2:45 PM - 3:00 PM
[SGC32-07] Halogen and noble gas compositions of mantle wedge and subducted sediment, recorded in metamorphic rocks from the Sanbagawa belt
Keywords:Mantle wedge, Sanbagawa belt, Halogen, Noble gas
Subduction of crustal rocks results in transportation of volatiles into the mantle, which can have a major effect on the chemical and physical properties of constituent rocks. Volatile transport also plays an important role in arc volcanism and seismicity. However, the details of volatile cycling process remain debated. The distinct elemental and/or isotopic compositions of halogens and noble gases in different reservoirs (e.g. MORB-source mantle, seawater, subducted sediments and igneous rocks) make them good tracers for fluid origin in subduction zones. We analyzed the halogen and noble gas compositions in a series of metamorphic rocks that recorded a paleo slab-mantle wedge boundary (Kawahara et al, 2016). Analyzed samples are ultramafic serpentinite (derive from the mantle wedge) and schist (derive from subducted sediment) located in the Mt. Shiraga region of the Sanbagawa belt in central Shikoku.
In order to determine the trace amounts of halogens, especially those contained in fluid inclusions, a portion of each sample was neutron-irradiated in the Kyoto University Research Reactor to convert halogens (37Cl, 79Br and 127I) to noble gas isotopes (38Ar, 80Kr,128Xe), which have lower detection limits than direct analyses of halogens with other methods. Stepwise crushing and heating processes were conducted in order to extract volatiles contained in different sites of the rocks. The irradiated and un-irradiated portions were analyzed by noble gas mass spectrometry at the University of Tokyo, to determine halogen and noble gas compositions respectively.
The halogen compositions of fluids obtained by crush extraction show relatively high I/Cl ratios with stable Br/Cl ratios compared with halogens in MORB-mantle or seawater. These features are similar to the data for the nearby Higashi-akaishi mantle wedge peridotite exhumed from ~100 km depth (Sumino et al, 2010), which are considered to partially preserve the signals of sedimentary pore fluids. In addition, the serpentinite body shows overall decreasing I/Cl ratios with increasing distance from the subduction boundary. This supports the idea that the original high I/Cl fluids were transported along the subduction interface and penetrated upward into the mantle wedge.
The isotopic data of the noble gases are complex. Neon and Argon isotopic compositions are almost overwhelmed by the seawater/atmosphere component. Helium shows distinct and diverse 3He/4He ratios, which is likely to be strongly influenced by the cosmic ray, producing 3He preferentially. Nevertheless, a radiogenic 3He/4He component possibly derived from subducted crust/sediment was identified in the fluids extracted from the serpentinite body, which is similar to slab-derived fluids preserved in the Higashi-akaishi peridotite (Sumino et al, 2010). This adds support to the idea that the shallow part of mantle wedge has been influenced by addition of volatiles from the subducted slab.
In order to determine the trace amounts of halogens, especially those contained in fluid inclusions, a portion of each sample was neutron-irradiated in the Kyoto University Research Reactor to convert halogens (37Cl, 79Br and 127I) to noble gas isotopes (38Ar, 80Kr,128Xe), which have lower detection limits than direct analyses of halogens with other methods. Stepwise crushing and heating processes were conducted in order to extract volatiles contained in different sites of the rocks. The irradiated and un-irradiated portions were analyzed by noble gas mass spectrometry at the University of Tokyo, to determine halogen and noble gas compositions respectively.
The halogen compositions of fluids obtained by crush extraction show relatively high I/Cl ratios with stable Br/Cl ratios compared with halogens in MORB-mantle or seawater. These features are similar to the data for the nearby Higashi-akaishi mantle wedge peridotite exhumed from ~100 km depth (Sumino et al, 2010), which are considered to partially preserve the signals of sedimentary pore fluids. In addition, the serpentinite body shows overall decreasing I/Cl ratios with increasing distance from the subduction boundary. This supports the idea that the original high I/Cl fluids were transported along the subduction interface and penetrated upward into the mantle wedge.
The isotopic data of the noble gases are complex. Neon and Argon isotopic compositions are almost overwhelmed by the seawater/atmosphere component. Helium shows distinct and diverse 3He/4He ratios, which is likely to be strongly influenced by the cosmic ray, producing 3He preferentially. Nevertheless, a radiogenic 3He/4He component possibly derived from subducted crust/sediment was identified in the fluids extracted from the serpentinite body, which is similar to slab-derived fluids preserved in the Higashi-akaishi peridotite (Sumino et al, 2010). This adds support to the idea that the shallow part of mantle wedge has been influenced by addition of volatiles from the subducted slab.