Helium isotope evidence for mixing of mantle-derived fluids and deeply penetrating surface waters in an obducted peridotite massif

We measured He and Ar in fresh peridotites from the Twin Sisters massif, Washington USA. 3He/4He ratios measured on >35 samples are highly variable (∼0.8 to ∼6 times the atmospheric ratio, RA). Step-heating of a subset of these samples in every case reveals a low 3He/4He component (∼1 RA) released at <1000 °C and a high 3He/4He component (>3 RA) above that temperature, but these components are not effectively isolated by crushing and powder fusion analysis. He-Ar systematics indicate an intimate association of two fluid-inclusion hosted components in the peridotites. The first is a ∼6 RA mantle component that is released at higher temperatures during step heating and is more abundant in dunite bands than surrounding harzburgites. The second component, released at low temperatures, has a 3He/4He ratio of 1.0 ± 0.5 RA, atmospheric 40Ar/36Ar, and 4He/40Ar far above atmospheric. It appears to be a mixture of mantle and radiogenic He sources introduced during obduction-related serpentinization, sometimes invisible, by surface-derived waters enriched with deeply-sourced helium.