11:45 〜 12:00
[SIT40-10] マントルウェッジにおける蛇紋岩化作用の進行：酸化還元状態への影響
キーワード：蛇紋岩化作用, 水素, 磁鉄鉱, 沈み込み帯, 酸化還元状態
Serpentinization of peridotite in the mantle is a key process that significantly changes the physical properties of the mantle. Serpentinization also produces hydrogen, which is essential not only for the activity of microbial systems in hydrothermal fields on the seafloor, but also for controlling the oxidation state of the mantle in subduction zones. Hydrogen is generated along with the formation of magnetite during serpentinization. However, there still remains controversy about what factors promote the mineralogical reactions responsible for magnetite formation during serpentinization in natural ultramafic rocks. Recent petrologic studies have proposed that serpentinization reactions proceed via a two-stage process involving the early formation of serpentine and brucite and subsequent magnetite formation. Many studies proposed that magnetite forms by the break down of ferrous brucite promoted by the addition of aqueous silica, but others proposed that magnetite forms by the breakdown of ferrous serpentine which releases silica component. To solve this controversy, we examined a number of variably serpentinized harzburgite and dunite samples taken from the Iwanaidake ultrama?c body in Kamuikotan belt, Japan (Miyoshi et al. 2014). Petrographic observations of these samples revealed that successive changes in textures, mineral chemistry, whole-rock H2O contents, and magnetic susceptibility with the progress of serpentinization of harzburgite involved two stages: replacement of olivine by serpentine and brucite, and subsequent formation of magnetite along with more-magnesian serpentine and brucite. The later reactions occurred concurrently with serpentinization of orthopyroxene, which supplied the silica component. In serpentinized dunite, which doesn't contain orthopyroxene, serpentinization involved replacement of olivine by serpentine and brucite, and the fraction of magnetite did not increase with the progress of serpentinization. These observations, and the fact that the Iwanaidake ultramafic body originated from the forearc mantle of the Northeast Japan arc, suggest that the silica supply from serpentinization of orthopyroxene is an essential factor for the formation of magnetite during serpentinization in mantle wedge.Our observations imply that serpentinization in the mantle wedge of subduction zone produces H2 along with magnetite if sufficient amounts of silica component are supplied from subducting slab, which will probably occur because dehydration in subducted sediments can supply silica-rich fluids. Since H2 is expected to exist as immiscible hydrogen-rich gas phases that coexist with H2O fluids in normal subduction zone conditions, it will be rapidly migrate upwards owing to its very low density. Then the remaining serpentinites will become oxidized. Such oxidation associated with serpentinization would occur in the shallow part of the wedge corner where temperatures are lower than ~600℃, but the oxidized (magnetite-bearing) serpentinite will be dragged downwards in the mantle wedge. Thus serpentinization reactions can be one of the main processes to increase the oxygen fugacity of the mantle wedge. On the other hand, the H2 gas removed from the wedge corner will produce highly reduced fluid phases, which may result in reducing the shallowest part of the forearc mantle and the lower part of the forearc crust. This could be the cause of rare presence of metal phases in subarc peridotite.Reference: A. Miyoshi, T. Kogiso, N. Ishikawa, K. Mibe (2014) American Mineralogist, in press.