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 U-238-Th-230 disequilibria

作者：Ankney Meagan E^*; Johnson Clark M; Bacon Charles R; Beard Brian L; Jicha Brian R

来源：Contributions to Mineralogy and Petrology, 2013, 166(2): 563-585.

DOI：10.1007/s00410-013-0891-4

• Mount Mazama
• Crater Lake
• Cascade arc
• Uranium-series isotopes
• Crustal interaction

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 Th-230-excess and U-238-excess components. U-Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected (Th-230/Th-232) and (U-238/Th-232) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % Th-230-excess to 8.8 % U-238-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 U-238-excess, but the HSr component, as well as more primitive lavas, are marked by Th-230-excess. Th-230-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high Th-230-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 U-238-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, U-238-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, U-238-excess in arcs is common and likely masks Th-230-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 U-238-excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating U-238-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.

• 出版日期2013-8