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Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

Hans-Joachim Massonne and Arne P. Willner
Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions (in From field observation to experimental petrology and back; a special issue to honour Werner Schreyer, Walter Maresch (editor), Edward Grew (editor) and Friedrich Seifert (editor))
European Journal of Mineralogy (October 2008) 20 (5): 867-879

Abstract

P-T pseudosections were calculated in the system Na-Ca-K-Fe-Mg-Al-Si-Ti-H-O with the PERPLE_X software package for the pressure-temperature range 1-25 kbar and 150-450 degrees C to gain a better understanding of the phase relations and the dehydration behavior of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe (super 2+) - and Fe (super 3+) -pumpellyite, Fe (super 2+) - and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H (sub 2) O bound in minerals at 150 degrees C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 degrees C/km, significant dehydration (1.5 wt.% H (sub 2) O release) of metapsammopelite can be noted in the temperature range 220-240 degrees C. We believe that this process leads to softening of the sedimentary cover of oceanic crust during early subduction so that this material can be scraped off the basic crust, which then would dehydrate at higher T, to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms (e.g., 12 degrees C/km) by dehydration of metapsammopelite between 260 and 300 degrees C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 degrees C/km, almost no water is released up to temperatures of 400 degrees C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 20
Serial Issue: 5
Title: Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions
Title: From field observation to experimental petrology and back; a special issue to honour Werner Schreyer
Author(s): Massonne, Hans-JoachimWillner, Arne P.
Author(s): Maresch, Waltereditor
Author(s): Grew, Edwardeditor
Author(s): Seifert, Friedricheditor
Affiliation: Universitaet Stuttgart, Institut fuer Mineralogie und Kristallchemie, Stuttgart, Federal Republic of Germany
Pages: 867-879
Published: 200810
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 59
Accession Number: 2009-003341
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: Ruhr-Universitaet, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200902
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