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Porosity, permeability, and basalt metamorphism

Craig E. Manning and Dennis K. Bird
Porosity, permeability, and basalt metamorphism (in Low-grade metamorphism of mafic rocks, Peter Schiffman (editor) and Howard W. Day (editor))
Special Paper - Geological Society of America (1995) 296: 123-140


The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of < or =550 degrees C; the pyroxene zone (T = 700-850 degrees C); and the olivine zone, within 10 m (T > 850 degrees C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H (sub 2) O as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock delta (super 18) O compositions are 1.7 per mil to 3.0 per mil and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 x 10 (super 3) mol cm (super -2) in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was approximately 10% higher in aa flow breccias.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 296
Title: Porosity, permeability, and basalt metamorphism
Title: Low-grade metamorphism of mafic rocks
Author(s): Manning, Craig E.Bird, Dennis K.
Author(s): Schiffman, Petereditor
Author(s): Day, Howard W.editor
Affiliation: University of California at Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA, United States
Affiliation: University of California at Davis, Department of Geology, Davis, CA, United States
Pages: 123-140
Published: 1995
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-2296-9
References: 70
Accession Number: 1995-032503
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. col., 5 tables, geol. sketch maps
N68°04'60" - N68°15'00", W31°45'00" - W31°34'60"
Secondary Affiliation: Stanford University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199512
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