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, R A). Step-heating of a subset of these samples in every case reveals a low 3He/4He component (∼1 R A) released at <1000 °C and a high 3He/4He component (>3 R A) 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 R A 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 R A, 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. These data indicate that mantle noble gas signatures can be retained in lithospheric peridotites against both diffusive loss and radiogenic ingrowth over at least 108 year timescales, likely due to concentration and immobilization of He in fluid inclusions. However, the mantle signature can be greatly modified by pervasive and potentially cryptic fluid alteration during emplacement. [ABSTRACT FROM AUTHOR]