TY - JOUR

T1 - The low-temperature shift of antigorite dehydration in the presence of sodium chloride: In situ diffraction study up to 3 GPa and 700 °C

AU - Likhacheva, Anna Yu

AU - Rashchenko, Sergey V.

AU - Semerikova, Anna I.

AU - Romanenko, Alexandr V.

AU - Glazyrin, Konstantin

AU - Safonov, Oleg G.

N1 - Funding Information: We thank anonymous reviewers for their profound and constructive criticisms, as well as E.M. Spiridonov (Moscow State University) for providing us with the antigorite sample. The authors acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. This work is performed on state assignment of IGM SB RAS and is supported by the RFBR project 18-05-00312. Diffraction experiments were carried at the DESY PETRA III research laboratory and supported by the approval of PETRA III regular proposal I-20190140. Publisher Copyright: © 2022 De Gruyter. All rights reserved.

PY - 2022/6/1

Y1 - 2022/6/1

N2 - 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.

AB - 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.

KW - Antigorite

KW - decomposition

KW - H2O activity

KW - high pressure

KW - high temperature

KW - in situ X-ray diffraction

KW - NaCl aqueous fluid

KW - subduction zone

UR - http://www.scopus.com/inward/record.url?scp=85131746691&partnerID=8YFLogxK

U2 - 10.2138/am-2022-7962

DO - 10.2138/am-2022-7962

M3 - Article

AN - SCOPUS:85131746691

VL - 107

SP - 1074

EP - 1079

JO - American Mineralogist

JF - American Mineralogist

SN - 0003-004X

IS - 6

ER -