Abstract
We performed argon isotopic composition measurements of MORB glassy samples from the Pacific, Atlantic and Indian Oceans. There is a very large scatter in the 40Ar 36Ar ratio, from 980 up to 24,400 for bulk rock analyses, which is mainly due to atmospheric contamination. Using the stepwise heating technique, very high ratios are obtained, from 15,000 up to 25,250 which is the highest 40Ar 36Ar ratio ever measured in MORB. We establish a negative correlation between the highest 40Ar 36Ar results from stepwise heating and 87Sr 86Sr ratios, which is perfectly c consistent with a two-layered mantle structure. From both 40Ar and 129Xe [1] MORB systematics a model is proposed for the kinetics of degassing: a very early and extensive burst, with a time constant of ≈ 4My, is followed by a slower process of present day type, with a time constant of ≈ 0.5Gy. The mean age of the atmosphere is so determined to be around 4.4 Gy.
Published Version
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