Helium as a tracer for fluids released from Juan de Fuca lithosphere beneath the Cascadia forearc

Abstract

Helium isotopic ratios (3He/4He) observed in 25 mineral springs and wells above the Cascadia forearc provide a marker for fluids derived from Juan de Fuca lithosphere. This exploratory study documents a significant component of mantle-derived helium within forearc springs and wells, and in turn, documents variability in helium enrichment across the Cascadia forearc. Sample sites arcward of the forearc mantle corner generally yield significantly higher ratios (∼1.2–4.0 RA) than those seaward of the corner (∼0.03–0.7 RA). 3He detected above the inner forearc mantle wedge may represent a mixture of both oceanic lithosphere and forearc mantle sources, whereas 3He detected seaward of the forearc mantle corner likely has only an oceanic source. The highest ratios in the Cascadia forearc coincide with slab depths (∼40–45 km) where metamorphic dehydration of young oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (∼25–30 km) beneath sites seaward of the corner. These observations provide independent evidence that tremor is associated with deep fluids, and further suggest that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through the ductile upper mantle and lower crust.