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Rate of antigorite dehydration at 2 GPa applied to subduction zones

David H. Eggler and Amy N. Ehmann
Rate of antigorite dehydration at 2 GPa applied to subduction zones
American Mineralogist (May 2010) 95 (5-6): 761-769

Abstract

A mixture of antigorite, forsterite, and enstatite was reacted at 2 GPa pressure, with water, to study kinetics of the reaction Mg (sub 48) Si (sub 34) O (sub 85) (OH) (sub 62) =10 Mg (sub 2) Si (sub 2) O (sub 6) +14 Mg (sub 2) SiO (sub 4) +31 H (sub 2) O. Reaction progress, F, which can vary between +1 and -1, was measured by comparing areas under X-ray diffraction peaks for run products with corresponding peaks for the starting material. Rates for dehydration and hydration can be regressed with the equation: r F (sub R) /t = K (sub r) A degrees (sub 32V) [-171(1-T (sub eq) /T0] (super n) mol/cm (super 3) (sub rock) /8. The function F (sub R) accounts for the decrease in A (sub theta ) , specific surface area, from A (sub theta ) degrees at F=0 to 0 at F=1: F (sub R) =1/1-p[1-(1-F) (super 1-p) ] where p, approximately 0.50 for elongate grains, characterizes grain shape. Regression of the rate equation for dehydration runs can be combined with A (sub theta ) degrees , measured on the antigorite starting material, to give reaction rate K (sub r) : -9.2(l.2)X10 (super -15) mol/s/cm (super 2) . With that rate, we calculate that well-defined conventional reversal brackets of 5 degrees C around the equilibrium temperature would require run lengths of 729(99) h, considerably longer than in this or any previous study. The rate equations can be applied to the question of overstepping of antigorite dehydration below arcs. One modeled geotherm 40 km below a slab surface crosses the antigorite dehydration reaction at about 2 GPa; the slab takes 3X10 (super 5) years to warm one degree. For grain sizes in a serpentinite in the 0.1 to 10 cm range, complete dehydration would take 10 (super 4) -10 (super 5) years. During that time, the plate would travel no more than a kilometer past the point of first dehydration. If earthquakes associated with dehydration occurred on timescales of 10 (super 3) -10 (super 4) years, complete dehydration of a volume of plate would require 10-100 separate dehydration events.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 95
Serial Issue: 5-6
Title: Rate of antigorite dehydration at 2 GPa applied to subduction zones
Affiliation: Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Pages: 761-769
Published: 201005
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 32
Accession Number: 2010-050156
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201027
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