Quaternary lavas from Mount Lassen, Mount Shasta, and Medicine Lake, which range in composition from olivine basalt to rhyolite, have Sr87/Sr86 values that fall between 0.7030 and 0.7043, corrected to 0.7080 for the Eimcr and Amend (MIT) SrCO3 standard. These values are very close to average (corrected) strontium isotopic values of some other circum-Pacific volcanic suites and of lavas of oceanic islands. The low ratios suggest a mantle origin. The notably high strontium content (1400 ppm) in the andesites from Mount Shasta also indicates a mantle origin at depths where plagioclase is absent as a stable phase in the residual material.

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