V23C-02
Helium as a Tracer for Fluids Released from Juan de Fuca Lithosphere Beneath the Cascadia Forearc

Tuesday, 15 December 2015: 13:55
305 (Moscone South)
Patricia A McCrory1, James E Constantz1, Andrew G Hunt2,3 and J Luke Blair1, (1)U.S. Geological Survey, Menlo Park, CA, United States, (2)U.S. Geological Survey, Denver Federal Center, Denver, CO, United States, (3)U.S. Geological Survey, Denver, CO, United States
Abstract:
Helium isotopic ratios (3He/4He) observed in mineral springs above the Cascadia forearc provide a marker for fluids derived from Juan de Fuca lithosphere. Sample sites arcward of the forearc mantle corner generally yield significantly higher ratios (~1.5–4.0 R/RA) than those seaward of the corner (~0.3–0.6 R/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 forearc coincide with slab depths (~40–45 km) where metamorphic dehydration of young, warm oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab (~ 25–30 km depth) beneath sites seaward of the corner.

High helium ratios are also observed in springs and wells in the Nankai and Hikurangi forearcs above the region where tremor and slow slip events are detected. This correlation provides independent evidence that tremor and slow slip are associated with deep fluids, and further suggests that high pore pressures associated with tremor may also serve to keep fractures open for 3He migration through the crust.

Even though our preliminary results document mantle-derived helium in surface waters of the Cascadia forearc, these results are based on sparse data from sample locations that are not optimally distributed. We have recently identified additional sample sites to investigate whether specific crustal structures in the Cascadia forearc might serve as conduits to speed the ascent of mantle-derived helium. Finally, the possibility of a 3He source related to westward flow of arc-derived fluids through the forearc mantle cannot be ruled out for some of the sites, nonetheless, the highest ratio (4.0 R/RA) is found >130 km from the nearest Cascade Arc volcano making a magmatic source unlikely.

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