We present U-Th and Sr isotopic data on young (Holocene) lavas from Lassen and Mt Shasta volcanic fields and surrounding areas. Andesitic lavas from Shasta have 232Th/238U ratios of 2.6–2.7 and, similar to those reported previously, have activity ratios (230Th/238U) of 1.01–1.03 close to secular equilibrium. A Holocene rhyodacite flow from Lassen has a 232Th/238U ratio of 2.9 and (230Th/238U) near secular equilibrium (0.99), similar to lavas from Shasta reported here and elsewhere. In contrast, basaltic andesite lavas from the Cinder Cone at Lassen show large U enrichments, 232Th/238U ratios between 1.4 and 1.7 and (230Th/238U) of 0.61–0.74. These are the highest 238U over 230Th enrichments seen in Cascade lavas, similar to many arc lavas. Lavas from Lassen, despite large differences in bulk composition of U/Th and (230Th/238U), have similar 87Sr/86Sr ratios (0.703954 – 0.704137). These Sr isotopic ratios are similar to values from other southern Cascade lavas. In contrast, the Shasta lavas have low 87Sr/86Sr ratios (0.702905 -0.703118), within the range of local MORB. The similarity in Sr isotopic composition between the more evolved lavas of Lassen and the Cinder Cone lavas argue against source variation to explain the U enrichment in the latter. The differences in (230Th/238U) are reflected in the 232Th/238U ratio, suggesting inherent U over Th enrichment of the Cinder Cone lavas, most likely due to fluid addition in the mantle.
Nicolas Slater, ’12 <
Dr. Yemane Asmerom
University of New Mexico, Albuquerque, New Mexico
Sponsor: Rhawn Denniston