124 D64. Advances in Stable Isotope Geochemistry–Applications to Earth Science Systems

Monday, 5 November 2018
1:30 PM - 5:30 PM
Room 135

FOREARC MANTLE HELIUM AS A CONSTRAINT ON LOCATION OF THE SEISMOGENIC TRANSITION ZONE ALONG THE CASCADIA SUBDUCTION INTERFACE

Paper number: 124-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

The ratio between helium isotopes (3He/4He) provides an excellent geochemical tracer for investigating the sources of fluids sampled at the Earth’s surface. Primordial 3He has a mantle source, whereas 4He is mainly produced from radioactive decay of uranium- and thorium-series elements in the crust. Consequently fluids derived from a crustal source will exhibit low 3He/4He values of less than 0.02 RA (RA is the observed 3He/4He value divided by the atmospheric value). Conversely, 3He/4He values greater than 1.0 RA may be interpreted as including a component of mantle-derived helium, once corrected for a potential 3He contribution from 3H decay. In subduction systems where helium isotope data are available, two bands of enriched mantle-derived helium can typically be resolved—one along the volcanic arc and a second within the forearc. The highest ratios in the Cascadia forearc coincide with slab depths (40–45 km) where metamorphic dehydration of young Juan de Fuca lithosphere is expected to release significant deep fluids and where tectonic tremor occurs. Lower ratios coincide with slab depths (25–30 km) seaward of the forearc mantle corner (FMC) where little slab fluid is expected to be released. Specifically, 3He/4He values observed in 40 mineral springs and wells above the Cascadia forearc document ratios of 1.2–4.0 RA within the tremor band arcward of the FMC as compared with ratios of 0.03–0.7 RA seaward of the FMC and tremor. This spatial correlation between high RA values and tectonic tremor (considered a marker for high fluid pressure) provides independent evidence that tremor is associated with deep fluids, and further suggests that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through ductile upper mantle and lower crust. A similar pattern is observed along the Nankai subduction margin, where high ratios are documented along the tremor band—just arcward of the FMC. In both subduction systems, the spatial correlation of tremor, high RA values, and the FMC coincide with the inferred down-dip edge of the seismogenic transition zone on the subduction interface, suggesting that they can serve as a priori constraints on its location.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-321366

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