Heterogeneous fluid release in subduction zones – evidence from experimental dehydration of brucite vein networks in serpentinite

Abstract

Aqueous fluids released by metamorphic dehydration reactions are key components for magmatism, seismicity, creep, and geochemical cycling in subduction zones. How these fluids drain and migrate towards the mantle wedge is not fully understood, partly because the recognition and interpretation of deep fluid pathways in the exhumed rock record is challenging. Serpentinites are among the most important H2O carriers in subducted slabs, with dehydration occurring for example during the lizardite to antigorite transition (ca. 300 – 350 °C), brucite breakdown (ca. 500 °C) and antigorite dehydration (620 – 670 °C). Fluid production and the related formation of interconnected porosity allowing fluid migration are influenced by pre-existing chemical and mineralogical heterogeneities, as well as microstructure and porosity. In oceanic and low-grade metamorphic serpentinites such heterogeneities are very common. To investigate their effect on metamorphic dehydration and fluid migration during subduction to forearc conditions, we experimentally dehydrated natural serpentinite that contains abundant brucite formed during the prograde lizardite–antigorite transformation [1]. In the starting material, brucite occurs as veins and as intergrowths with serpentine in the matrix. We performed piston-cylinder experiments at conditions of brucite dehydration in subduction zones (520 – 570 °C; 1.5 GPa), coupled with micro-tomography (µ-CT), Raman, electron microscopy and microstructural analysis. The experimental results show the formation of olivine as (i) veinlets along the rims of brucite veins, (ii) surrounding and replacing Fe-oxides, and as (iii) tabular grains growing in the serpentinite matrix at the hot spot of the sample cylinder. All olivine types are related to newly formed porosity visible at the resolution of the µ-CT (1.2 µm voxel size). Broad-ion beam polished FE-SEM analysis of the starting material indicates that veins of brucite (± serpentine, Fe-oxides) have significantly more nano-porosity than the serpentine matrix. This observation and the formation of olivine veinlets along the previous brucite vein walls in the experiment suggest that the presence of brucite veins –formed early during shallow forearc metamorphism of serpentinite– will influence fluid production and migration pathways during brucite and antigorite dehydration at deep forearc conditions. Incidentally, our results further demonstrate that preferential dehydration occurs when an external reducing agent (in the case of the experiment H2 most likely derived from the graphite heater) triggers the replacement of serpentine + magnetite by olivine, in line with previous experimental and natural observations [2,3]. Incipient fluid release from serpentinite is thus heterogeneous at the microscale, and will cause local fluid pressure variations that may lead to flow and ultimately drainage. This process, possibly in combination with deformation, deviatoric stress and/or external fluid flux, may favour the development of commonly mono-mineralic olivine veins, which are inferred to form by an interplay of brucite dehydration and reactive fluid flow [4,5].   [1] Menzel et al., 2018, Lithos [2] Eberhard et al., 2023, Journal of Petrology [3] Padrón-Navarta et al., 2023, Nature Geoscience [4] Plümper et al., 2017, Nature Geoscience [5] Huber et al., 2022, G-cubed   Funding: M.D.M: Junta de Andalucía (Postdoc_21_00791) and project “RUSTED”, MCIU Spain (PID2022-136471N-B-C21 & 22). LE: NWO (VI.Vidi.193.030), EXCITE (TNA-C3-2023-13).