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Dehydration of phengite inferred by electrical conductivity measurements; implication for the high conductivity anomalies relevant to the subduction zones

Chen Sibo, Guo Xinzhuan, Takashi Yoshino, Jin Zhenmin and Li Ping
Dehydration of phengite inferred by electrical conductivity measurements; implication for the high conductivity anomalies relevant to the subduction zones
Geology (Boulder) (January 2018) 46 (1): 11-14

Abstract

Phengite is a critical carrier of water and potassium (K) in sediment as well as in the basaltic layers of subducted slabs. However, although phengite dehydration can potentially be used to interpret high conductivity in the mantle overlying such slabs at depths between 250 km and 300 km, no experimental constraints on the electrical conductivity of this mineral have so far been performed. We investigated the electrical conductivity of natural phengite before and after dehydration at pressures (P) as high as 12 GPa and at temperatures (T) as high as 1200 K. The results of this study confirm positive T and negative P effects on the electrical conductivity of phengite before its breakdown; subsequently, at 12 GPa and 850 K, electrical conductivity dramatically increased from 10 (super -4) S/m to 10 (super 0.5) S/m. Characteristic impedance spectra, as well as post-experimental textural observations and sample chemical analyses, show that these abrupt conductivity changes are due to K-rich fluids released by phengite dehydration. These high conductivities of K-rich fluids are in close agreement with geophysical observations at depths of between 250 km and 300 km in backarcs beneath the North Philippine Sea and central Argentina. Comparisons between the geotherms of these areas and the stability field of phengite defined by phase equilibrium experiments suggest that the descending crust is almost completely dehydrated and should not cause further K-metasomatism subsequent to the breakdown of phengite at depths of approximately 300 km.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 46
Serial Issue: 1
Title: Dehydration of phengite inferred by electrical conductivity measurements; implication for the high conductivity anomalies relevant to the subduction zones
Affiliation: China University of Geosciences, State Key Laboratory of Geological Processes and Mineral Resources, Wuhan, China
Pages: 11-14
Published: 201801
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 35
Accession Number: 2018-039533
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2018002
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Okayama University, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2018
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