Abstract
Before attaining the mantle wedge, where they trigger partial melting, volatiles released from dehydration reactions in the slab have to migrate across a relatively cold (< 750 °C), peridotite-layer above the incoming slab. In order to unravel the mechanisms allowing for this initial stage of fluid transport, we performed a detailed field and microstructural study of metamorphic prograde peridotites in the Cerro del Almirez ultramafic massif (Betic Cordillera, Spain), where evidences of one of the most important dehydration reactions in subduction zones, the high-pressure antigorite breakdown ( P = 1.6–1.9 GPa and T ≈ 680 °C), can be mapped in the field. This reaction led to arborescent growth of centimeter-size olivine and orthopyroxene, producing a chlorite–harzburgite with a spinifex-like texture. Microstructural observations and crystal preferred orientations (CPO) mapping show no evidences of solid-state deformation during the prograde growth of olivine and orthopyroxene at the expenses of antigorite. However, a few tens to a hundred meters away from the reaction front, the metamorphic texture is partially obliterated by grain-size reduction in roughly planar conjugate zones, a few mm to meters wide. Grain size reduction zones (GSRZ) are characterized by (1) sharp contacts with undeformed spinifex-like texture domains, (2) important reduction of the olivine grain size (60–250 μm), (3) olivine color change from brownish to colorless, (4) decrease in the modal amount of orthopyroxene, and (5) at the mm- to cm-scale, irregular shapes and abrupt terminations. Field and microstructural observations exclude that relative displacement took place across these GSRZ. Changes in modal composition imply reactions with fluids undersaturated in silica. Analysis of olivine crystal-preferred orientations (CPO) in GSRZ shows patterns similar, but more dispersed, than those in neighboring spinifex-like domains. It also reveals mm- to cm-scale discrete domains with rather homogeneous crystallographic orientations suggesting inheritance from the preexisting spinifex-like olivines in the host peridotite. Misorientation angles between neighboring grains in the GSRZ show peaks at ∼ 5–10° and ∼ 20°, but rotations are not crystallographically controlled. Based on these observations, we rule out the formation of the GSRZ by dynamic recrystallization during dislocation creep and propose that they record brittle deformation (microcraking) of the spinifex-like chlorite–harzburgite, probably induced by hydrofracturing at high pressure and relative low temperature conditions (680–710 °C). High-pressure hydrofracturing can, thus, be invoked as an efficient mechanism for fluid flow across the cold top-slab mantle layer, hence allowing the slab-derived fluids to ingress in the wedge.
Highlights
Transfer of fluids from the subducting slab to the mantle wedge is an essential process in subduction dynamics, as it controls the onset of the partial melting process and, the location and the chemical characteristics of the arc magmatism
The uncorrelated misorientation distribution of olivine, that is the misorientation between randomly chosen measurement points, in most samples matches closely the random distribution, except for samples Al06-34 and Al06-35 that show a higher proportion of misorientations b40°. These uncorrelated misorientation distributions are consistent with the measured olivine crystal-preferred orientations (CPO), which are slightly stronger in the Grain size reduction zones (GSRZ) of these two samples (Fig. 7)
Based on the field and microstructural observations presented above, we propose that hydrofracturing is a plausible mechanism for the transport of fluids produced by dehydration reactions across an otherwise almost impermeable cold, top-slab mantle wedge
Summary
Transfer of fluids from the subducting slab to the mantle wedge is an essential process in subduction dynamics, as it controls the onset of the partial melting process and, the location and the chemical. In this paper we present a combined field and microstructural study of chlorite–harzburgite from the km-scale Cerro del Almirez ultramafic massif (Betic Cordillera, SE Spain) where the antigorite-out isograd produced by high-pressure serpentinite dehydration, the most important devolatilization reaction in subduction zones, can be mapped at the outcrop scale (Trommsdorff et al, 1998; Garrido et al, 2005; Padrón-Navarta et al, 2008). Detailed petrographic and mineral chemistry studies, as well as thermodynamic calculations, lead Trommsdorff et al (1998) to interpret the chlorite–harzburgite as a result of the high-pressure dehydration of the antigorite serpentinite during the Alpine metamorphism of the Nevado–Filábride Complex This event occurred during the Middle Miocene, as revealed by SHRIMP U–Pb zircon ages of 15.0 ± 0.6 Ma in a meta-clinopyroxenite just outside the antigoriteout isograd (López Sánchez-Vizcaíno et al, 2001). We focus on those developed at the expenses of the spinifex-like texture, because they allow for a better characterization of the changes relative to the original texture
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