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

Abstract

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 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.

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