle-derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid-Atlantic and Southwest Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction-induced cracks developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of $60 mm but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100-400 mm in size. Apparent mesh rim thickness is on average 33 6 19 mm corresponding to serpentinization extents of 70-80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution.

Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite

“…1, 2) were preferentially exploited by 607 serpentinization reactions (cf. Rouméjon and Cannat, 2014). The stress increase due 608 to volume expansion was sufficient to induce further cracking during our 609 experiment, in particular along cone-shaped etch pits (e.g., Figs.…”

“…An intriguing 118 explanation for this observation is persistent reaction-driven fracturing, which may 119 Ernst, 1983), limited only by water access to primary mineral surfaces (Macdonald 124 and Fyfe, 1985; Malvoisin and Brunet, 2014). While the use of powdered starting 125 materials in laboratory serpentinization experiments has provided valuable insight 126 by allowing reactions to go to completion, this approach does not permit testing of 127 whether serpentinization is a self-sealing process (Lister, 1974), or whether 128 serpentinization can go to completion via thermal, tectonic, and reaction-driven 129 fracturing (Plümper et al, 2012, Rouméjon andCannat, 2014). Moreover, it remains 130 unclear how the putative self-sealing nature of serpentinization or fracturing affects 131 rates of mineral dissolution and H2 release.…”

Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

“…Serpentinization in young oceanic lithosphere is generally believed to be limited to shallow depths of 4-6 km below detachment faults, but its extent is generally difficult to estimate from seismic velocities or rock samples gained from dredging or drilling 24 . Our data suggest that in the extensive amagmatic regions of ultraslow-spreading ridges serpentinization and fluid circulation may reach far deeper into the mantle than previously assumed.…”

“…The upper limit of seismicity in amagmatic regions may therefore image the serpentinization front at previously unknown depths of up to 15 km. This implies in turn that fluid circulation extends to these depths to alter the mantle rocks, exploiting major shear zones or a network of microfractures 24 .…”

“…1; site 1, 75-115 km along the profile, site 2). Serpentinization in these mantle domains is apparently less pronounced and cannot effectively reduce the shear strength, so these regions of the mantle behave more like normal ocean lithosphere, where serpentinization is commonly confined to the uppermost mantle 23,24 . We therefore speculate that differences in lithospheric composition favour serpentinization of the upper mantle in amagmatic lithosphere but limit serpentinization of magmatic lithosphere at the same depth levels.…”