Fluid-rock interaction in eclogite-facies meta-peridotite (Erro-Tobbio Unit, Ligurian Alps, Italy)

In subduction zones, fluids released by dehydration reactions strongly influence rock rheology and seismicity. In particular, the occurrence of deep Episodic Tremor and Slow Slip events (deep ETS) along the subduction interface, at 25-60 km depth¹, is likely fostered by the simultaneous presence of fluctuating fluid pressure and rheological heterogeneities, that allow for strain partitioning into low-strain domains radiating tremor, and high-strain domains accommodating slow slip events.

The Erro-Tobbio meta-peridotite (Ligurian Alps) records fluid-rock interactions and associated deformation that occurred within the deep ETS depth range. Heterogeneous serpentinization of the original mantle peridotite resulted in partitioning of the eclogite-facies deformation into high-strain domains of antigorite mylonites and low-strain domains of undeformed meta-peridotites. Both mylonites and meta-peridotites contain veins/reaction bands of metamorphic olivine (Ol₂) and Ti-clinohumite (Ti-chu), formed by breakdown of brucite (Brc) and antigorite (Atg) at estimated P-T of 1.5 GPa and 500 °C³. Ol₂ + Ti-chu reaction bands are arranged into two main sets, mutually oriented at ~50°: (i) Set₁, steeply-dipping around 320°, (ii) Set₂, trending N-S and parallel to the mylonites. The mylonites include: (i) type₁ mylonites, composed of a planar foliation marked by Set₂ reaction bands, and (ii) type₂ mylonites, displaying a chaotic structure.

Within the undeformed domains, hydration and dehydration events occurred statically. In such domains, Al-rich Atg (Atg₁) epitaxially replaced mantle olivine (Ol₁), and was in turn epitaxially overgrown by Ol₂, that crystallized in radial aggregates and along Set₁-Set₂ reaction bands. Along the mylonitic horizons, Atg₁ is affected by ductile deformation, and Set₂ reaction bands mark a foliation parallel to that of Atg₁. In this case, Ol₂ is rarely crystallographically related to Atg₁ and is mostly oriented with a-axis parallel to the reaction bands. Atg₁ and Brc relics are preserved along Set₁ and Set₂. The absence of Brc in the wall rock suggests that formation of Ol₂ localized along original Brc-rich layers. Later stage, Al-free serpentine locally extensively (up to 70% volume) replaces Ol₂ along a pervasive network of microcracks that exploited the previous Set₁-Set₂ structures. These observations suggest the occurrence of localized Brc ± Atg₁ dehydration to Ol₂ along specific planes, likely related to Brc distribution and Atg deformation, and subsequent Ol₂ hydration localized along serpentine-bearing microcracks.

In-situ LA-ICP-MS reveals an enrichment in fluid-mobile elements (As, Sb, Ba, W, Li, B) in prograde Ol₂ and retrograde Al-free serpentine. This information provides evidence of infiltration of external fluids, indicating open system conditions during eclogite-facies deformation, in agreement with the literature^2,4, and during retrogression.

References

1: Behr et al., 2021, What’s down there? The structures, materials and environment of deep-seated slow slip and tremor. Phil. Trans. R. Soc. A 379: 20200218.

2: Clarke et al., 2020, Metamorphic olivine records external fluid infiltration during serpentinite dehydration. Geochem. Persp. Let. 16, 25–29.

3: Hermann et al., 2000, The importance of serpentinite mylonites for subduction and exhumation of oceanic crust. Tectonophysics 327, 225±238.

4: Scambelluri et al., 2012, Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle. Geology 40, 10,  907–910.