TY - JOUR
T1 - Competitive hydration and dehydration at olivine-quartz boundary revealed by hydrothermal experiments
T2 - Implications for silica metasomatism at the crust-mantle boundary
AU - Oyanagi, Ryosuke
AU - Okamoto, Atsushi
AU - Hirano, Nobuo
AU - Tsuchiya, Noriyoshi
N1 - Funding Information:
The authors thank Masaoki Uno, Tomoyuki Mizukami, and Tatsu Kuwatani for helpful discussions, and Takahiro Watanabe for help in analyses of solutions by ICP–AES. We also thank an anonymous reviewer for constructive comments. This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (no. 24109701 , 25120005 ) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by a Grant-in-Aid for Challenging Exploratory Research from the Japan Society for the Promotion of Science (JSPS, no. 23654180 ) awarded to A. Okamoto, and by a Program for Leading Graduate Schools from JSPS awarded to R. Oyanagi.
Publisher Copyright:
© 2015 Elsevier B.V.
PY - 2015/9/1
Y1 - 2015/9/1
N2 - Serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust. Given that such a fluid has a high silica activity compared with mantle peridotite, it is thought that serpentinization and silica metasomatism occur simultaneously at the crust-mantle boundary. In this study, we conducted hydrothermal experiments in the olivine (Ol)-quartz (Qtz)-H2O system at 250 °C and vapor-saturated pressure under highly alkaline conditions (NaOHaq, pH = 13.8 at 25 °C) to clarify the mechanism of silica metasomatism at the crust-mantle boundary. Composite powders consisting of a Qtz layer and an Ol layer were set in tube-in-tube vessels. After the experiments, the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the Ol-Qtz boundary.The mineralogy of the reaction products in the Ol-hosted region changed with increasing distance from the Ol-Qtz boundary, from smectite + serpentine (Smc zone) to serpentine + brucite + magnetite (Brc zone). Olivine hydration proceeded in both zones, but the total H2O content in the products was greater in the Brc zone than in the Smc zone. Mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone. In contrast, the Smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions. In the Smc zone, smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration, and around the boundary of the Smc and Brc zones, serpentine formation occurred by olivine hydration and brucite dehydration. The relative extent of hydration and dehydration reactions controlled the along-tube variation in the rate of H2O production/consumption and the rate of volume increase. Our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure, possibly affecting the mechanical behavior of the crust-mantle boundary.
AB - Serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust. Given that such a fluid has a high silica activity compared with mantle peridotite, it is thought that serpentinization and silica metasomatism occur simultaneously at the crust-mantle boundary. In this study, we conducted hydrothermal experiments in the olivine (Ol)-quartz (Qtz)-H2O system at 250 °C and vapor-saturated pressure under highly alkaline conditions (NaOHaq, pH = 13.8 at 25 °C) to clarify the mechanism of silica metasomatism at the crust-mantle boundary. Composite powders consisting of a Qtz layer and an Ol layer were set in tube-in-tube vessels. After the experiments, the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the Ol-Qtz boundary.The mineralogy of the reaction products in the Ol-hosted region changed with increasing distance from the Ol-Qtz boundary, from smectite + serpentine (Smc zone) to serpentine + brucite + magnetite (Brc zone). Olivine hydration proceeded in both zones, but the total H2O content in the products was greater in the Brc zone than in the Smc zone. Mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone. In contrast, the Smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions. In the Smc zone, smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration, and around the boundary of the Smc and Brc zones, serpentine formation occurred by olivine hydration and brucite dehydration. The relative extent of hydration and dehydration reactions controlled the along-tube variation in the rate of H2O production/consumption and the rate of volume increase. Our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure, possibly affecting the mechanical behavior of the crust-mantle boundary.
KW - Crust-mantle boundary
KW - Hydrothermal experiments
KW - Pore fluid pressure
KW - Serpentinization
KW - Silica metasomatism
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U2 - 10.1016/j.epsl.2015.05.046
DO - 10.1016/j.epsl.2015.05.046
M3 - Article
AN - SCOPUS:84934943267
SN - 0012-821X
VL - 425
SP - 44
EP - 54
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -