TY - JOUR
T1 - Dehydration of phengite inferred by electrical conductivity measurements
T2 - Implication for the high conductivity anomalies relevant to the subduction zones
AU - Chen, Sibo
AU - Guo, Xinzhuan
AU - Yoshino, Takashi
AU - Jin, Zhenmin
AU - Li, Ping
N1 - Funding Information:
We are grateful to three anonymous reviewers for constructive comments that greatly improved this manuscript. We thank Mouchun He and Shuiyuan Yang for their assistance in Raman and electron probe microanalyzer analysis, respectively. This study was supported by the National Natural Science Foundation of China (41590623), the National Key Research and Development Project of China (Project 2016YFC0600309), the National Natural Science Foundation of China (41472040), the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (G1323531510), and the Fundamental Research Funds for the Central Universities (CUGL150801).
Publisher Copyright:
© 2017 The Authors.
PY - 2018/1/1
Y1 - 2018/1/1
N2 - 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 10-4 S/m to 100.5 S/m. Characteristic impedance spectra, as well as postexperimental 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 ~300 km.
AB - 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 10-4 S/m to 100.5 S/m. Characteristic impedance spectra, as well as postexperimental 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 ~300 km.
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U2 - 10.1130/G39716.1
DO - 10.1130/G39716.1
M3 - Article
AN - SCOPUS:85038845845
VL - 46
SP - 11
EP - 14
JO - Geology
JF - Geology
SN - 0091-7613
IS - 1
ER -