Tracing changes in mantle and crustal influences in individual cone-building stages at Mt. Shasta using U–Th and Sr isotopes

摘要: Abstract 230Th-excess is rare in most arc lavas, but common the Cascades, yet origin of such excesses remains unclear. At Mt. Shasta, age-corrected (230Th/232Th) and (238U/232Th) activity ratios range from 1.108 to 1.290 0.987 1.309 (27.3% 6.1% 238U-excess), respectively. Although small degrees zircon crystallization ( destruction ancestral cone (Sand Flat) was followed by four cone-building stages, three which lie age U-series geochronology. Lavas within individual eruptive stages have relatively constant (230Th/232Th)0 that are interpreted reflect specific mixtures mantle (m) lower crustal (lc) melts characteristic a stage M m : l c ). High identify higher proportions crust Misery Hill = ∼ 85 15 ), whereas low more mantle-like composition Shastina lavas 95 5 ); case very 87Sr/86Sr ratios, down 0.7029, support substantial contribution. Changes correlate with volume, where voluminous (Misery Hill) inferred largest proportion melt highest ratios. Variable (230Th/238U)0 within, between, groups likely combination residence time differential assimilation bulk, non-garnet-bearing had (230Th/238U) 1. The volume-(230Th/232Th)0 relations accompanied correlations radiogenic Sr associated volumes, indicating magmatic episodes produced amount interaction. new U–Th isotope measurements this study, along data for other Cascade centers suggest interaction exerts greater control on magma chemistry than previously recognized. Indeed, dry nature Cascades may offer unique opportunity better understand influence deep young continental arcs, as larger subduction components settings overprint any signature produce large 238U-excesses.