Mathematics – Logic
Scientific paper
Dec 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..475v&link_type=abstract
Earth and Planetary Science Letters, Volume 107, Issue 3-4, p. 475-486.
Mathematics
Logic
35
Scientific paper
We use 238U-series nuclides and 230Th/232Th ratios measured by mass spectrometry to constrain processes and time scales of calc-alkaline magma genesis at Mount St. Helens, Washington. Olivine basalt, pyroxene andesites and dacites that erupted 10-2 ka ago show 3-14% (230Th)-(238U) and 6-54% 226Ra-230Th disequilibria. Mineral phases exhibit robust (226Ra)-(230Th) fractionation. Plagioclase has large 65-280% (226Ra) excesses, and magnetite has large 65% (226Ra) deficits relative to (230Th). Calculated partition coefficients for Ba, Th, and U in mineral-groundmass pairs, except Ba in plagioclase, are low (<= 0.04). Correlation between (226Ra/230Th) activity ratios and rmBa/Th element ratios in the minerals suggests that 226Ra partitions similar to Ba during crystallization.
Internal (230Th)-(238U) isochrons for 1982 summit and East Dome dacites and Goat Rocks and Kalama andesites show that closed Th-U system fractionation occurred 2-6 ka ago. Apparent internal isochrons for Castle Creek basalt (34 ka) and andesite (27 ka) suggest longer magma chamber residence times and mixing of old crystals and young melt. Mineral (226Ra)-(230Th) disequilibrium on Ba-normalized internal isochron diagrams suggests average magma chamber residence times of 500-3000 years. In addition, radioactive (226Ra/230Th) heterogeneity between minerals and groundmass or whole rock is evidence for open-system Ra-Th behavior. This heterogeneity suggests there has been recent, post-crystallization, changes in melt chemical composition that affected 226Ra more than 230Th. Clearly, magma fractionation, residence and transport of crystal-melt before eruption of chemically diverse lavas at Mount St. Helens occurs over geologically short periods.
Hammond Paul E.
Volpe Alan M.
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