High Pressure-Temperature Single-Crystal Elasticity of Grossular: Implications for the Low-Velocity Layer in the Bottom Transition Zone

摘要

We determined the single-crystal elasticity of Fe~(3+)-bearing grossular using Brillouin scattering up to 22 GPa and 1,000 K. Using the obtained results, we modeled mantle velocity profiles to investigate the origin of low-velocity zones (LVZs) in the bottom 100 km of the mantle transition zone. Our modeling revealed that although a water-saturated transition zone could be seismically detected as the LVZs, it would also cause a negative velocity jump across the 520-km discontinuity, which has not been seismically detected. However, mid-ocean ridge basalts (MORB) with an FeO content of 10-17 wt.% in garnet will have V_S 2.0(5)%-3.0(5)% lower than that in the pyrolitic mantle. Oxidizing (FeO = 17 wt.%, Fe~(3+)/(Fe~(2+) + Fe~(3+)) = 0.3-0.8) or lowering the Ca content of garnet to 1.4 wt.% can further decrease the V_S of MORB. Such Fe-enriched, oxidized MORB or Ca-depleted, oxidized MORB, with a 3.5(5)%-7.6(5)% lower V_S and distinct dln ρ/dln V_S ratio, have a seismic signature consistent with the observed LVZs in the region.