Introduction

Here we report anhydrous chemical (CO₂, H₂S, N₂, H₂, CH₄, O₂, Ar, He, Ne) and isotopic (³He/⁴He, ⁴⁰Ar/³⁶Ar, ¹³C of CO₂, ¹³C of CH₄, ¹⁵N) compositions of virtually air-free gas samples collected between 1994 and 1998 from 12 quiescent but potentially restless volcanoes in the Cascade Range and Aleutian Arc (CRAA). Sample sites include ≤173°C fumaroles and springs at Mount Shasta, Mount Hood, Mount St. Helens, Mount Rainier, Mount Baker, Augustine Volcano, Mount Griggs, Trident, Mount Mageik, Aniakchak Crater, Akutan, and Makushin. The chemical and isotopic data generally point to magmatic (CO₂, Ar, He), shallow crustal sedimentary (hereafter, SCS) (CO₂, N₂, CH₄), crustal (He), and meteoric (N₂, Ar) sources of volatiles. CH₄ clearly comes from SCS rocks in the subvolcanic systems because CH₄ cannot survive the higher temperatures of deeper potential sources. Further evidence for a SCS source for CH₄ as well as for non-mantle CO₂ and non-meteoric N₂ comes from isotopic data that show wide variations between volcanoes that are spatially very close and similar isotopic signatures from volcanoes from very disparate areas. Our results are in direct opposition to many recent studies on other volcanic arcs (Kita and others, 1993; Sano and Marty, 1995; Fischer and others, 1998), in that they point to a dearth of subducted components of CO₂ and N₂ in the CRAA discharges. Either the CRAA volcanoes are fundamentally different from volcanoes in other arcs or we need to reevaluate the significance of subducted C and N recycling in convergent-plate volcanoes.