Fluid-rock interaction in eclogite-facies meta-peridotite (Erro-Tobbio Unit, Ligurian Alps, Italy)
Authors: Cacciari Serena , Pennacchioni Giorgio , Cannaò Enrico , Scambelluri Marco , Toffol Giovanni 
Published: March 2024

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 &#176;C3. Ol2 + Ti-chu reaction bands are arranged into two main sets, mutually oriented at ~50&#176;: (i) Set1, steeply-dipping around 320&#176;, (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 &#177; 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 literature2,4, and during retrogression.References1: Behr et al., 2021, What&#8217;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&#8211;29.3: Hermann et al., 2000, The importance of serpentinite mylonites for subduction and exhumation of oceanic crust. Tectonophysics 327, 225&#177;238.4: Scambelluri et al., 2012, Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle. Geology 40, 10,&#160; 907&#8211;910.&#160;