[S-MP40] Thermal structure of subduction zones: modeling and the rock record

[SMP40-P01] Brucite as a major phase in the shallow mantle wedge

*Takayoshi Nagaya¹, Simon Richard Wallis¹, Kazuhiro Kawahara², Shunsuke Endo³, Hiroshi Mori⁴, Yoshihiro Asahara⁵ (1.Graduate School of Science, The University of Tokyo, 2.Japan Oil, Gas and Metals National Corporation, 3.Shimane University, 4.Shinshu University, 5.Nagoya University)

Keywords:Brucite, Shallow forearc mantle, Si metasomatism, Serpentinite, Antigorite, Sanbagawa metamorphism

The shallow part of the mantle wedge in convergent margins is relatively low temperature and influx of water from the subducting slab will cause peridotite mantle to become serpentinized. If this serpentinization affects olivine-rich mantle and proceeds under low SiO2 activity, brucite can be a major phase where temperatures are <450°C. However, only very few natural examples have been documented mostly from blocks witin serpentinite diapirs associated with subduction along the Izu-Bonin arc and the proportion and distribution of this mineral in the mantle wedge is not well known. The formation of brucite is generally thought to be suppressed by the supply of silica in excess of that present in olivine, either by the break down of pyroxene or the inflow of SiO2 in aqueous solution. The presence of brucite causes a drastic lowering of the strength of serpentinite even at proportions of a few per cent and determining its distribution and proportion is a key issue in developing physical models for the shallow mantle wedge. The subduction-type Sanbagawa metamorphic belt contains a large number of units derived from the wedge mantle. One of the largest of these units is the Shiraga metaserpentinite body. Brucite is present in two types (brucite I and II) that can be distinguished by their chemical zonation and microstructure. Brucite I shows clear reaction microstructures with antigorite to form olivine. In contrast, brucite II shows no reaction relationships with antigorite and is interpreted as a retrograde product. The original amount of brucite I was estimated based on both thermodynamic modeling combined with bulk rock composition and point counting of constituent minerals combined with metamorphic reactions. The results of the two techniques show the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~10–15 vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H2O budget and mechanical properties of serpentinite, these should be taken into consideration when examining the behavior of the shallow mantle wedge.