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

Abstract

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.