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The concentration and isotopic composition of carbon in basaltic glasses from the Juan de Fuca Ridge, Pacific OceanThe abundance and C-13/C-12 ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 C and 600-650 C is interpreted to be secondary, and only the carbon recovered during a final combustion step at about 1200 C is thought to be indigenous to the samples. For carbon released at about 1200 C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta C-13 values of -4.8 to -9.3 per mil, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta C-13 values of -6.1 to -9.2 per mil. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta C-13 ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta C-13 values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.
Document ID
19930045396
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Blank, Jennifer G.
(California Inst. of Technology Pasadena, United States)
Delaney, John R.
(Washington Univ. Seattle, United States)
Des Marais, David J.
(NASA Ames Research Center Moffett Field, CA, United States)
Date Acquired
August 16, 2013
Publication Date
February 1, 1993
Publication Information
Publication: Geochimica et Cosmochimica Acta
Volume: 57
Issue: 4
ISSN: 0016-7037
Subject Category
Geophysics
Accession Number
93A29393
Funding Number(s)
CONTRACT_GRANT: NSF OCE-79-25041
CONTRACT_GRANT: NSF OCE-83-09812
CONTRACT_GRANT: NSF OCE-81-11413
Distribution Limits
Public
Copyright
Other

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