Assessment of the usefulness of lithic clasts from pyroclastic depositsfor paleointensity determination

摘要

Paleomagnetic and rock magnetic measurements were carried out on lithic clastsfound within pyroclastic deposits to assess their potential for paleointensitydeterminations. The use of multiple lithologies in a single paleointensity determinationwould provide confidence that the result is not biased by alteration within one lithology.Lithic clasts were sampled from three historically active volcanoes: Lascar in the ChileanAndes, Mt. St. Helens, United States, and Vesuvius, Italy. At Lascar, triple heatingpaleointensity experiments allow development of new selection criteria for lithic clastsfound within pyroclastic deposits. Using these criteria, the Lascar data yield a meanpaleointensity of 24.3 ± 1.3 μT (1σ, N = 26), which agrees well with the expected valueof 24.0 μT. This indicates that pyroclastic rocks have promise for paleointensitydeterminations. Pyroclastics, however, still suffer from the range of problems associatedwith conventional paleointensity experiments on lava flows. Samples from Mt. St. Helensare strongly affected by multidomain (MD) behavior, which results in all samples failing topass the paleointensity selection criteria. At Vesuvius, MD grains, magnetic interactions,and chemical remanent magnetizations contributed to failure of all paleointensityexperiments. Rock magnetic analyses allow identification of the causes of failure of thepaleointensity experiments. However, in this study, they have not provided adequatepreselection criteria for identifying pyroclastics that are suitable for paleointensitydetermination.