The uranium and thorium decay series nuclides in Mt. St. Helens effusives |
| |
| Authors: | JT Bennett S Krishnaswami KK Turekian WG Melson CA Hopson |
| |
| Institution: | 1. Department of Geology and Geophysics, Yale University, Box 6666, New Haven, CT06511, U.S.A.;2. Department of Mineral Sciences, Smithsonian Institution, Washington, DC20560, U.S.A.;3. Department of Geological Sciences, University of California at Santa Barbara, Goleta, CA93106, U.S.A. |
| |
| Abstract: | The concentrations of the radionuclides238U,230Th,226Ra,210Pb,210Po,232Th,228Ra and228Th 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 average232Th/238U weight ratio in the rocks is 2.4 and the230Th/232Th activity ratio equals the238U/232Th activity ratio indicating no fractionation of U from Th during magma genesis.226Ra activity is in excess (～40% on average) of its parent230Th whereas228Ra is in radioactive equilibrium with its parent232Th, constraining the time of magma formation between 30 and 104 years prior to eruption. The210Pb/226Ra activity ratios in the samples average 1.0, with a 20% scatter on either side, but allowing for volatile210Pb loss at time of eruption excess210Pb over226Ra 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 of210Po 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. The222Rn activity volatilized should have been comparable to the210Po activity released. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
