Detection of mantle-derived fluid from the Makimine mélange in the Shimanto accretionary complex: Evidence from helium isotope analysis on mineral veins
Abstract
The detection of mantle helium from hot/cold spring waters in forearc regions suggested that subduction fluids have returned to the Earth surface after its interaction with mantle wedge. However, it remains uncertain how mantle-derived fluids have returned. To examine fluid flow processes along subduction plate boundary, we measured noble gas isotope compositions (He, Ar, Kr, and Xe) of mineral veins in the Makimine mélange of the Late Cretaceous Shimanto accretionary complex in southwest Japan. The mélange was deformed at the frictional-viscous transition zone at 10-15 km depth and 300-350 °C. The analyzed veins are sigmoidal extension vein arrays and shear veins. The former represents Riedel shear zones formed during subduction under vertical σ1. The latter records short recurrence time of low-angle brittle thrusting at near-lithostatic fluid pressures within viscous shear zones, which is interpreted to be geological manifestation of episodic tremor and slow slip (ETS). The both types of veins are filled with quartz with a small amount of calcite. The measured 3He/4He ratios range 1.6-2.5 Ra (Ra is the 3He/4He ratio of air), regardless of types of veins. This suggests the presence of mantle-derived helium in the veins. Since the mantle-originated rocks are absent in the Makimine mélange, the presence of mantle-derived helium in the veins reflects that the fluids derived from near mantle wedge and/or from slab mantle migrated along plate boundary to shallow depths of 10-15 km. Argon, Kr, and Xe data suggest that the vein-forming fluid is a mixture of seawater and the fluids dehydrated from sediment, altered oceanic crust, and/or serpentinized slab mantle. If the seawater is attributed to local pore fluid and the dehydrated fluids to the mantle-derived fluid, a mixing model yields ≥23-25 % and ≥11-26 % contributions of the mantle-derived fluids for the shear veins and the extension veins, respectively. Therefore, the mantle-derived fluids may lead to increase in fluid pressure during subduction, which could also contribute to generation of ETS under near-lithostatic fluid pressures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T31G0394N
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8045 Role of fluids;
- STRUCTURAL GEOLOGYDE: 8170 Subduction zone processes;
- TECTONOPHYSICS