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Evidence for hydrous high-MgO melts in the Precambrian

William E. Stone, Etienne Deloule, Michelle S. Larson and C. Michael Lesher
Evidence for hydrous high-MgO melts in the Precambrian
Geology (Boulder) (February 1997) 25 (2): 143-146


Prevailing petrogenetic models for Precambrian high-MgO melts such as komatiites invoke crystallization from nearly anhydrous melts (<<0.5% H (sub 2) O) generated by partial melting of mantle peridotite at temperatures of (< or =1900 degrees C and pressures of (< or =18 GPa. However, ultramafic cumulate and gabbro zones of komatiitic and other high-MgO units in Precambrian greenstone belts contain vesicles and minor to major amounts (< or =25%) of igneous amphibole. The textures (oikocrysts, rims on intercumulate pyroxene, and mineral inclusions within orthocumulate olivine) and the water-rich compositions (1.00%-2.50% H (sub 2) O) of igneous amphiboles from the Archean Abitibi belt indicate crystallization in situ from significantly hydrous melts while the melt fraction was still as high as 40%-50%. Comparisons to experimental phase equilibria suggest that the residual melts from which the amphiboles crystallized contained 3%-4% H (sub 2) O, and adjustments for fractional crystallization suggest that the initial melts may have contained as much as 2% H (sub 2) O. H (sub 2) O contents of this magnitude would require substantial revision of the nearly anhydrous models for Precambrian high-MgO melts, possibly permitting generation at lower temperatures and pressures, lowering their densities and viscosities, increasing their eruptibility, and enhancing the formation of spinifex textures.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 25
Serial Issue: 2
Title: Evidence for hydrous high-MgO melts in the Precambrian
Affiliation: University of Alabama, Department of Geology, Tuscaloosa, AL, United States
Pages: 143-146
Published: 199702
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 28
Accession Number: 1997-020220
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch maps
N49°00'00" - N49°00'00", W80°00'00" - W80°00'00"
N49°30'00" - N49°30'00", W78°00'00" - W78°00'00"
Secondary Affiliation: CNRS, Vandoeuvre les Nancy, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 199707
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