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

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, doc...

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Bibliographic Details
Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2016-06, Vol.17 (6), p.2434-2449
Main Authors: McCrory, P. A., Constantz, J. E., Hunt, A. G., Blair, J. L.
Format: Article
Language:English
Subjects:
Cascadia forearc
Geophysics
Helium
Isotopes
Lithosphere
Magma
mantle helium
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Summary: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. Key Points Springs enriched in mantle‐derived helium occur in the Cascadia forearc Springs with highest helium ratios (RA) occur above the tectonic tremor band, similar to Nankai forearc Mantle‐derived helium in the forearc provides evidence of link between deep fluids and tremor
ISSN:1525-2027
1525-2027
DOI:10.1002/2015GC006198