HELIUM AS A TRACER FOR FLUIDS RELEASED FROM JUAN DE FUCA LITHOSPHERE BENEATH THE CASCADIA FOREARC

Helium isotope ratios in mineral springs provide an indication of the sources and pathways for magma ascending beneath volcanic arcs and are used as a tracer for fluids associated tectonic processes occurring in subduction systems. We sampled a series of mineral springs to define fluids derived from Juan de Fuca lithosphere beneath the forearc as the subducting slab dehydrates and densifies with increasing depth. Surface springs above the slab depth of 25–30 km have ³He/⁴He ratios of ~0.3 (R/R_A); above the slab at a depth of ~40 km the ratio is ~4.0; and for springs above the slab at depths of 50–55 km the ratio ranges from ~0.7–1.6.

The springs situated trenchward of the forearc mantle corner (FMC; varying from 35 to 43 km deep), yield the lowest ratios, thus indicating only a minor component of mantle-derived helium within spring waters. Springs situated arcward of the FMC yield intermediate (0.8–1.2 R_C/R_A ) to high (>1.2 R_C/R_A ) ratios, indicating a significant component of mantle-derived helium. Although helium isotopes do not allow us to differentiate between oceanic and forearc mantle sources, the lowest values are situated above the region that lacks forearc mantle, suggesting that either little slab-derived fluid is released at shallow slab depths, or that forearc mantle is the major source of ³He and acquired as the fluids rise to the surface. Sample sites range from 40 km to more than 200 km from the nearest Cascade Arc volcano. For the closer sites, we cannot rule out that ³He may be partially derived from westward migration of arc related fluids. The highest value occurs ~130 km from the nearest arc volcano, thus likely does not reflect arc related fluids.

These preliminary observations provide geologic evidence that slab-derived fluids can migrate through the forearc mantle wedge to the surface even though the mantle is typically considered a sink for fluids owing to serpentinization processes. Likely pathways consist of fractures in the forearc mantle lined with serpentinized material that would serve to seal the fractures from the rising fluids.