The dehydration of serpentine mineral antigorite, Mg2.8Si2O5(OH)3.6, is regarded as the key step in metamorphic transformation of ultramafic hydrated rocks in subduction zones, which affects seismicity and feeds volcanic activity. The abundance of alkali-chloride brines derived from deep subduction/upper mantle sources implies the possibility of a large control of the H2O activity by the dissolved salts. The present study examines the effect of alkali chlorides, lowering the H2O activity in fluid, on antigorite stability at high pressure. The decomposition of natural antigorite (Ural) in the presence of a halite-saturated NaCl-H2O fluid was studied up to 3 GPa and 700 °C by in situ X-ray diffraction combined with resistively heated diamond-anvil cell. Reference experiments were also performed on salt-free sample. At 1.5–3 GPa in the presence of halite-saturated fluid (XNaCl ≈ 0.15), antigorite decomposes to an intermediate product assemblage of talc+forsterite at about 550 °C, which is ≈150 °C lower compared to salt-free H2O-unsaturated system. Such a low-temperature shift supports the previous models of a broadened P-T area of serpentinite dehydration in the subducting slab. In addition, the present experiments reveal active dissolution of the product Mg silicates, first of all forsterite, in the NaCl-H2O fluid at 600–700 °C/1.5–3 GPa. This implies that dehydrated serpentinites are a potential source of fluids enriched in MgO and SiO2, which play an important role in deep metasomatic processes.

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