Distinguishing lower and upper crustal processes in magmas erupted during the buildup to the 7.7 ka climactic eruption of Mount Mazama, Crater Lake, Oregon, using 238U–230Th disequilibria

• Published in: Contributions to mineralogy and petrology Vol. 166; no. 2; pp. 563 - 585
• Main Authors: Ankney, Meagan E., Johnson, Clark M., Bacon, Charles R., Beard, Brian L., Jicha, Brian R.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.08.2013
• Subjects: Earth and Environmental Science; Earth Sciences; Geology; Mineral Resources; Mineralogy; Original Paper; Uranium-series isotopes; Crater Lake; Mount Mazama; Cascade arc; Crustal interaction
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-013-0891-4

Uranium-series isotope ratios determined for 35 volcanic rocks and 4 glass separates erupted from ~36 to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify both 230 Th-excess and 238 U-excess components. U–Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected ( 230 Th/ 232 Th) and ( 238 U/ 232 Th) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % 230 Th-excess to 8.8 % 238 U-excess), respectively. The most distinctive aspect of the data set is the contrast in U–Th isotope ratios between low and high Sr (LSr, HSr) components that have been previously identified in products of the 7.7 ka caldera-forming climactic eruption and preclimactic rhyodacite lavas. The LSr component exclusively contains 238 U-excess, but the HSr component, as well as more primitive lavas, are marked by 230 Th-excess. 230 Th-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high 230 Th-excesses observed in the more primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition that contained garnet in the residuum that was produced through dehydration melting of amphibolite that was initially garnet free. Dehydration melting in the lower crust offers a solution to the “hot-slab paradox” of the Cascades, where low volatile contents are predicted due to high slab temperatures, yet higher water contents than expected have been documented in erupted lavas. The 238 U-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, 238 U-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, 238 U-excess in arcs is common and likely masks 230 Th-excess resulting from lower crustal interaction. Isotopic and trace element data, however, suggest a relatively minor role for slab fluid fluxing in the Cascades. We propose that 238 U-excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating 238 U-excess are likely important features at Mt. Mazama that accompanied development of a large-scale silicic magma chamber that led to the caldera-forming eruption.