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[SCG54-04] Noble gas contents of serpentinite-related metasomatic rocks and the origin of subduction fluids: Example of the Cretaceous Sanbagawa metamorphic belt, SW Japan
Keywords:subduction zone, noble gas, serpentinite, metasomatism
Fluids released as plates subduct play an essential role in active magma generation and seismic activity in convergent margins. High-pressure low-temperature metamorphic belts preserve rocks that record processes occurring along the plate interface in subduction zones and commonly contain units of ultramafic rocks derived from the mantle wedge. These ultramafic rocks are generally surrounded by metasomatic zones composed mainly of hydrous minerals. Such metasomatic zones are direct geological records of hydrous fluids moving along the slab-mantle interface and causing transport of different chemical species. Both the origin of these metasomatic fluids and the pathways followed once fluids are released by dehydration are not well known.
In this study, we selected several 101 –103 m scale ultramafic bodies and their surrounding metasomatic reaction zones in the Sambagawa metamorphic belt for detailed study: Kamabuseyama, Saitama; Ryumonzan, Wakayama; Hatobayama, Wakayama; and Nushima, Hyogo. The Sanbagawa belt is an example of a warm subduction-related metamorphic belt. The origins of the fluids in these rocks were inferred by applying noble gas mass spectrometry to fluid inclusions and crystalline water of hydrous minerals. Three major reservoirs have been identified as potential sources of subduction fluids: oceanic sediment pore fluid, altered oceanic crustal basalt, and serpentinized lithospheric mantle. Each of these reservoirs has characteristic isotopic and elemental ratios of noble gases (He, Ne, Ar, Kr, and Xe). These known relationships, coupled with the inert chemical behavior of noble gases, make them very powerful as tracers for inferring the origin of fluids.
Our geological survey revealed metasomatic reaction zones mainly consisted of serpentine (chrysotile and antigorite), amphibole, talc, and chlorite. The noble gas compositions (3He/4He, 84Kr/36Ar, 130Xe/36Ar) of the metasomatic rocks revealed two distinct fluid inflow events.
(1) Serpentinization of the wedge mantle
Fluids of altered oceanic crustal basalt origin caused the initial serpentinization of ultramafic bodies in many localities. This indicates that dehydration of altered oceanic crustal basalts continues to be an important source of subduction fluids at depths beneath the wedge mantle and confirms proposals of previous studies (e.g., Hyndman and Peacock, 2003).
(2) Boundary rock formation (formation of talc, amphibole, and chlorite)
Analyses of a range of different metasomatic lithologies suggest relationships between the type of hydrous mineral and the origin of the fluid. This suggests that in addition to the distinct noble gas compositions, fluids derived from different sources also had distinct major chemical species. Moreover, the origin of the fluids involved in the metasomatism was heterogeneous from place to place and may have changed with time.
In this study, we selected several 101 –103 m scale ultramafic bodies and their surrounding metasomatic reaction zones in the Sambagawa metamorphic belt for detailed study: Kamabuseyama, Saitama; Ryumonzan, Wakayama; Hatobayama, Wakayama; and Nushima, Hyogo. The Sanbagawa belt is an example of a warm subduction-related metamorphic belt. The origins of the fluids in these rocks were inferred by applying noble gas mass spectrometry to fluid inclusions and crystalline water of hydrous minerals. Three major reservoirs have been identified as potential sources of subduction fluids: oceanic sediment pore fluid, altered oceanic crustal basalt, and serpentinized lithospheric mantle. Each of these reservoirs has characteristic isotopic and elemental ratios of noble gases (He, Ne, Ar, Kr, and Xe). These known relationships, coupled with the inert chemical behavior of noble gases, make them very powerful as tracers for inferring the origin of fluids.
Our geological survey revealed metasomatic reaction zones mainly consisted of serpentine (chrysotile and antigorite), amphibole, talc, and chlorite. The noble gas compositions (3He/4He, 84Kr/36Ar, 130Xe/36Ar) of the metasomatic rocks revealed two distinct fluid inflow events.
(1) Serpentinization of the wedge mantle
Fluids of altered oceanic crustal basalt origin caused the initial serpentinization of ultramafic bodies in many localities. This indicates that dehydration of altered oceanic crustal basalts continues to be an important source of subduction fluids at depths beneath the wedge mantle and confirms proposals of previous studies (e.g., Hyndman and Peacock, 2003).
(2) Boundary rock formation (formation of talc, amphibole, and chlorite)
Analyses of a range of different metasomatic lithologies suggest relationships between the type of hydrous mineral and the origin of the fluid. This suggests that in addition to the distinct noble gas compositions, fluids derived from different sources also had distinct major chemical species. Moreover, the origin of the fluids involved in the metasomatism was heterogeneous from place to place and may have changed with time.