Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites

Abstract

[1] Deformation and hydration processes are intimately linked in the oceanic lithosphere, but the feedbacks between them are still poorly understood, especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and/or mass transfer. Serpentinization is accompanied by abundant veining marked by different generations of vein-filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation. We use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow-spreading ridges. We have selected a representative set of veins from ocean floor serpentinites of the Mid-Atlantic Ridge near Kane transform fault (23°N) and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy (SEM, TEM) with electron microprobe analyses. Four main veining episodes (V1 to V4) accompany the serpentinization. The first episode, identified as vein generation V1, is interpreted as the tectonically controlled penetration of early seawater-dominated fluid within peridotites, enhancing thermal cracking and mesh texture initiation at 3-4 km up to 8 km depth and at T <300-350°C. The two following vein stages (V2 and V3) formed in a closed, diffusive system and accommodate volume expansion required to reach almost 50% serpentinization of the protolith. The cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault. Degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization, and this capacity is in turn linked to the exhumation rate and local tectonics. During this period, water consumed by hydration may prevent the establishment of convective hydrothermal cells. The onset of an open hydrothermal system in the shallow lithosphere (<2 km), where brittle fracturing and advective transfer dominate and enable the completion of serpentinization, is marked by the last vein generation (V4). These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events, chemical conditions, and modes of hydrothermal alteration of the lithosphere.

Deformation Processes, Textural Evolution and Weakening in Retrograde Serpentinites

“…Andreani et al [74] described the kinetic control on serpentine vein mineralogy, with an overall chronological evolution from poorly crystalline protoserpentine to conical serpentine, chrysotile, polygonal serpentine, polyhedral serpentine and lizardite. According to these authors, the main serpentine veins found in natural faults and shear zones are (formed from the first to last stages of serpentinite evolution): isolated sigmoidal chrysotile veins (V2 in their classification); crack-seal veins formed by variable mixtures of protoserpentine, chrysotile and polygonal serpentine fibres (V3; see also [53] for this kind of vein); and polyhedral serpentine/lizardite veins (V4).…”

“…At the outcrop scale, fibrous serpentine veins (type V3 in [74]) and splintery antigorite veins represent the most commonly encountered vein types. These veins often constitute the typical anastomosing slickenfibre faults that surround lens-shaped massive blocks (e.g., Figure 3).…”

Expedition 357 summary

“…Research on detachment faulting and OCCs indicates that this style of oceanic spreading is intimately linked to hydrothermal circulation and encompasses a wide variety of fluid flow and hydrothermal regimes (McCaig et al, 2007). High-temperature fluid circulation is well documented mineralogically and geochemically (Schroeder and John, 2004;Boschi et al, 2006;McCaig et al, 2010), and uplift along detachments may promote circulation and alteration within the footwall (Andreani et al, 2007). McCaig et al (2007) propose temporal evolution in the style of hydrothermal circulation associated with the development of OCCs: from high-temperature systems hosted in the basaltic hanging wall within the rift valley (e.g., Trans-Atlantic Geotraverse [TAG]-type), to high-temperature ultramafic-hosted systems within the footwall (Rainbowtype), and ultimately to off-axis ultramafic-hosted systems within the footwall (Lost City-type).…”

MORB melt metasomatism and deserpentinization in the peridotitic member of Variscan ophiolite: an example of the Braszowice–Brzeźnica serpentinites (SW Poland)

“…At low-pressure conditions, formation of secondary olivine is expected at 380 °C, whereas secondary clinopyroxene appears at 450 °C in the system CaOMgO-SiO 2 -H 2 O (Fig. 13, Nozaka 2005; Andreani et al 2007;Plümper et al 2012 and references therein). If true, this confirms general low-grade metamorphic conditions for the BBM serpentinites, similarly to lithologies from other Central-Sudetic Ophiolites (Kryza and Pin 2010).…”