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GEOREF RECORD

An exsolution origin for Archean mantle garnet

Emma L. Tomlinson, Balz S. Kamber, Brendan C. Hoare, Clare V. Stead and Benoit Ildefonse
An exsolution origin for Archean mantle garnet
Geology (Boulder) (February 2018) 46 (2): 123-126

Abstract

It is well established that the cratonic subcontinental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond approximately 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene; however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic, and chemical data for an exceptionally well preserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogeneously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are subcalcic Cr-pyrope, and the orthopyroxene host is high-Mg enstatite; these phases equilibrated at 4.4 GPa and 975 degrees C. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of approximately 6 GPa and 1750 degrees C. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most subcalcic garnets in depleted peridotites.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 46
Serial Issue: 2
Title: An exsolution origin for Archean mantle garnet
Affiliation: Trinity College Dublin, Department of Geology, Dublin, Ireland
Pages: 123-126
Published: 201802
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 33
Accession Number: 2018-041852
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2018027
Illustration Description: illus.
S27°00'00" - S18°00'00", E18°00'00" - E30°00'00"
Secondary Affiliation: Universite Montpellier, FRA, France
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: 201810
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