Serpentine and brucite of ultramafic clasts from the South Chamorro Seamount (Ocean Drilling Program Leg 195, Site 1200): inferences for the serpentinization of the Mariana forearc mantle

摘要

Serpentine minerals and brucite in ultramafic rocks from the South Chamorro Seamount were characterized chemically to investigate the serpentinization of the Mariana forearc mantle. Relict primary minerals of the seipentinites are olivine, enstatite and minor Cr-spinel and diopside. The secondary minerals are mostly serpentine and brucite with minor magnetite. The seipentine minerals, mostly lizardite and chrysotile, display large compositional variations. A1_2O_3 and Cr_2O_3 contents depend generally upon the nature of the primary mineral from which the serpentine was derived. Both serpentine minerals and brucite exhibit wide Mg, Fe and Mn substitution: the Mg# ranges are 95.1-77.2 and 88.9-60.8, respectively. These mineralogical and chemical features allowed us to estimate an upper temperature limit for serpentinization of approx 200-300 deg C, in agreement with recent thermal models which suggest that the seipentinized mantle wedge of the Izu-Bonin-Mariana subduction zone is cold. The high degree of serpentinization (40-100 percent, average >75 percent), and the serpentine + brucite paragenesis of these ultramafics imply that the Mariana forearc mantle has a significantly reduced density and strength down to ~30 km, which provides a driving mechanism for serpentinite diapirism. Pervasive serpentinization of the forearc by fluids released from the decollement zone also explains the low seismicity of the Izu-Bonin-Mariana subduction zone.