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[MZZ42-05] Volatile recycling in the mantle revealed by ultra-trace halogen analysis using neutron irradiation and noble gas mass spectrometry
Keywords:halogen, noble gas mass spectrometry, neutron irradiation, volatile
Heavy halogens (Cl, Br, and I) are highly concentrated in surface geochemical reservoirs because they are highly volatile and partitioned into the liquid phase. Since elemental ratios of halogens vary widely depending on the geochemical reservoirs, they are expected to be good tracers of the material exchange between the Earth's surface and mantle, especially the water recycling into the mantle [1]. Halogens in mantle materials have been challenging to analyze by conventional methods due to their low concentrations, but recently, highly sensitive analysis has become possible by a combination of neutron irradiation and noble gas mass spectrometry (NI-NGMS) [2].
The halogen compositions of slab fluids trapped in serpentine and fluid inclusions in exhumed metamorphic rocks from the Sanbagawa metamorphic belt and mantle-derived rocks from subduction zones (Japan, Kamchatka, and Philippine) are similar to those of sedimentary pore fluids, which is most prominently characterized by an elevated I/Cl ratio [3-5]. The slab fluids' halogen signature also resembles forearc and seafloor serpentinites [6]. These suggest that serpentine carries sedimentary pore fluids into the mantle in the subducting plate's oceanic lithosphere. In addition, samples from a serpentinite body in the Sanbagawa belt show overall decreasing I/Cl ratios with increasing distance from the subduction interface, suggesting that the original high-I/Cl fluids were transported along the subduction interface and penetrated upward into the mantle wedge [5].
Halogen signatures found in the mantle-wedge-derived rocks reveal that serpentine supplies a significant amount of water to the mantle wedge beneath the volcanic front. On the other hand, halogen compositions of the slab fluids are different from those in mantle-derived xenoliths and diamonds brought to the surface by intraplate volcanoes or kimberlites and from those of MORB- and OIB-source mantle, suggesting that the pore-fluid-like halogens would not subduct beyond arcs [7-10].
[1] Clay and Sumino, Elements, 2022, 18: 9-14. [2] Kobayashi et al., Chem. Geol., 2021, 2021, 582: 120420. [3] Sumino et al., EPSL, 2010, 294: 163-172. [4] Kobayashi et al., EPSL, 2017, 457: 106-116. [5] Ren et al, Goldschmidt Abstracts, 2021 6698. [6] Kendrick et al., EPSL, 2013, 365: 86-96. [7] Kobayashi et al., G-cubed, 2019, 20: 952-973. [8] Kendrick et al., Nature Geo., 2017, 10: 222-228. [9] Broadley et al., Nature Geo., 2018, 11: 682-687. [10] Toyama et al., Am. Mineral., 2021, 106, 1890-1899.
The halogen compositions of slab fluids trapped in serpentine and fluid inclusions in exhumed metamorphic rocks from the Sanbagawa metamorphic belt and mantle-derived rocks from subduction zones (Japan, Kamchatka, and Philippine) are similar to those of sedimentary pore fluids, which is most prominently characterized by an elevated I/Cl ratio [3-5]. The slab fluids' halogen signature also resembles forearc and seafloor serpentinites [6]. These suggest that serpentine carries sedimentary pore fluids into the mantle in the subducting plate's oceanic lithosphere. In addition, samples from a serpentinite body in the Sanbagawa belt show overall decreasing I/Cl ratios with increasing distance from the subduction interface, suggesting that the original high-I/Cl fluids were transported along the subduction interface and penetrated upward into the mantle wedge [5].
Halogen signatures found in the mantle-wedge-derived rocks reveal that serpentine supplies a significant amount of water to the mantle wedge beneath the volcanic front. On the other hand, halogen compositions of the slab fluids are different from those in mantle-derived xenoliths and diamonds brought to the surface by intraplate volcanoes or kimberlites and from those of MORB- and OIB-source mantle, suggesting that the pore-fluid-like halogens would not subduct beyond arcs [7-10].
[1] Clay and Sumino, Elements, 2022, 18: 9-14. [2] Kobayashi et al., Chem. Geol., 2021, 2021, 582: 120420. [3] Sumino et al., EPSL, 2010, 294: 163-172. [4] Kobayashi et al., EPSL, 2017, 457: 106-116. [5] Ren et al, Goldschmidt Abstracts, 2021 6698. [6] Kendrick et al., EPSL, 2013, 365: 86-96. [7] Kobayashi et al., G-cubed, 2019, 20: 952-973. [8] Kendrick et al., Nature Geo., 2017, 10: 222-228. [9] Broadley et al., Nature Geo., 2018, 11: 682-687. [10] Toyama et al., Am. Mineral., 2021, 106, 1890-1899.