9:45 AM - 10:00 AM
[SIT26-04] Preferential orthopyroxene serpentinization in the fore-arc mantle and implications for seismic velocity interpretation
Keywords:fore-arc mantle, serpentinization, orthopyroxene, talc, seismic velocity
Seismic velocity calculations indicate that Vp and Vs of hydrated harzburgite decrease monotonically with increasing reacted H2O content; the rate of decrease does not depend on the extent of the preferential Opx reaction. However, the harzburgite hydration reaction reaches completion at a relatively low H2O concentration (as low as 2.6 wt%) in the preferential Opx reaction because the talc H2O concentration (4.8 wt%) is lower than that of serpentine (13 wt%). For example, Vp decreases from 8.3 to 7.7 km/s when all the Opx reacts (i.e., 100% reaction progress) at 580°C and 1.3 GPa. However, it corresponds to a reaction progress of only 33% when the hydration reaction proceeds stoichiometrically to produce serpentine alone. Another important effect of talc formation is that, due to its lower Vp/Vs (∼1.60) compared with that of serpentine (∼1.78), the bulk rock Vp/Vs does not change significantly with talc formation along with serpentine. Regions with low Vp and Vs but no conspicuous Vp/Vs perturbation might be explained by the presence of talc, which may contribute to weakening of the mantle (Hirauchi et al., 2016). We also found a case of preferential Opx reaction and resultant talc formation in natural samples; peridotite from the Lower Zone of the Horoman complex, which is assumed as hydrated at pressures below 0.5 GPa. This observation supports the possibility of preferential Opx reactions occurring in the fore-arc mantle.