The volatiles record of a ``popping'' rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles

Details The volatiles record of a ``popping'' rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles The volatiles record of a ``popping'' rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles

Dec 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..598j&link_type=abstract

Earth and Planetary Science Letters, Volume 107, Issue 3-4, p. 598-611.

Computer Science

84

Scientific paper

The popping rock dredged at 13.5°N and 3770 m deep on the Mid Atlantic Ridge by the ship Akademik Boris Petrov (sample 2πD43), has the largest gas content so far reported in MOR basalts. Given its vesicularity of 17% and gas chemical composition, this rock contained, when erupted, 7.56 cm3 STP g-1 of gas, most of which is CO2. The corresponding carbon concentration is 3850 ppm, mostly contained in the vesicles. The water content, mostly from the glass, is 5400 ppm, giving a CO2/H2O molar ratio of 1.07. The initial helium and argon contents were 2.46 and 1.80 10-4 cm3 STP g-1 respectively. The average chemical composition of the vesicle gas is CO2 = 94.95%, H2O = 4.93%, N2 = 0.12% and He = 32.6 ppm, Ar = 23.9 ppm, H2 = 28.8 ppm, CO = 7.2 ppm and CH4 = 1.8 ppm. Because of its very low degree of outgassing and very typical MORB rare gas isotopic composition [3] (3He/4He = 8 Ra, 40Ar/36Ar up to 28,000 ), this rock provides probably the best sample so far of upper mantle gas, with negligible (3-7%) atmospheric and seawater contamination (40Ar/36Ar > 6000 in all aliquots). The isotopic compositions of carbon and nitrogen are -3.68‰ PDB, and -3.54‰ ATM, respectively. The average C/N ratio is 400, similar to the lower end of the diamond range (400-10,000). The ratios C/He of 29 000 (C/3He = 2.7 109) and He/Ar of 1.36 are similar to the computed radioactive production ratios for the upper mantle. However, within this single sample significant variations of these ratios occur, and are more or less related to the vesicle size: a factor of 2 for C He, 40% for He Ar and 30% for C N, probably reflecting outgassing effects. Whether it is considered as a concentrate of MORB volatiles at the top of a magma column or chamber, or as a virtually undegassed magma sample, this sample provides new estimates of upper mantle isotopic compositions: δ13C around -4‰ PDB, significantly higher than previous estimates, and δ15N around -3.5‰ ATM, a little higher than the average of ordinary diamonds sampled so far. If the sample's carbon ( ~ 4000 ppm) and nitrogen ( ~ 12 ppm) contents are representative of primary upper mantle magmas, they correspond to mantle concentrations of ~ 300 and 1 ppm respectively, and suggest a mantle carbon flux around 1.5 1013 mole y-1.

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