The uranium and thorium decay series nuclides in Mt. St. Helens effusives

Details The uranium and thorium decay series nuclides in Mt. St. Helens effusives The uranium and thorium decay series nuclides in Mt. St. Helens effusives

Aug 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..60...61b&link_type=abstract

Earth and Planetary Science Letters, Volume 60, Issue 1, p. 61-69.

Computer Science

31

Scientific paper

The concentrations of the radionuclides 238U, 230Th, 226Ra, 210Pb, 210Po, 232Th, 228Ra and 228Th and the abundances of major elements were determined in samples from all major eruptions of Mt. St. Helens from May 18, 1980 through June 21, 1981. During this time the effusives changed from plagioclase-phyric dacite to a more andesitic composition but the concentrations of U and Th series nuclides were measurably invariant. The average 232Th/238U weight ratio in the rocks is 2.4 and the 230Th/232Th activity ratio equals the 238U/232Th activity ratio indicating no fractionation of U from Th during magma genesis. 226Ra activity is in excess (~40% on average) of its parent 230Th whereas 228Ra is in radioactive equilibrium with its parent 232Th, constraining the time of magma formation between 30 and 104 years prior to eruption. The 210Pb/226Ra activity ratios in the samples average 1.0, with a 20% scatter on either side, but allowing for volatile 210Pb loss at time of eruption excess 210Pb over 226Ra is inferred, indicating that the time of magma formation was within the last 150 years. 210Po was virtually absent in the samples immediately after eruption, indicating its total loss by volatilization during eruption. The quantity of 210Po volatilized during the May 18, 1980 event is estimated to be in the range of 300 Ci from the effusives and as much as 5000 Ci total including losses from heated slide material. The 222Rn activity volatilized should have been comparable to the 210Po activity released.

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